Tuesday, December 26, 2023

Forty Years of Personal Computing - MC6809 V2

MC6809 CPU card, version 2.
By March 1988, the MC6809E V1 card I designed in 1983 needed updates. I built an entirely new card with new features intended to run OS-9 more effectively. 


A MC6809 chip simplified things with the on-chip clock oscillator. The chip handled M.RDY without extra logic, and the rising edge of the Q clock did not need delay.


The MC6809E V1 card had no on-board RAM. There wasn't room. By 1988, a number of manufacturers had 32 KB static RAMs in 28-pin packages. 64 KB of memory is realized with a couple of chips. 

For the V2 board, I allowed for eight chips, totaling 256 KB of memory. This was a good compromise between cost and the space available. The memory is logically separate from the rest of the card -- decoding from the physical address and data bus, using appropriate buffers. In this way, the memory can be accessed by a bus master other than the CPU. It responds to physical addresses C0000-FDFFF or FEFFF, jumper selectable. For years, it held two chips -- 64 KB on the board -- with only 56 KB accessible. The six remaining chips were added recently, making 248 KB or 252 KB accessible. 


20-pin bus driver chips reduced the chip count, even with two sets of bus drivers, one for the CPU, and one for the memory array.

Program ROM

The design allows for a much larger ROM. The MC6809E V1 card originally had two 2KB 2716-compatible sockets -- one for a ROM and another for ROM or RAM. To make swapping OS-9 and BBUG easier, I changed this to a single 4 KB 2732-compatible ROM socket

For the MC6809 V2 board, the ROM can be a 2764, 27128 or 27256-compatible device, holding 8 KB, 16 KB or 32 KB, respectively. The larger ROM permitted more OS-9 modules to reside there, if desired. 

As built, a 2764-compatible EPROM is used, containing a BBUG image in one 4 KB half, and the OS-9 ROM image in the other 4 KB half. A jumper selects which half is active. This is much easier than swapping chips to go between BBUG and OS-9.

Accessing the correct amount of the ROM requires clever decoding. 


A hard-wired decoder would limit the flexibility of the system, and it would be complex and difficult to change. Rather than discrete logic, the decoder consists of a Cypress Semiconductor CY2C291 2Kx8 EPROM. This is a fast device with a 70ns access time. The CPU address lines A5 to A15 are connected directly to A0 to A10 on the chip. The decoder is enabled with the logical OR of E and Q, which asserts during three quarters of the memory cycle. This way, the eight data output pins can be used as decoder selects programmable on every 32-byte segment of memory.

Three select lines are used: one for bus access (including the on-board memory array), one for the program ROM, and one for the DAT. Each select line is pulled up to +5v. Placing a 0 bit in the decoder ROM data array makes the select line active for that 32-byte memory segment. 

Modifying the memory map becomes a simple matter of programming the decoder ROM. I programmed the following logical memory map:
  • 0000-EFFF - Bus
  • F000-F77F - Program ROM
  • F780-F7FF - Bus
  • F800-FFFF - Program ROM
  • FFE0-FFFF - DAT (writes only)
This configuration is compatible with the existing ROMs for BBUG and OS-9, which require I/O at E000-E07F. It has 4KB of program ROM, except for the hole at F780-F7FF. This hole deserves a bit of explanation. 

I/O Port Address Migration

BBUG occupies the top 2 KB of ROM. The OS-9 ROMs take up nearly 4KB. However OS9p2 doesn't use the last 128 bytes of that space. This unused space became an alternate location for the I/O ports. If the I/O ports moved from E000-E07F to F780-F7FF, the MC6809 could use RAM in the logical E block (E000-EFFF), for a total of 60 KB of RAM, up from 56 KB. 

Moving the I/O address requires motherboard decoder changes and software changes to the BBUG and OS-9 ROMs, as well as revision to Flex09 and OS-9 I/O configurations. The V2 board decoder ROM would work with the existing motherboard, or with the motherboard and ROMs altered for the new I/O addresses.

Larger ROM

Once the I/O addresses are moved, the decoder can be reprogrammed to allow for more ROM space. This opens the option of moving OS-9 modules into ROM. The decoder allows the lower limit of the ROM to be changed in 32-byte increments. This allows an OS-9 system to be entirely in ROM. OS-9 would start from the reset button without requiring a boot disk.


Back side of MC6809 V2 card.

The DAT configuration is similar to the MC6809E V1 board, with one important difference. In the SWTPc MP-09 board, as well as my V1 board, the outputs of the DAT are inverted on the lower four bits (A12-A15), but non-inverted on the higher four bits (S0-S3). 

This means that values programmed into the DAT must be one's complemented on the lower four bits (A12-A15), with the higher four bits (S0-S3) not complemented. 

For the V2 board, all eight bits of the DAT are inverted on the bus. Thus, the value programmed into the DAT is the one's compliment of the highest eight physical address bits (A12-A15, S0-S3). 

Which makes programming correct DAT values simpler, since the entire byte is complemented.

I introduced a hardware bug in the DAT decoder. More on this later.


Rather than wirewrap, I opted to try something new. A technician from work gave me a couple of 3M Scotchflex Breadboarding kits. This breadboarding system was brilliant. Chip sockets connected to IDC pins. Wiring is accomplished by forcing wire-wrap wire between the IDC pins with a special tool. 

It is way  easier than wire-wrap, because there's no tedious cutting, stripping, threading and winding of wire. One lays the wire down and pushes it on to the pins. Wiring several connections in succession, such as with a bus, is a breeze. The results also look great. The IDC pins are low profile, so there's less chance of shorting a connection than with wire-wrap.

It's sad 3M discontinued this product. It was great. 3M has since re-used the Scotchflex brand on three other products.

Fixing the Bug

The MC6809 V2 board worked great. There were no wiring errors. I did find a problem with the DAT.

In the default BBUG and OS-9 configuration, the DAT is written once during reset and never touched. And that seemed to work just fine.

Then I started playing with an OS-9 driver called VDisk. It created a virtual disk from selected extended memory blocks. At the time, I had 56 KB of memory from the MC6809 V2 card, plus another 60 KB from the Digital Research Computers / Tanner card. That made possible a 60 KB virtual disk.

Every time I tried to access the virtual disk, the computer would crash. This took a while to track down. 

I eventually realized the new decoder did not take into account the clock cycle when accessing the DAT. Transients on the R/W* line early in the clock cycle could cause bad data to be written to the DAT. After I added the missing gate, the Disk driver worked perfectly. 


Like the MC6809E V1 board, this V2 board was exactly how I wanted it. There are only two jumpers. 

The jumper at the top edge of the board selects the 4KB portion of the EPROM. This makes it easy to switch between OS-9 and BBUG. No more hassle of changing out chips - just move a jumper.

The jumper in the middle of the board, just above the decoder ROM enables the FE000-FEFFF block of on-board memory. This would be installed once the motherboard I/O addresses are moved out of the E-block of memory and would allow 60 KB of RAM to be used.


Moving the I/O addresses out of the E-block gains 4KB more usable memory for OS-9. Perhaps I'll try that in my retirement.

Another fun project would be to put a full OS-9 Level I system into ROM. Unfortunately, all of the essential modules take up just over 16 KB of memory, so the division doesn't fall on a natural 4 KB boundary. This might cause a conflict accessing extended memory with the DAT.  I'd also have to figure out how to program the decoder ROM. There are not many EPROM programmers that can program the Cypress Semiconductor CY2C291 devices, and I no longer have access to the ones I originally used. 

OS-9 Level II

This design works well for OS-9 Level I. To run OS-9 Level II, which allows each process to have a full 64 KB address space, requires more hardware. First, a second set of DAT memory chips allows the user and supervisors states to have separate memory maps. Second, a means of switching between those maps automatically -- like when servicing and returning from interrupts. Third, would require ROM to be accessible from an extended memory address, and then mapped into the supervisor space. 

Those requirements go beyond the scope of this design. Perhaps there's room for a V3 board. All of this assumes access to a copy of OS-9 Level II, which may be difficult to find. 

    Thursday, November 30, 2023

    Forty Years of Personal Computing - 5 1/4" WD2797 Disk Controller

    WD2797 controller card for 5 1/4" drives
    To work on OS-9, I borrowed some 5 1/4" drives, and used the SWTPc DC-2 controller. This allowed me to boot up OS-9. Single-sided, single-density, 40-track diskettes hold about 100 KB -- they were quite limited on space.

