Monday, March 8, 2021

Forty Years Of Personal Computing - SWTPc CT-64

CT-64 keyboard with switch options.
In the Fall of 1977, I was sixteen and a half years old. A classmate of mine had a 1970 Ford Torino that he was willing to sell for $1,500, and I had money saved from my paper route. While I didn't yet have my license, I considered purchasing this car. My father informed me that if I bought my own car, I'd be solely responsible for fuel, maintenance and insurance. After some deliberation, I made the wisest decision of my life -- I decided against the car, and instead spent my savings on computer equipment.

The Computer

After the mis-step with the Viatron 2111, I became selective. The KIM-1 experience was useful, but I wanted something more than a single-board computer. I wanted something that could grow into a larger machine without spending a fortune.

That meant a slot-based system, such as the S-100 bus computers. These devices cost in excess of $2000 once you bought all the components to do something useful -- beyond my budget. Plus, you needed a teletype or a terminal plus storage equipment. I needed something more affordable.

Southwest Technical Products Corporation (or SWTPc, as it was abbreviated) came out with the 6800 computer system in November 1975. I read all about the introduction in the October 1975 issue of 73 magazine. At $450, this machine was substantially cheaper than the S-100 computers of the day. It had no front panel, with the switches and LEDs, and the motherboard used far less expensive connectors. A 4K static memory board was $100, compared to an S-100 8K static memory board at $250. 

The Motorola MC6800 processor was no slouch. My older brother had spent some time at college programming a Tektronics 4051. It used the same processor, and seemed plenty capable.

The Terminal

By the time I was ready to spend money in the fall of 1977, I had decided on the SWTPc machine. But, where to start? It wouldn't do any good to buy the 6800 computer system first, because you needed a terminal to use it (no front panel, remember?). Therefore, my first order was the SWTPc CT-64 Terminal System.

Powered up after two
decades on the shelf
The CT-64 was a direct descendent of Don Lancaster's Television Typewriter. SWTPc had revised that design with their CT-1024 kit, which featured 16 lines of 32 upper-case only characters. (The CT-1024 was referred to as the TV Typewriter II) The CT-64 revised the design further with fewer boards and sported 16 lines of 64 characters, including upper and lower case. Displaying 64 characters per line required more video bandwidth than your typical television, so a composite video monitor is required. I had a small black and white TV that I though I could modify, so in October 1977, I ordered a kit.


By 1977, I had built several electronic kits, starting with Heathkits, with their easy-to-follow instructions. The SWTPc kit was a new experience. The instructions were basically:
  • Insert all the resistors. Solder.
  • Insert all the capacitors. Solder.
  • Insert all the transistors and diodes. Solder.
  • ...
CT-64 main board.
You get the idea. The instructions were minimalist. The CT-64 the most difficult piece of equipment I'd ever assembled. The main board was very complicated with 47 integrated circuits, and covers most of the base of the CT-64. The memory board, serial board, keyboard and power supply were much easier to assemble. 

Nevertheless, I completed it the kit and got it working. I know I had a few difficulties like solder bridges to track down, but it worked as advertised and I was able to type characters and see them on the screen. That worked OK on my little black and white TV in 16x32 mode. 16x64 mode was illegible. I bought a 9" Sony black and white video monitor that worked perfectly.

I had one assembly problem -- the KBD-5 brackets are not mounted to the bottom chassis. If I put those screws in, I never could get the top case to go on. By leaving them out, the keyboard could "float" and find the most favorable position for the top case. 

In the first year of using the CT-64, I encountered two chip failures, something that has never happened with any other piece of equipment. The first chip to go was the AY5-2376 keyboard encoder. I ordered a replacement that has worked ever since.

Customization and Modification

The second failure happened gradually. IC22, the MCM6575L character-generator chip, slowly became flaky, and pixels of characters would flash on an off. Eventually, the chip failed entirely and every character became a solid block. I believe this was a thermal failure. 

