Thursday, February 3, 2022

Forty Years of Personal Computing - MC6800 Years - Hardware Accessories

MP-8M sans modifications.
During the summer of 1978, I ordered more parts for my SWTPc 6800 computer. 


I ordered an 8KB memory board, the MP-8M. The board used 4Kx1 chips, compared to the 1Kx1 chips on the MP-M. Even though it held twice as much memory, the MP-8M looks barely populated by comparison. It also drew far less power. Notice the LM7805's have no heat sinks. The kit came with them, but they really aren't needed.

The original MP-M boards drew about 1.5 amps from the 8 volt supply for each board. SWTPc recommended running a maximum of four MP-M boards to avoid exceeding the capacity of the power supply. With the MP-8M, the machine could be populated to the 32 KB of continuous memory the MIKBUG memory map allowed. 

At some point, I tried expanding the 8 KB board to 16 KB. I did this by buying 16 more 4Kx1 RAM chips and piggy-backing them on the original devices. All the pins were soldered except for the CE - Chip Enable (pin 10), which was raised to the side and wired together in two groups of eight. A couple of additional chips were used to drive the CE pin for the upper and lower 4 KB banks. 

In theory, it should have worked. When the CE pin was high, the chips were unselected and all their outputs were in a tri-state mode. The decoder should select only one set of 8 chips at a time (or none at all, depending on the memory address).

I never could get it to work. The original 8KB worked fine, but the additional 8KB never responded properly. I suspect there was something incorrectly wired in my additional decoder. I removed the decoder circuits and left the piggyback chips. Eventually, I removed those as well.

One feature of the MP-8M was the write-protect switch. Flipping the switch made the memory unwritable. I never found a use for this feature, so I removed the switch.


Two other purchases I made in the summer of 1978 -- SWTBUG and the MP-LA. SWTBUG was a MIKBUG-compatable ROM with several extra features. Perhaps the most important feature was that it supported the MP-S serial interface, and not just the MP-C serial interface. More on this in a later article.


MP-LA board.
The MP-LA was a parallel interface card using a 6821 PIA. I didn't have anything to hook this up to, but I wanted to experiment with parallel interfaces. So I built something.

The Kreepie-Peepie

I cobbled together a small aluminum box that contained 8 red LEDs, plus a small audio amplifier. The amplifier design was taken from the May 1977 Kilobaud article, "Adding 'Plop' to Your System" (page 98).

Kreepie Peepie - says so with dry-transfer letters.
The original amplifier design used a 7406 hex inverter with open-collector outputs. One inverter buffers the input, the next creates a 180 degree phase difference, and the rest are hooked in pairs to drive either side of a 500:8 ohm transformer. The 500 ohm winding is center-tapped and fed through a 1 k ohm potentiometer as a volume control.

For some reason, I used two 7400 NAND gate chips. Probably because it was what I had one hand. This did not work the same, since the 7400 does not have open-collector outputs. But it worked despite my error. 

The 8 LEDs connected to the B port of the MP-LA, and the audio amplifier was hooked to the CB2 control pin. The whole unit was powered by stealing 5 volts off the MP-LA's LM7805 regulator.

While it sounds simple, this tiny box was fascinating. I spent the next several weeks writing and modifying small assembly language programs to exercise it. Programming different patterns of the 8 LEDs was easy and fun. Animating the lights at different speeds required mastering delay loops in the code.  

Inside the Kreepie Peepie.
Even more challenging was the audio interface. Whenever the CB2 pin was changed, it would make a "tick" sound in the speaker. Making sounds required one to constantly change the state of the CB2 pin. By toggling CB2 rapidly, one could create tones, the frequency of which was controlled by the speed of toggling. Experimenting, I found that I could create more than one tone simultaneously by using two numbers and computing the delay loop. Changing the delays algorithmically produced weird rising and falling sounds.

I spent a lot of time hand-modifying machine language programs and running them to see what new sound or LED pattern I could produce. One Saturday, my mother got really annoyed that I was playing with the computer instead of coming to dinner. She complained that I was spending "too much time with the kreepie peepie." And thus the little box was dubbed the Kreepie-Peepie. I even applied rub-on letters to the box with this name.