    Running OS-9 on single-sided, single-density 8" disks, the situation was a little better, as each drive has about 300 KB of storage. But my two-drive system was limited. Plus, I was something of an island. None of my friends using OS-9 had 8" disks, so I couldn't exchange data with them. It was time to consider 5 1/4" drives.

    5 1/4" disk drives went through considerable evolution since their 1976 introduction. The early drives were single-sided, single-density with only 35 tracks. By 1987, double-sided, double-density drives sporting 80 tracks were common. These disks could hold about 640 KB, more than twice what my single-sided, single-density 8" drives held. (And more than single-sided, double-density 8" drives could as well)

    Disk Controller

    In August 1987, I designed a 5 1/4" floppy disk controller. The 5 1/4" controller is very similar to the 8" design, with appropriate changes for the disk interface. 

    A MOTOR ON* signal is generated any time the WD2797 is accessed, with a one-shot multivibrator holding that signal for 10 seconds. Another one-shot asserts the READY signal on the WD2797 after a second of MOTOR ON*. 5 1/4" disks always have the heads loaded, so HLD is tied to HLT.
    Back side of 5 1/4" controller

    Double-density is jumper-selectable to either follow drive select bit 7, or the SSO output. Side selection is controlled by drive select bit 6. Write pre-compensation isn't used, as it was unnecessary for 5 1/4" disks. 

    I built the controller the same piece of 0.1" perfboard that originally held the FD1771 disk controller for 8" disks. The board is a little bit smaller than the WD2797 controller for 8" disks, so it appears more densely packed. Wire-wrap techniques are used for the wiring, and a handful of connectors and discrete parts are soldered.


    For initial troubleshooting, I borrowed the two drives and power supply from a Sage II computer from work, which I had to return. I needed my own drives.

    How many drives did I need?  I decided three drives would be sufficient -- one boot disk, and two working disks. This would allow me to copy disk to disk, while still having the boot disk with commands in place. (and no crazy disk-swapping for copies like the original Macintosh that had one disk drive!)

    I bought two Tandon TM100-4 drives at a local hamfest. These were common surplus from Lanier word processing units at that time. When I went to buy a third drive, I could no longer find any. I ended up with a Mitsubishi M4853 drive. The specs of the drives are virtually identical, except the Mitsubishi is a half-height drive.  

    Drive Cabinet

    5 1/4" Drive Cabinet
    Finding a cabinet to house three drives was a problem. New metal cabinets are very expensive, particularly in larger sizes, and I couldn't find anything suitable on the surplus market. 

    September 1987, I built a wooden cabinet to proper dimensions for three TM100-4 drives. I used 1/4" plywood, reinforced at the corners with 1x1/2 strips. The bottom, back, sides and one quarter front panel are all glued together as one unit. The top screws on to the four corner posts. The finished unit is quite sturdy. 

    As originally built, the cabinet was plain unfinished plywood. I recently sanded and finished it with a couple of coats of polyurethane.
    Inside the box, plenty of room.

    Power comes from a 12 volt, 5 amp supply. 5 volts is provided from a single LM7805 regulator mounted to that supply. In retrospect, the LM7805 might be a bit over-taxed. I suspect the drives draw less power than their maximum specifications. Heat is removed from the cabinet by a small (but noisy) muffin fan on the back panel.

    A power switch and neon pilot light round out the front panel, giving a clear indication the unit is on.

    The controller and drives work great, easily formatting  double-side, double-density disks using 80 tracks. 

    Drives & Software

    In April of 1989, I revised all the disk drivers to handle double-density, double-sided drives. The BBUG monitor "D" command code was updated to look for double-density sectors, and the boot loader for Flex09 updated to read double-density, double-sided disks.

    For OS-9, I modified an existing driver (FD2) for the Processor Technology PT69 to work with my disk controller and created a new boot disk with several drive descriptors. The drivers and descriptors allowed for 40-track disks (which required double-stepping of tracks, and adjusting the track register), and SWTPc format, where track 0 is formatted single-density -- as well as the standard, double-density, double-sided, 80-track format.

    I updated the Boot module to handle double-density, double-sided disks and burned a new OS-9 ROM. 

    The result is a smart, efficient unit roughly the same size as the SWTPc 6800 Computer System cabinet. The fan is a little noisy, but was typical for the day. 


    The Tandon and Mitsubishi drives only require 250 ms to get up to speed after MOTOR ON*. I can shorten the timing on the one-shot driving the READY signal.

    If I can manage to find a second Mitsubishi M4853 drive, four drives would fit into the cabinet. I'd need to add a second LM7805 regulator for the 5-volt supply, and split the 5-volt output across two drives for each.

    One limitation of the WD2797 is the track to track and head settling time. These drives can move track to track in 3 ms and need 15 ms for the head to settle. The WD2797, using a 1 MHz clock for 5 1/4" drives, can only do 6 ms and 30 ms, respectively.

    Western Digital did manufacture another device, the WD1772-00. This was a 28-pin floppy disk controller for 5 1/4" drives that is software compatible with the WD179x and WD279x devices. The WD1772-00 allows faster track to track and head settling times -- up to 2 ms and 15 ms. 

    The biggest problem is finding one, as the WD1772-00 wasn't used in a lot of designs, and Western Digital stopped manufacturing them over a decade ago. Might be interesting for a V3 floppy disk controller card.

    Sunday, November 26, 2023

    Halfway through the DXCC Challenge

    Twenty years ago, when I first started uploading my logs to Logbook of the World, I began to pursue the DXCC Challenge award. I created lists of confirmations that I had, and began to try to fill in the band / countries I was missing. This has continued for years. 

    In April of 2016, I gathered sufficient confirmations to earn the DXCC Challenge award. Since then, I've continued to pursue new band / countries practically every time I am on the air.

    This month, I passed another milestone. Currently, there are 340 entities on the DXCC list. And the DXCC Challenge counts on ten bands, from 160m through 6m. That makes 3400 total items for DXCC Challenge. 

    I recently collected confirmations over 1700 items on the DXCC Challenge. That's the half-way point. It's only going to get harder after this.

    Tuesday, October 31, 2023

    Forty Years of Personal Computing - OS-9 Level I

    I learned about OS-9 in early 1983, when it was new. What I heard mainly concerned BASIC09 and at that time BASIC didn't interest me. That was unfortunate. OS-9 is a miniature Unix clone, optimized for the 6809.

    Baud rate generator and
    counter/timer board


    By fall of 1986, I tired of the limitations of Flex09, and started looking at OS-9. Bringing up an OS-9 system didn't have the same challenges as Flex09,  since OS-9 can format 8" diskettes. OS-9 does have additional hardware requirements. It needs a periodic source of interrupts. 

    Interrupt Logic

    The OS-9 CLOCK module has logic for several interrupt sources, using chips available at the time. The MC6840 programmable counter/timer chip, with three 16-bit programmable counter/timers, was one option. The MC6840 fit nicely on the bit rate generator board. The driver allowed two circuit variations.

    The circuit I chose exercises all three counter/timers.  Timer 1 counts 50,000 cycles, then trips Timer 2 and 3. Timer 2 counts twice and signals an interrupt. Timer 3 counts down from 90. In this way, Timer 2 provides regular interrupts every 50 ms on a 2 MHz system. Timer 3 counts interrupts and adjusts the system clock whether or not the Timer 2 interrupt is serviced every 50 ms.


    The OS-9 kernel has two modules burned into ROM: OS9p1 and OS9p2. I obtained two 2KB ROMs and programmed them with the images. OS9p1 resides at F800. OS9p1 initializes the kernel, then searches for installed modules, which are position-independent. The second ROM contains OS9p2, Init and Boot. Once OS9p1 finds the OS9p2 module, it initializes it. OS9p2 looks for certain key modules, like IOMan. If they cannot be found, it uses the Boot module to load the rest of OS-9 from a floppy disk.

    Once initialized, OS9p2 uses the information in the Init module to start executing. During a soft reset, OS-9 does not always load from disk. If the modules are not altered, OS9p2 can find them and bypass the boot process. 

    The modular structure of OS-9 allows great flexibility. Modules can be in ROM or loaded from storage devices. The Init module provides the configuration to execute the first module.