Character set with MCM6571L
Finding chips is easier today, but in those days, I could not find a direct replacement. I did find an MCM6571L. This is a similar chip, with two important differences. The control character codes printed as greek characters, so it changed the control-character print function. The other difference was the row select lines were reversed. This meant the four row select lines had to be inverted before they made it into the MCM6571L. I added a small daughter board using an 74LS00 to invert these signals.

When I built my CT-64, I wanted to take advantage of every option. In addition to the standard UPPER / U + L and C.C. PNT switches, I added more switches:
  • Video memory PAGE 1 / AUTO / 2
  • RVSE - reverse characters - which sets the 8th bit
  • Switch to the right of the POWER LED - No longer connected, I think this might have connected to DTR to allow one to stop scrolling. Honestly, I don't remember.
In retrospect, I never used the CT-64 in paged mode, so I could have dispensed with the PAGE 1 / AUTO / 2 and PAGE / SCROLL switches.
Legend for added

The keyboard has other visible modifications. The KBD-5 lacks access to several characters. I modified the RCVE / XMIT switch so it was no longer locking, and that key sends the \ and | characters. The unmarked key in the lower right of the keyboard was wired to produce @ and `. (To modern users, it would seem impossible to use a keyboard that did not have an "@" key, but things were different in the mid-70s) The final character in the ASCII set is accessed by the ugly red pushbutton in the lower right corner. Pressing it gives an underscore _ and the DEL character. I painted the symbols beside each key.

My CT-64 was also set up with a number of control characters. Beyond the standard control characters of BEL, BS (cursor left), HT (cursor right), LF (cursor down), VT (cursor up) and CR (cursor to column 1), I wired in additional controls:

- Ctrl-F (ACK) toggles screen inversion - white and black or black on white
- Ctrl-L (FF) cursor to top of screen
- Ctrl-N (SO) toggle scrolling or page mode
- Ctrl-O (SI) clears the screen
- Ctrl-P (DLE) cursor to top of screen
- Ctrl-U (NAK) erase to end of line
- Ctrl-V (SYN) erase to end of screen
- Ctrl-Y (EM) toggles the cursor on or off
- Ctrl-\ (FS) toggle memory page

It's unusual that Ctrl-L and Ctrl-P perform the same function. I originally wired this function for Ctrl-L. However, the SWTBUG monitor has Clear Screen command that sends Ctrl-P followed by Ctrl-V. To be compatible, I added the additional wire.

While I was at Georgia Tech, this was enough control that I was able to create a profile for a full-screen editor for the CT-64 on the Software Tools Subsystem on the Pr1me minicomputers, and had fun editing at 300 baud via dial-up. I'm not sure why there are two different control characters for the home function.


CT-64 Insides
As terminals go, the CT-64 was limited. Terminals in that day were more typically 24 or 25 lines by 80 characters. The CT-64's 16 lines by 64 characters is a little more than half that space. Plus, it had a maximum speed of 1200 baud. Nonetheless, the CT-64 was a reasonable value in 1977. The kit sold for $325 when a typical terminal would cost $1000 or more. 

I used the CT-64 for several years, from 1977 until about 1985, when I bought a Wyse WY-85 -- a much more capable terminal. (24 or 25 lines by 80 or 132 columns, and 38,400 baud!)

The original Sony monitor from 1977 I used until 1983, when I loaned it to someone who took it to the People's Republic of China. It never came back. The gentleman who borrowed the Sony monitor bought a replacement Sanyo monitor. 

The CT-64 has sat on a shelf for a couple of decades until I recently pulled it out for this article. It mostly works -- the KBD-5 has some switch issues with the O, T and left Shift keys - they tend to stick in the up position. 

These days, computers don't even come with serial ports, much less require a terminal, so there's little demand for a slow, limited device such as the CT-64. It gave good service in the late 70s and early 80s.