    With a little help from a working OS-9 system, bootstrapping was straightforward. The ROMs I started with were pretty generic SWTPc system ROMs. The MC6840 occupied the bit rate generator board. I borrowed a 5 1/2" disk drive and plugged the DC-2 controller into I/O slot 1, with the 8" controller in I/O slot 2. Armed with a single-sided, single-density 5 1/4" boot disk, I successfully booted OS-9. That was the hard part.

    From there, I created a new 5 1/4" boot disk with drivers and configuration for my 8" drives. Booting from this new disk, I formatted 8" disks and moved the OS-9 files to them. I then created an 8" OS-9 boot disk with a new I/O configuration and drivers for both 8" and  5 1/4" drives. At that point, I swapped the floppy disk controller slots, with the 8" controller in I/O slot 1, and the DC-2 in I/O slot 2. (The Boot module is configured to find a WDC-compatible floppy disk controller at the address for slot 1)

    At that point, I could boot OS-9 from my 8" drives, and was able to copy files from the 5 1/4" disks. Compared with bringing up Flex09, this was easy.

    I tailored my configuration to suit my hardware, and updated the ROMs with customized modules.

    Swapping BBUG/Flex09 and OS-9

    While I was using OS-9, I would swap back to BBUG and Flex09 on occasion. This was a pain. I would swap out the two 2 KB ROMs and use a different boot disk. 

    In late October of 1986, I modified the MC6809E V1 board to use a single 4 KB 2732 ROM. This put all of the OS-9 kernel on one chip, and allowed room to expand BBUG. With this modification, only one chip was swapped.

    Extended Memory

    Working with OS-9 uncovered an issue with extended memory addressing. December 1986, I installed a 74LS21 4-input NAND gate on the SWTPc motherboard to decode the top address bits S0-S3. This placed the I/O addresses at FE000. With the MC6809E V1 board, this worked great with BBUG and Flex09. BBUG initialized the E-block of the DAT with a value F1 -- which the board would interpret as physical address FE000. 

    However, I found I could not boot into OS-9 any more. Turns out, OS-9 initializes the E-block of the DAT using a value 01, which the board interpreted as physical address 0E000. With the extended addressing decoder on the motherboard, the OS-9 Boot module could not communicate to the I/O devices. This forced me to disable the 74LS21 decoder.

    User Experience

    OS-9 Level I uses a single 64 KB memory space for the operating system, programs and data. That's not a lot of memory. Many OS-9 programs are small, being written in assembly language. Larger programs, like a compiler, load in as multiple passes, to conserve memory use.

    Using OS-9 is cool. It is a real-time, multi-tasking operating system, first available in 1982. Windows wouldn't have comparable functionality until 1989 (Windows NT), and the Mac in 1999 (MacOS X). Like Unix, you can spawn off programs to run concurrently in the background.

    Using a second serial port, I ran two users simultaneously, one from the main terminal, and one from the second serial port. I used a Wyse-85 terminal on the main port, and the old CT-64 on the second port. Amazing on an 8-bit machine with 56 KB of memory! 

    At some point, I hung a modem on the second port. I could leave the machine running at home and dial into it from work. 

    Saturday, September 30, 2023

    Forty Years of Personal Computing - Wyse-85 Terminal

    By the summer of 1985, my original CT-64 terminal felt limited. Sixteen rows of 64 characters didn't seem like enough. Especially when at work I regularly used screens with at least 25 rows of 80 characters. In 1977, terminals with such capabilities were around $1000 -- way beyond my modest budget. By 1985, much more capable terminals were available for about half that price. It was time to upgrade.

    August of 1985, I purchased a Wyse-85 terminal for about $700 -- a good price for the time. The terminal offed a DEC VT-220, VT-100 and VT-52 emulator, so it was plenty capable. It sported 24 or 25 rows of 80 or 132 columns on the screen. I purchased the green phosphor screen.

    The most important thing, however, about the Wyse-85 compared to the CT-64 was speed. The CT-64 was limited to a paltry 1200 bps. The Wyse-85 had a top speed of 38400 bps. Thirty-two times faster. The CT-64 would take more than eight seconds to write every character on the 16 x 64 screen. The Wyse-85 could write an entire 25 x 132 screen in less than a second. 

    The Wyse-85 was such a joy to use compared to the CT-64, I couldn't believe I hadn't done this sooner. 

    I did have trouble with this terminal when I tried to use it in the shack back in the late 1980s. The keyboard scan generated a fair amount of RFI. Putting several ferrite toroids on the keyboard cable helped a little, but did not eliminate the problem. 

    I still have this terminal. It's been stored in the original box since November of 1994. I hope it still works.

    Tuesday, September 19, 2023

    Demise of one 80/40/20m Dipole

    I was QRV in Gordon county briefly - only a couple of weeks. I managed to erect the 80/40/20m dipole I had up in Warren county, which previously flew over Fulton county. It was a cobbled-together mess, made from wire left over from the original 80/40m dipole, newer traps, and old insulators and rope.

    Using the Mark III Antenna Launcher, I did a good job casting over a tree in the front yard. Weight sailed up over the tree and came right down beside the trunk. The 1/16" guide line went back out to the antenna launcher, and then the 1/4" nylon halyard came back over. Perfect.

    At the far end, I had more trouble. Not wanting to crawl over a fence, I cast sideways to branches overhanging the edge of the yard. The first toss wasn't great, so I pulled it down. Second toss got stuck in the tree, and I lost the weight. I was down to my last antenna weight. I confidently tied it on, pulled back, let it fly, only to watch it sail off the end of the fishing line and into oblivion. Nuts. 

    With no weights handy, I couldn't use the antenna launcher. I opted to use a small hammer and toss the halyard over a branch about 20 feet up in the tree. At least I didn't lose the hammer. 

    The resulting installation sloped the dipole from about 25 feet on the south end, to about 60 feet on the north end. No matter - it would work. At least, until I could make more weights and get it higher in the air. 

    I used it to make about 100 contacts for the NAQP Phone in August, plus a little casual operating. Then I found most of it lying on the ground after a few windy days. Inspecting the remains showed that the wire between the 20 and 40 meter traps had broken. That particular segment was pretty old, being part of the original 80/40m dipole, and might have used wire from the ancient untuned doublet before it.

    This meant that one of the 40m traps was still up in the tree. Looking carefully, I could see it about 50 feet up. Untying the rope, I could not get it to drop, and instead pulled the halyard to recover the rope. The wire ended up coming off the insulator, leaving wire and one trap stuck in the tree. Drat.

    The rest of the antenna lay across the yard and lower driveway. I don't use that driveway, so I didn't think about it. However, some folks came to visit the parsonage and apparently didn't see the traps laying there. Two of the trap forms got crushed in the process. Doggone it.

    I guess I have to rebuild this antenna from scratch, using new wire and traps. That will take some doing, as most of the parts are back in Gwinnett county. Plus, I have to make more antenna weights to put it back up. 

    In the meantime, I'm off the air in Gordon county.

    Thursday, August 31, 2023

    Forty Years of Personal Computing - V2 Floppy Disk Controller

    WD2797 controller card for 8" Pertec drives in
    the Icom Peripherals FD360
    The FD1771 disk controller works well with the Pertec 8" drives. The single-density drives each hold around 300 KB of data. 

    Single-density encoding is FM, which has regular clock pulses, with a data pulse placed between. A data pulse indicates a "1", a missing pulse is a "0".

    Double-density uses Modified FM (MFM) encoding. It eliminates the clock pulses entirely, leaving only the data pulses. To keep synchronization during runs of zeros, extra pulses are inserted between each pair of zeros. Encoded in this way, the clock can be recovered from the data pulses alone.

    Western Digital followed the FD1771 with the WD179x chips, which support double-density with a two device solution. The later WD279x chips offer the same features on a single device. Double-density allows 500 KB on the same disks, with the data transfer rate also doubled.

    WD2797 8" Controller

    Back side of the 8" controller
    September 1986, I built a new controller using the WD2797 to support the Pertec FD400 drives. While the drives were designed for single-density, I hoped they would work using double-density.

    In keeping with my other home brew cards, it's built on a piece of 0.1" perfboard with the Molex connectors epoxied to the bottom edge. Wire wrap sockets are used.

    Naturally, I broke out the WWARP program I used years before to build the MC6809E V1 card

    The WD2797 design borrows from the FD1771 design. I kept the latching data bus buffer, but eliminated the redundant data bus buffers in front of them. The WD2797 performs the clock/data separation, which eliminates several gates. Fourteen total chips on this board, whereas the FD1771 board used more like eighteen. 

    Double-density is enabled through an option jumper. The SWTPc DC-4 controller used the SSO output to drive the DDEN* pin through an inverter. (SWTPc offered double-density before double-sided disks) One side effect of using SSO is the sector address markers will have side 0 for single-density sectors and side 1 for double density sectors.

    Other designs used bit 7 of the drive select latch, controlled through software. SSO isn't connected to anything, as the Pertec drives only have one side.

    A jumper at the top of the card chooses the DDEN* signal source: the SSO pin, or bit 7 of the drive select latch. Both paths go through an inverter, so double-density is selected with a 1 on either the SSO pin or bit 7. 

    Bit 3 of the drive select latch controls ENP - the pin for write pre-compensation. Generally, ENP would connect to the TG43 output of the floppy drive interface. Using a separate bit allows write pre-compensation to be enabled or disabled at any time, through software. I didn't know if write pre-compensation would be required or desired. It seemed like a good plan to allow write pre-compensation on any track, since the Pertec drives weren't designed for double-density.

    Reading the drive select latch address returns the state of the INTRQ* and DRQ* pins, on bits 7 and 6, respectively. Using these separate bits allows more efficient loops than reading and interpreting the status bits of the WD2797. The SWTPc DC-4 introduced this feature, and is common to controllers of that era.

    The WD2797 calibration starts by grounding the TEST* pin and checking three signals with a scope.  A set of four pins at the base of the WD2797 chip bring these signals out making calibration easier. 

    To support the new controller, I re-wrote the Flex09 disk drivers to allow double-density operation. 

    Do the Pertec drives work at double-density? I don't know. Supporting double-density meant re-writing NEWDISK to initialize in double-density format. Before I figured that out, my interest shifted from Flex09 to OS-9, and I did not complete that project. But the card works great with single-density.

    Monday, July 31, 2023

    Forty Years of Personal Computing - RTTY Receiving Program

    September 1985, I purchased a Kenwood TS-430S and became more active in amateur radio. In the apartment where I was living, I snuck wires out of a second floor window and began to make contacts. 

    In October, I got the notion to try some Radio Teletype (RTTY). I built a demodulator using a circuit I've forgotten. Perhaps it used a couple of NE567 chips. Having a demodulator, I needed to translate the five-level Baudot characters into ASCII that I could display on the terminal.

    (I purchased a Wyse 85 VT-220 emulator terminal in August of 1985, so I was no longer constrained by the 64x16 screen and 1200 bps limitations of the CT-64)

    RTTY Decoder

    I wrote a program for Flex09 to decode 45 Baud RTTY by bit-banging a PIA pin. I couldn't use the MC6850 ACIA, because it does not support 5 bit characters.

    A delay loop established character timing: 

    LOOP    LEAX -1,X
                  BNE LOOP

    Each pass through the loop consumes 8 clock cycles. With the right value loaded in X, fairly precise timings could be accomplished. A value close to 250 would be 1 ms on a 2 MHz machine. By calling this loop repeatedly, timings of 11 and 22 ms are measured. 

    I connected the demodulator output to PIA Port B, pin 0. The program looks at this pin, waiting for a zero. Finding one, it calls the delay loop for 1 ms and checks again. If the pin is still zero, it waits 10 ms and checks Port B pin 0. A continued zero at this point indicates a start bit. The 11 ms total delay places us right in the middle of the start bit.

    The next sequence waits 22 ms and then samples of value of Port B, pin 0. It does this five times. These samples are shifted into a byte value, which used to look up an ASCII character in one of two tables -- one for letters, and one for figures -- according to the shift mode. This character is then sent to the terminal, and we go back to waiting for a start bit.

    The resulting program is about 300 bytes long. Despite the simplicity,  I had little success decoding RTTY signals. 

    In hindsight, there are several reasons for this. 

    • Decoding signals off the air that might have been noisy.
    • Demodulator circuit was completely untested and might not have worked.
    • No experience with RTTY, so signals might not have been properly tuned.
    • Precise value of the 1 ms time delay not known. I used values of 230 and 240, allowing cycles for other program logic. 

    At some point, I distinctly copied "RY RY RY RY RY RY RY" from someone, but not much else. Later, I figured out this meant my program, at least, was working. 

    Hardware Solution

    In November 1986, I decided to use serial chip that could do five-level Baudot. The MC6850 only allows 7 and 8 bit characters, so I needed a different chip. The NS8250 could do 5, 6, 7 and 8 bit characters, and sports a programmable bit rate generator for all the common RTTY rates. Hence, I added an NS8250 UART to the baud-rate generator board. 

    Funny, though -- I never wrote software to use the NS8250. In February 1989, I removed the NS8250 and its associated circuitry. 

    I didn't become active in RTTY on the air until 2005, using Cocoamodem.

    Friday, June 30, 2023

    Forty Years of Personal Computing - The Big Toss Out

    Some time in 1985, I had one of those moments I regretted. 

    I'd gotten my MC6809 system running in late 1983, running the Flex09 operating system on 8" floppy disks. I had a full set of documentation for several pieces of hardware and software. My new job kept me busy, so I didn't have much time to work with my system at home.

    August of 1984, I began programming the Macintosh at work. We published our first product in March of 1985. The Macintosh was a revelation -- it completely changed the metaphor for computing. I saw early on that it was the future. But, I couldn't afford one right away. I wouldn't buy my first Mac until the summer of 1987 -- a Macintosh SE. I've only purchased Macintosh computers since that time. 

    In the late spring of 1985, I prepared to move to a new apartment. I had bulky boxes of documentation in my closet. I figured that I'd never do anything more with that MC6809 system -- the Macintosh was the future. 

    I threw nearly all of the documentation away. 

    Two months later, in the new apartment, I regretted my decision. There were things I could do with my MC6809 computer. I had to replace the missing documentation. 

    To this day, I'm not sure I found replacements for everything I had.

    Wednesday, May 31, 2023

    Forty Years of Personal Computing - Tanner/Digital Research 64KB Memory Board

    Digital Research Computers 64 KB SS-50 Board
    I graduated from Georgia Tech in the fall of 1983 and got a full-time job. By 1984, 20 KB of memory didn't seem like enough for the MC6809E V1 board. My attempt to expand the  8 KB MP-8M to 16 KB didn't work. And I never built the dynamic RAM circuits I designed. I wanted more memory.

    Digital Research Computers marketed an SS-50 card designed by Tanner Computers in the early 80s. It sported thirty-two sockets for 2 KB RAM or ROM chips. These 2716-compatible chips were quite popular at the time.

    I bought a kit for about $225 with a full 64 KB of RAM in June 1984.

    Assembling the kit was straightforward, along the lines of the SWTPc kits. The board worked right away, with no soldering issues -- largely due to the excellent solder mask on the board.

    For SS-50 systems, this board has several flexible options. The first 48 KB presents as three 16 KB banks that are enabled individually. Each 2 KB segment in the top 16 KB is enabled individually, allowing one to navigate conflicts in the C, D, E and F blocks of memory. This allowed for I/O on the motherboard, or perhaps RAM or ROM on the CPU board.

    The board supports extended addressing on the S0-S3 pins. When enabled, the entire board responds as one 64 KB block. Each socket can contain either RAM or ROM chips, selectable by the jumper next to each chip.

    Initially, I used this board without extended addressing as a 56 KB board. I later enabled extended addressing to access the full 64 KB, after modifying the MP-B motherboard to decode the 20-bit address for the I/O slots. This allowed me to use that 8 KB of RAM for a virtual disk drive, briefly.

    I discovered some  extended memory issues between BBUG/Flex09 and OS-9, so I disabled the MP-B decoding.

    As pictured, the board has the E000 and E800 blocks disabled, with F000 enabled, and F800 disabled. This configuration was appropriate for the MC6809E V1 CPU board and MP-B motherboard without the 20-bit address decoding, although, technically, the E800 block could be enabled, and the F000 block would not be accessible after I modified the MC6809E V1 CPU board for a 4K ROM.

    Sunday, April 30, 2023

    Forty Years of Personal Computing - MC6809 Meets Floppy Disks

    The massive ICOM "Frugal Floppy" FD360
    By August 1983, I had a working MC6809 computer with no storage. My BBUG monitor eschewed audio cassette tape. Floppy disk drives were expected -- BBUG included a Disk Boot command. Any disk controller needed to be compatible with that code. 

    In 1983, a friend of mine gave me an Icom Peripherals FD360 Dual Floppy drive box. A client of his had purchased the Icom, but opted to use another disk system instead. His client couldn't use the Icom box, but perhaps I could.

    Icom was one of the first floppy disk suppliers with their "frugal floppy" system. The FD360 contained two Pertec FD400 8" disk drives, a couple of interface boards, and a large power supply. It is huge and heavy, larger than the SWTPc 6800 Computer System, and weighed about 70 lbs.

    Pertec FD400

    The Pertec drives differ from the popular Shugart SA800/801 of the same era. They have a 44-pin edge connector, as opposed to Shugart's 50-pin edge connector. The signals and power supply requirements are different. Icom's interface boards translated those connections into a 34-pin IDE connection, quite similar to what was later used with 5 1/4" floppy disks. There were a few important differences:
    Data plate from the Icom FD360

    - Step In and Step Out signals, instead of Step and Direction.
    - Erase Enable in addition to Write Enable.
    - 8" media data rates (250 kHz), instead of 5 1/4" data rates (128 kHz)
    - Head Load signal (not present on 5 1/4" disks)

    The FD400 has a single metal head, designed for single-density recording. 

    Western Digital FD1771

    The FD1771 was the first in a series of LSI floppy disk controllers from Western Digital. It simplified the interface and operation of floppy disk drives. Designed for Shugart-style drive signals, it could accommodate the signals from the Icom with a bit of logic.

    I had schematics for the SWTPc DC-2 controller, and borrowed from that design. The FC1771 data sheet convinced me that the data bus hold times meant data bus latching was required, so I had an extra layer of buffering on the data bus using a 74LS373 matched with a 74LS244 (for reads). These were on top of the pair of 74LS242s that buffered the data bus to the card.

    Following the FD1771 data sheet recommendations, I used an external hardware clock/data separator. (The SWTPc DC-1,-2 and -3 all used the inadequate internal clock/data separator) I used the circuit from Figure 6 of the FD1771 application note. It has a 4 MHz clock, 74LS175, 74LS161 and a few other gates. Dividing the clock oscillator by two fed the FD1771 with 2 MHz necessary for 8" drives. A 74LS221 one-shot multivibrator supplied the head-load timing (HLT) signal after about 40 ms. A 74LS175 captures data bus D0 and D1 to a two-to-four decoder to allow drive selection -- up to four drives, but I only have two.

    I built this controller using wire-wrap techniques on a piece of perfboard, with 30 pin Molex connectors glued to the edge. The same construction as I had used on the MC6809E V1 board. A 34-pin dual pin connector provided the connection to the Icom drives.

    In my schematics, I included a 74LS221 one-shot to trigger the M.RDY line on the SS-50 bus when accessing the FD1771. But this pin is not available on the SS-30 bus. So I omitted the circuit during construction. The FD1771 did not need this additional delay. 

    The I/O slot memory map is simple:

    01xx - Drive select latch (bits D0 and D1)
    1000 - Command / Status Register (FD1771)
    1001 - Track Register (FD1771)
    1010 - Sector Register (FD1771)
    1011 - Data Register (FD1771)

    After building, I tested the controller by writing a few commands to the FD1771 by hand using the BBUG monitor. I could select each drive, move and load/unload the head. It seemed to work. Using the drives required a disk operating system.

    Disk Operating System

    At that time, 6809 systems generally ran TSC's Flex09 or Microware's OS-9. BBUG's disk boot command was designed for Flex09. OS-9 required a completely different set of ROMs. My friend Carl was already using Flex09, so that was my choice.

    Upgrading from using cassette tape, a disk operating would seem like a bit step upwards. But Flex09 was very primitive, especially compared to the work I was doing with MS-DOS at the time.


    SWTPc DC-2 Controller
    Carl had Flex09 on 5 1/4" disks. As my disks were 8", bringing up Flex09 took some doing. 

    Carl gave me a SWTPc DC-2 disk controller, and loaned me his 5 1/4" drives for a couple of weeks. The DC-2 was surplus to his needs. He'd upgraded to double-sided, double-density controller, and the old controller was useless to him.

    With the controller and disk drives, Flex09 booted up easily from the floppies Carl supplied. I just needed to get them on the 8" drive. The motherboard gave each controller a separate address, as they  plugged into different slots. Some configuration changes let Flex09 know about the additional drives. Before anything could be copied to an 8" diskette, it had to be initialized. The 5 1/4" NEWDISK program didn't work for 8" disks as their formats were different.

    I wrote my own version of NEWDISK. This required understanding the FD1771 Write Track command. NEWDISK generated all of the data for each track, with the proper address marks, gaps and codes that permitted the FD1771 to properly read and write the disk. Then it would issue a Write Track command to the FD1771 and the track data delivered. This process repeated for each track on the disk.

    At the time, there was no defined format for 8" disks. Flex09 required 256 byte sectors. The 5 1/4" format used ten sectors per track in single-density. 8" disks had room for more. With very generous gap sizes, fifteen sectors per track fit easily. 

    After track initialization, NEWDISK would write the volume information, and then the boot loader into the first sector. A disk was made bootable simply by copying the Flex09 system files to the disk.


    Reaching the point where I could boot Flex09 from my 8" disks, I reconfigured the system and gave 5 1/4" disks back to Carl. The work done on NEWDISK led me to experiment. With 77 tracks of fifteen sectors per track, my 8" drives held a little more than 288 KB each. 

    Reducing the gap sizes, seventeen sectors would fit on each track. This increased the storage capacity to 327 KB for a 77 track disk. 

    Doing some tests, I found that the Pertec drives were mechanically capable of 78 tracks. Trying to step out to the 79th track resulting in a very alarming klunk as the head reached its mechanical limit.

    While I confirmed that diskettes formatted for 78 tracks were valid and reliable, I decided this was not recommended. Such diskettes might be unreadable on other disk drives. My final version of NEWDISK formatted 77 tracks, with either fifteen or seventeen sectors per track.


    By the fall of 1983,  I had a working 6809 system, complete with 8" floppy disks drives running Flex09. 

    Thursday, March 30, 2023

    Forty Years of Personal Computing - Das Blinkenlights

    SWTPc 6800 front panel, sporting sixteen LEDs.
    Each LED indicates a 4 KB address block access.
    You can also see a yellow power LED was added.
    The SWTPc 6800 Computer System sports a rather spartan black and silver front panel. Unlike contemporaries of its day, there were no lights.

    I'd already modified the front panel of my computer. I mounted the CIS-30+ cassette interface inside the case, which accounts for the assortment of holes in the upper right. There's a yellow power LED next to the reset button. Before I went to the ISEF 1979, I replaced the power switch with a key switch. I added the yellow LED so I could tell when the machine was on. 

    While building the MC6809E V1 board, I decided to add some lights. Hooking up LEDs to the address bus didn't seem useful, since addresses change quickly. Plus, the DAT complicated the issue -- do you show logical or physical addresses?

    I ended up with a simple solution. Running the top four CPU address lines into a 4:16 decoder, each LED indicator represents a logical 4 KB address block. This connects to the CPU board directly, before the address signals enter the DAT. 

    Board holding 74LS154 and LEDs.
    I drilled the front panel for sixteen LEDs. On the backside of the front panel, I mounted a 74LS154 4:16 decoder and wired up the LED cathodes. The board is held in place by the LED wires. A 16-pin ribbon cable goes back to the CPU board and attaches to a socket.

    This socket has address pins A12-A15 from the CPU, the VMA* signal, the DAT write decoder signal, +5 volts, and ground. The VMA* and DAT write signals go to the two Enable* pins on the 'LS154. This means the address must be valid, and not writing to the DAT to enable.

    The LEDs anodes are connected together to a resistor to +5 volts. Originally, this was a 470 ohm resistor. I found that the LEDs lit very dimly. Each LED is not asserted all the time. VMA* is not asserted at all times, and pulses every clock cycle, which limits how bright the LED can illuminate. I decreased the resistor value and experimentally arrived at a value of 22 ohms.

    The resulting front panel is informative. I can see when the CPU is busy-waiting on I/O ports, if it is crunching away at some calculation, or if it has crashed entirely. And if the CPU executes a CWAI instruction to wait for interrupts, all the LEDs go dark.


    Tuesday, February 28, 2023

    Forty Years of Personal Computing - MC6809

    The bare board that held the HB 6809 V1 circuitry. 
    Waiting for the next SS-50 project.
    In computings early days, advancements happened quickly. Which mean purchases went out of date quickly. I built my SWTPc 6800 Computer System in November of 1977. In 1978, Motorola introduced the MC6809 processor, and by March 1979, SWTPc had the MP-09 CPU board available.


    For those who programmed the MC6800, the MC6809 fixed many problems. It sports 16-bit arithmetic operations, more index registers, many additional addressing modes and a very orthogonal instruction set. It was a powerhouse of an 8-bit processor. Early chips were costly, and the marketplace moved quickly beyond 8-bit chips to 16-bit ones. The MC6809 never became popular enough to dominate the market. For that brief time in the late 70s, it was the most powerful 8-bit processor. I wanted one.

    Georgia Tech Symposium

    In early 1983, Georgia Tech hosted a symposium where Terry Ritter, one of the designers of the MC6809 spoke. I attended, and the room was packed. Ritter talked about the design decisions that went into the architecture of the MC6809, about the importance of reusable software and position-independent code. Their design lead to the creation of BASIC09 and OS-9.

    At the time, I wasn't much interested in any variety of BASIC, so I didn't pay much attention to OS-9. I should have.

    The last of Ritter's talk was about an unannounced product, termed the General Motors Custom Microprocessor (GMCM). It built on the foundations of the MC6800. At that time, GM was using the MC6802 in their vehicles. The GMCM added some features to the MC6800, such as 16-bit arithmetic, an extra index register and both multiply and divide instructions. (Divide was something that the MC6809 did not have) But the GMCM lacked all the complex, orthogonal addressing modes of the MC6809. The next year, Motorola introduced the MC68HC11.


    SWTPc offered the MC6809 with the MP-09 board. The MP-09 draws from the design of the MC6800-based MP-A2 board. Both have four 2716-compatable sockets that can hold 2 KB ROM, PROM or EPROMs. While the primary difference is the CPU, there are other differences:
    • SS-50 bus changes for the 6809 processor
    • NMI and RESET switch connections relocated to a CPU board connector, which frees up two bus lines for BUSY and M.RDY, respectively
    • ROM socket at address F800
    • ROM/RAM sockets at address E000, E800, and F000
    • No on-board RAM. (the MP-A and MP-A2 had 128 bytes of RAM)
    • 16 bytes of writable memory at address FFFx that translate the top 4 bits of address
    This last feature is known as the Dynamic Address Translator or DAT. The DAT uses a fast 16x4 bit RAM - the 74LS189 chip. It sits between the top 4 address lines A12-A15 of the CPU and the physical bus. Thus it translates address values in 4KB blocks. It is the reason the MP-09 has no on-board RAM. The ROM monitor SBUG-E, on reset, scans addresses to determine which 4 KB blocks have RAM. It then writes the DAT to position those 4 KB blocks as desired. The first block is relocated at D000-DFFF, and the second block at C000-CFFF. These two blocks are used for the stack, and any operating system such as Flex09. The rest of the memory blocks are arranged from address 0000 upward.

    The board has a socket for an additional 74LS189 chip for the DAT, which adds the four higher-order bits to the DAT RAM. This extends the address bus with four more lines (S0-S3) allowing the MC6809 to address 1 MB of memory. The S0-S3 lines take the place of the bit rate signals on the SS-50 bus, so these signals have to be supplied elsewhere for the SS-30 bus. SWTPc did this with their MP-MB motherboards, a component of the SWTPc S/09 systems.

    The MP-09 board runs on the original MP-B/B2 motherboards with modifications. More on these later. 

    During my years in college, I eagerly wanted to upgrade my computer to the MC6809. I obtained a copy of the SBUG-E source code and studied it. By April of 1981, I modified SBUG-E, removing code for features I didn't need and adding some features of my own. I called my monitor BBUG. I used a cross-assembler on the Pr1me minicomputers to build my code. But I could not run BBUG without hardware.

    MC6809E V1

    My last months at Georgia Tech, I took a required course that involved building a working microprocessor system. It had to have ROM, RAM and I/O. Many of my colleagues used the computing laboratory that provided prototyping plugboards and components for their circuits.

    This was a challenging course for many computer science majors. It was their first experience with digital logic. And the prototyping plugboards had been used and re-used so many times that their connections were not always reliable. This made it difficult to determine if problems were programming bugs, wiring errors, or just flakiness in the hardware.

    I didn't relish dealing with flaky prototyping boards. I had fulfilled many of the requirements of this class when I built the SWTPc 6800 system in 1977! This seemed an opportunity to create my own MC6809 processor board.

    I had read  articles about the MP-09, and had copies of the schematics, which I studied in-depth. There were  electronic parts stores and friends around Atlanta where I could obtain components. It seemed right.

    MC6809 V1 Design

    April 1983, I drew up schematics. My design borrowed from the MP-09, and included the DAT, but there were several differences.

    I did not include an MC14411 bit rate generator or crystal. I intend to use extended physical memory right away. I moved the bit rate generator and crystal to a card plugged into the SS-30 bus. There were no bit rates on my SS-50 bus, only S0-S3 address lines.

    Instead of the MC6809 chip, I opted to use the MC6809E. The MC6809 has a built-in clock oscillator and bus control logic. The MC6809E requires an external oscillator as well as a few chips to handle bus control for things like DMA. In trade, it has an LIC (Last Instruction Cycle) and AVMA (Advance VMA) pins. I made use of AVMA in my design.

    Instead of four 2716-compatible 2KB sockets, I only had two. A ROM at Address F800, and ROM or RAM at address F000. Having ROM or RAM at E800 or E800 didn't make sense to me, since those addresses conflicted with the I/O ports used by SBUG-E. (In October 1986, I would revise this board to use a single 2732-compatible 4KB chip, covering F000-FFFF)

    The memory map for this design was pretty straightforward:
    • 0000-EFFF - accessing off the CPU board to the SS-50 bus
    • F000-F7FF - ROM/RAM socket #1
    • F800-FFFF reads - ROM socket #2
    • FFF0-FFFF writes - DAT RAM
    I had no option switches or jumpers in this design. It was exactly how I wanted.

    Dynamic RAM Design

    The same month, I drew diagrams for dynamic RAM boards. One was a 64 KB board using  4116 chips. Another design provided 256 KB memory using 4164 chips. I intended to add one of these boards to my project. The 4116 design was complicated, with multiple voltages involved. The 4164 design was simpler, but the chips were more expensive. I never build either design.

    MC6809E V1 Building

    Although I had made some simple PC boards, creating one for the MC6809 design was outside my skill set. Plus, it would have been expensive for my student budget. I had constructed a seven-chip CMOS keyer in 1979 using point-to-point wiring. That was difficult. I built a duplicate keyer for my brother in 1980 using wire-wrap construction. Wire-wrap was a heck of a lot easier. Plus, I already had the tools. A friend supplied wire-wrap wire and sockets and I was in business.

    Wiring a design this complex directly from the schematic worried me. Wiring mistakes were easy to make, and hard to track down. Then a classmate asked to join my project. Denise Messerschmidt worked with me in the Users Assistance office at the Computer Center, and she didn't want to have to fuss with the flaky prototype boards either. She asked if she could help, and I figured should could do the wiring of the hardware. This required good instructions.

    I solved that problem with a bit of software. I used the LL(1) parser generator written by fellow student Roy Mongiovi and defined a 28 production computer "language" that described the chips and devices and the wiring between them. Given a well-described design, my program produced three lists as output.

    The device list showed each device and all of the signals connected to each pin. A device was either a chip or a plug. Chips listed two pins to a line, just as it would appear on a DIP device. Plugs listed one pin to a line. The device list made it easy to verify the design was correctly programmed in the language.

    A cross-reference list related the signals connected to each device. Another cross-reference list showed the signals by each device and pin. One would follow this final list to do the wiring. The Wire Wrap parser was about 2500 lines of Pascal code, and it worked beautifully.

    The board used 0.1" perf-board, cut to 8 1/2" by 5" -- a little smaller than standard SWTPc boards. Molex connectors epoxied on the edge connected to the SS-50 bus. I installed the wire wrap sockets, and wired up the voltage regulator and the SS-50 pins,  plus a handful of discrete components. That took care of everything that required soldering. After training Denise in wire-wrap technique, I gave her the board, wire, tools and the wiring list.

    About two weeks later, she came back with the finished product. Every wire had to be manually stripped and wrapped. Denise had done an excellent job - better than I would have. 

    MP-B Modifications

    The MP-B motherboard required modification to work with my 6809 CPU board. The I/O ports on the MP-B decode as eight, four byte ports starting at address 8000. The SBUG-E (and hence BBUG) monitor expects eight, sixteen byte ports starting at address E000.

    The modifications aren't difficult:
    • Connect SS-50 A2 and A3 to SS-30 UD3 and UD4, respectively
    • Move the ABC inputs in U3 from SS-50 A2, A3 and A4 to A4, A5 and A6, respectively
    • Cut the trace on U3 pin 5 (Enable*) and connect to SS-50 A9
    • U5 has an unused NOR gate, connect the inputs to SS-50 A10 and A11, and the output to U3 pin 6 (Enable)
    • Move the U6 pin 4 (Enable*) from SS-50 bus A6 to A12
    • Move the U6 decode output on pin 4 (4*) to pin 7 (7*) this signal goes to U3 pin 3 (Enable*) and  U5 pin 13 (Enable* for data bus drivers)
    Modifications on underside of
    MP-B. The brown wires on the 
    right go to the 74LS21.
    To support extended addressing using S0-S3, the traces between the SS-50 bus to the SS30 bus for the bit rate lines 110, 150, 300, 600 and 1200 must be cut. 

    I added extended address decoding for the I/O ports. I mounted a 74LS21 4-input AND gate on a daughter board, and connected the inputs to S0-S3, the output goes to U6 pin 6 (Enable).

    With these modifications in place, the I/O ports show up in memory as eight, sixteen byte ports, from physical address FE000 to FE1FF. The ports show up in memory four times in sequence, because address lines A7 and A8 are not decoded. 

    Bit Rate Generator

    Bit Rate Generator Board
    Since I used the extended addressing lines S0-S3 on the SS-50 bus, the bit rate signals must be provided on the SS-30 bus. 

    For this, I built a simple card that had just four components: MC14411, LM341-5 regulator, 1.8432 MHz crystal and a capacitor. You can see them lined up on the left side of the board. The MC14411 is the original device from the MP-A CPU board with a date code of 7718 - the 18th week of 1977.

    Later, I added the MC6840 timer chip and the SN74LS04. At some point, this board also held an Intel 8250 UART, which is the reason for the molex connector in the upper right. But those parts have been removed. There's certainly room for an MC6850 ACIA or other I/O device on this board.

    Bit rates are configured as follows:
    • 110 = 38400 baud
    • 150 = 2400 baud
    • 300 = 300 baud
    • 600 = 9600 baud
    • 1200 = 1200 baud
    This represents a good collection of rates across a wide range. As configured other rates are available on the MC14411 from 300 to 115200 baud.

    MC6809E V1 Debugging

    The beautifully wrapped board Denise wired didn't work.

    When I built the SWTPc 6800 Computer System, it didn't work the first time, either. I borrowed an oscilloscope and figured out the issues. For the SWTPc 6800 boards, the problem was nearly all solder bridges. But those boards started with a working design. Mine was untested.

    I had access to test equipment on campus, but I couldn't take any of it with me. I had to bring the whole computer and terminal into one of the computer center labs in the evenings, when the labs weren't in use. An oscilloscope wasn't enough. I was introduced to a new device, an HP logic analyzer -- although it took me a while to figure it out. Those evening sessions in the lab were very productive.

    Dragging my computer and terminal in and out of he lab each day, I isolated six different wiring errors on the board. Had Denise let me down? No, the wiring errors were all in the original wiring listing! Denise had done her job to perfection -- she wired it exactly as specified. 

    Once the wiring errors were fixed, the debugging process went smoother. I also had to correct some defects in BBUG. At one point, I had all 24 channels of the HP logic analyzer engaged. 16 pins went to the address bus, and the other 8 to the data bus. This way, I could watch each memory cycle of the MC6809 and determine what was happening.  

    Eventually, I fixed the issues and had the system running. I received an "A" for the course. Denise, because of her limited involvement, received a "B", but we were both happy.

    End of an Era

    In 1988, I built another MC6809 CPU board. It replaced the V1 board, and once working, I stripped the V1 board down for parts. All that is left is a board with edge connectors and a voltage regulator, which you see at the beginning of the article.

    Sunday, February 26, 2023

    Moving the A50-3S Closer to Home

    A3S/A743 on tower, A50-3S nearby.
    Nearly five years ago, I mounted a Cushcraft A50-3S out in the yard. I figured this would work for a few months until I found a more permanent solution. This lasted way longer than I expected.

    It was a good move. I worked 48 states, plus 39 countries using that antenna, despite feeding it with 120 feet of RG-8X which likely adds 3 dB loss.

    In July 2021, I asked the SEDXC email reflector for advice on how to work Europeans on 6m. I heard others working them, and even heard a few myself. For the most part, however, I could not hear them, or I couldn't get them to hear me.

    The first piece of advice was to get a better feed line. RG-8X is not a good choice for VHF, especially with over 100 feet. The second bit was to mount the A50-3S a little higher. It's taken me a year and a half to get there. I've finally taken the first step.

    The first question: where to put the antenna? The mount in the yard used a mast concreted into the ground that originally supported a Cushcraft R7000. I considered moving it to my 50 foot tower below the Cushcraft A3S/A743. The option allowed for easy rotation, and would have been convenient. However, with antennas in close proximity at five feet away, I believed there would be too much destructive interaction. I needed a mount point further away.

    View of installation.
    Without room for additional towers, I opted to mount a mast against the house, about 30 feet away from the tower. This location is close to where coaxial cables exit the house, which meant a shorter feed line. An older but serviceable piece of 9913 had sufficient length, so the feed line problem was solved.

    How to mount a mast to the house took a bit of figuring. I used a small 6" wall mount on the eave of the house, just below the gutter. This gives the mast enough distance to clear the gutter. The bottom of the mast sits in a pole mount on the railing of the deck blow. The mast is the same 19 feet using two 10 foot pieces of rigid EMT I used to mount the antenna in the yard. 

    Erecting the antenna was a little bit of a challenge. I used a rope and pulley hooked on the wall mount to raise the mast into position, then lifted the mast up on the railing. The weight of the rigid EMT made this harder. 

    Reflector askew
    In the process, the reflector of the antenna brushed up against the roof, which knocked it out of alignment. It doesn't affect the operation of the antenna terribly much, but I will fix it eventually.

    The mast bracket is not cinched on the mast, to allow for rotation. Jam nuts are used to keep the bracket U-bolt from loosening.

    The result has the antenna around 27 feet (8m) high, next to the house, fed with about 50 feet of 9913. This should be a substantially better than out in the yard.

    I plan to replace the mast with some aluminum tubing, as well as adding a rotator, which should put the antenna a couple of feet higher. That will also give me an opportunity to straighten out the reflector alignment. 

    In the meantime, I'm ready for this year's Es season in plenty of time.

    Monday, February 6, 2023

    Nine-Band Worked All States

    Nearly twelve years ago, I wrote about completing Worked All States on six bands. I'd worked all states on 160, 80, 40, 20, 15 and 10m. About three years ago, I finished up 30m, so now it was seven bands. However, finishing 17 and 12m seemed like it would take forever. I felt stalled out.

    A couple of months ago, it occurred to me that I was only four band-states away from Ten-Band Worked All States. I needed Delaware on 17m, Kentucky on 12m, and Alaska and Hawaii on 6m. 

    The 6m states would have to wait -- I'd need very special conditions to work either state. But with the recent rise in sunspots, working those close-in states on 17 and 12m seemed do-able. The biggest problem would be operating the Gwinnett station. That was solved after I configured the RemoteRig devices to allow remote operation

    Indeed, the first afternoon operating remotely, I was able to work Kentucky on 12m and the LoTW confirmation came the next day. Finishing off 17m took a month longer.

    It was surprising to me how calling CQ DEL AA4LR EM83 would gather so many responses from people who were not in Delaware. I worked at least one station in Delaware, but the LoTW confirmation was not forthcoming. Then the RemoteRig Control device no longer powered up.

    I got lucky one Friday afternoon when I was in Gwinnett county and managed to get a legitimate answer to my CQ DEL message and a confirmation later that day. I'd done it. Worked All States on Nine Bands. 

    Now, I just have to wait for those special conditions in order to work Alaska and Hawaii on 6m....

    Monday, January 30, 2023

    Remote Operation - Level 1 (RemoteRig RRC-1258MkII)

    RemoteRig RRC-1258MkII at Radio

    Last spring, I wrote about using RealVNC to remote control a computer in my shack allowing me to make FT8 contacts on 6m. I have made many contacts using that remote system, including several new countries and grids.

    I want to be able to operate the Gwinnett county station remotely -- on any mode or band, as if I were sitting there. Doing this required several connections over the internet, and, being behind on other software projects, it seemed a daunting one. 

    A company called Microbit (www.remoterig.com) has a solution. The RRC-1258MkII is a pair of devices that establish multiple audio, serial and control links over the internet. One unit sits with the Radio, the other is called the Control. They are similar boxes, with subtle differences: the Control box as a CW speed knob, but the Radio box does not. These units work with a number of radios, including the Elecraft K3. 

    One operating mode is K3 Twin. In this mode, the Control K3 acts as a front-end to the remote Radio K3. All the knobs and buttons operate the remote radio. Indeed, Elecraft made special, stripped down, non-RF versions of the K3 for this purpose (K3/0, later the K3/0-mini).

    This seemed perfect, as I owned two K3 radios. Obtaining the RRC-1258MkII was more difficult. Microbit is based in Sweden. Due to the pandemic and subsequent supply chain issues, they no longer sold them in the USA. I had to find them used. 

    I managed to find Kirby, VE6IV, who had a set surplus to his needs, and we agreed on a price. Then ensued a much longer negotiation on how to get the funds to Kirby in Canada. Eventually, we figured it out, and a week later, the devices were delivered.


    Configuration was not plug-and-play, by any means. These boxes are designed to connect to 10/100BaseT networks. I found an old router and set up a local network to do the initial configuration. First step was to update the firmware. This did not work over the network, and I had to use the USB connection. After a few tries, I successfully updated both units to the latest firmware. 

    The local web server in each box allows configuration of the other parameters. There are dozens of settings, and the manual leaves a bit to be desired explaining all the details. 

    First order of business was the IP configuration. I opted to define a static IP addresses on my local network. for the Radio, for the Control. You can use DHCP for the Control, but it is easier to change configuration settings when you know the address. 

    The Radio device needs to be accessible from the public internet. Since I don't have a static public IP address, I opted to use dynamic DNS. RemoteRig supplies such a service, at ddns.remoterig.com. They automatically set up a host address based on your serial number. 

    I set the web site username and password, as well as the SIP password. Audio was set for 16 bits and 8 kHz dual channel. The COM ports were set with COM1 inactive, COM2 in logical parallel with COM0, and COM3 inactive. 


    Next step was to integrate the Radio unit into the Gwinnett station. I still needed to use the station locally. I found that I could connect the local computer through the COM1 port on the Radio unit and still be able to run WSJT-X locally. One caveat - the RemoteRig devices don't pass through the DTR and RTS signals, so you can't do CW keying from the serial port. You also can't update K3 firmware. Both of these require direct connection of the computer to the rig.

    The manual shows the Radio unit connected with seven different cables, but only six of them are described in the manual. The seventh cable connects from the I/O port on the back of RemoteRig to the ACC jack on the K3. The Radio unit turns the K3 off when you disconnect remotely. Without this seventh cable, you cannot turn the K3 on when you connect. That took some experimentation to figure out.

    Initial Connection

    Puzzling out the rest of the operation was easier on the local network. I programmed my Control unit to connect directly to the Radio unit's local IP address. Initially, I couldn't get anything to work. I would have brief periods where the Control unit would connect. I could hear the audio of the radio, and then it would disconnect. Nothing was happening with the Control K3.

    Part of the problem is my Control K3 had been upgraded to a KIO3B, so it did not have an RS-232 jack. Generally, I used the USB port. The KIO3B has an RJ-45 jack labelled RS-232/P3, and I had a cable designed to plug into the P3. I used that cable, but it didn't work. I decided I needed a special cable from Elecraft, part #E980297, an RJ-45 to DE-9S. 

    The new cable didn't work either, and that lead to more sleuthing. I tried using the K3 Utility on this cable, and it didn't work either. I then discovered that the CONFIG:RS-232 menu had to be set to 38400 b for the serial connection to work. That was part of the problem.

    A bit more checking and I determined that the RemoteRig COM2 jack required a null modem cable between it and the K3. I had that, and it required a male/male DB-9 adapter. Both these items were in the batch of cables that Kirby shipped me. 

    With CONFIG:RS-232 and the correct cabling, the Control unit placed the Control K3 into TERM mode. I successfully connected locally. 

    Remote Connection

    Connecting through the dynamic DNS address was the next step. I figured I had to change the SIP Contact parameter on the Control unit. That was correct, but it would not connect. Then I thought perhaps it didn't work because I was trying to connect on the same IP address on the public network. So, I packed up the K3 and the Control unit and took them back to Warren county. But, it didn't work there, either.

    This was frustrating. Then it occurred to me that perhaps I had to re-program the firewall of my Gwinnett county router to let certain traffic pass. I lamented that those experiments would have to wait until I could pack it all up and go back to Gwinnett county to fix. Then it dawned on me that I could use the RealVNC connection to my shack computer to re-program the firewall remotely.

    Programming the router was not simple. I used the IP Allocation mode of Default Server to direct all incoming traffic to the Radio unit IP address. That worked! I was able to connect and control the station.

    This configuration worried me. RemoteRig uses four UDP ports, plus TCP port 80 for the web server configuration and port 23 for telnet configuration. Having those TCP ports open on the public internet seemed like a bad idea. A single password protected access, which seemed to invite hacking.

    Instead, I wanted to pass the traffic on the four UDP ports and block everything else. This was accomplished by setting up four custom "gaming" services for each UDP port. I then assigned these services to the RemoteRig Radio IP address. Bingo.


    Operation is pretty straightforward, even though the RemoteRig manual isn't. To activate the system, you simply turn the Control K3 on. Within 20-30 seconds, the devices connect across the Internet, the remote K3 is turned on and the Control K3 goes into TERM mode, and audio starts streaming into the Control unit. All of the knobs, buttons and indicators on the Control K3 operate the remote K3. 

    When you are finished, you turn the Control K3 off, and the remote K3 also turns off. If you happen to lose your internet connection, the remote K3 turns off in about a minute. 

    During my experiments, I was able to confirm operation of WSJT-X using the remote shack computer. I've also been able to transmit CW, using the paddle check on the Control unit. So far, though, I haven't figured out how to transmit voice signals. Most likely, I have another cable or configuration problem. 


    The system has a few rough spots. The audio volume is controlled from the Control K3 volume control. Certain operations stop the audio stream -- switching into or out of SUB receiver mode, or into or out of DIVersity reception. Moving the volume control brings the audio stream back. 

    In my set up, the audio occasionally has small drop-outs, perhaps when a UDP packet fails to arrive in time. For this reason, I would not recommend using RTTY across the audio connection. One can operate a remote computer to run RTTY, just as I do for FT8. There may be a configuration option to help this.

    Next Steps

    While I can operate my Gwinnett K3 remotely now, I need to automate other parts of the station. I cannot change antennas, rotate rotators or switch the K9AY direction. I'm working on solving those problems.

    Update: Sad News

    Unfortunately, about two weeks after I started writing this article, my Control unit fails to power up. I apply power, but the PWR LED does not come on. I've tried with two different power supplies, no luck. RemoteRig support indicates this could be a failure of the CPU. Sadly, they don't offer repairs in the USA, and will have no replacement units available until May, 2023. 

    In the meantime, I'm back to Remote Operation - Level 0.