Chapter 6: Marion Had a Micro Chroma
"Good morning, Dr. Brown."
Dr. Brown grinned at me through the door of the lab where he was preparing for class. "Top of the AM to you, Marion Joseph. Are you early for tomorrow's classes?"
"Yep. Early for tomorrow." I grinned back. "I've got that prototyping board running."
"Well that's good. Is it running laps?"
"It'll probably run rings around these 8080 trainers." I indicated the trainer kits stored on the shelves against the wall behind me.
"Yeah, the trainers are clocked pretty slow."
"Can you take a look at it tomorrow, to see if it will be okay for the microprocessors class?"
"I think I can, if you bring it."
"Okay. I'll bring it. And I was thinking maybe I could use a scope and a frequency source to check and tune the cassette interface while it's here, if it's okay?"
"You want to do that in class so the other mages can take a gander at the process?"
"Don't know if it will go right or not."
"Not going right gives us more opportunities to learn ..."
"Okay, sure. I'll do it in class."
"Just for the record, have you given a peek at the lab book for the class yet?"
"Yeah, I scanned through it at the bookstore. Will you be using the same book next year?"
"Not sure, but those are the kinds of labs you'll be doing."
"I guess I should go look at it some more."
On the way home, I went to the bookstore to look at the lab book again, and decided to buy it, even though it was expensive and entirely focused on the 8080.
Then I went to the library to hunt in the recent issues of Byte, Kilobaud, Popular Electronics, and Radio Electronics for advertisements for software for the 6800. Found an ad for Technical Systems Consultants in Kilobaud, so I took a Xerox copy of it home with me.
At home, I got permission from Mom for a long distance call at the expensive daytime rates before businesses in the eastern time zone would be shutting down, and called TSC to ask whether they could send the tapes for the software I wanted in the Kansas City format. They said they could, and told me how to write the order.
After delivering the newspapers, I got out a pencil and some paper, and sketched out a box to hold the Micro Chroma 68 mainboard, the power supply, a fan, and three perfboards mounted above the mainboard. Taking a clue from the Apple II and Radio Shack's TRS-80 Color Computer (and typewriters), I sketched a slanted seventh face on one end to mount the keyboard and ten-key pad on, leaving about two thirds of the top face as a removable cover to set the TV monitor on.
I noted that, depending on how I mounted the keyboard, there might be just enough space between the keyboard and the top cover to mount a perfboard with breadboard and signal headers on. (I wasn't thinking in terms of needing someplace to rest my wrists, but, at that time, most typewriters had the same deficiency.)
I showed the sketch to Dad.
"Looks good. Just getting it in a box will make it less scary for Mom and me, I think."
I had to agree, for a variety of reasons, static electricity among them.
Setting the sketch aside, I picked up the microprocessors text and looked more carefully through the units, writing down rough guesses about the parts I'd need for the labs for each subject. (The real me did not think this far ahead.)
After ten o'clock, when the cheapest phone rates began, I called Denny.
"This phone call is expensive.
"Not as expensive as it could be. Don't rub it in. I changed my mind."
"Did you? About what?"
"Could you send me the editor and the assembler on cassette? I want to test drive it."
"I snuck it into your stuff. Go look for it."
"No kidding? I'll go look."
There it was, an extra cassette in the stuff I had brought back from Austin. In fact, he had slipped in some other things as well, including a rail of other chips that I had asked him about, including a couple of direct memory access controllers (6844 DMAC) and extra parallel and serial ports (6821 and 6850), and some blank EPROMs.
I called him back to say thanks.
I tried loading the assembler, but it kept giving me errors. With a couple of tries adjusting the tank circuit, I was able to get what seemed like a clean load, but it wouldn't run.
So I called it a night and shut things down, reading my usual scriptures before I slept.
*****
Wednesday morning, after breakfast and exercises (more healthy habits I'd gotten better at during my mission), I wrapped the Micro Chroma 68 mainboard in aluminum foil, to give it some protection from static, and loaded it with the manuals, some blank cassette tapes, and my cheap cassette recorder into the hardside briefcase I often used as a toolbox. Left the tools that had been in the briefcase on my desk.
I had also considered using the briefcase as a box for the Micro Chroma 68, but it would have been a tight squeeze, and then I would have also had to buy a real toolcase.
I put the briefcase and my TV into the back seat of the Colt and drove to school. General parking was free back then, but I usually rode my bicycle to save gas money and get exercise. Carrying the prototyping board was reason enough to drive.
Arriving at the digital circuits class early, I set the computer up and checked a dual channel oscilloscope, a reference frequency generator and some other equipment out, and set up the computer, getting it ready for the demonstration. Some of the other students wandered over as they came in, to see what I was doing.
Dr. Brown decided to let my demonstration start the class, so, after the rest of the students had gathered, I explained what I understood of what I was doing, first showing the main clock and signals from several testpoints on the 'scope, then focusing on the cassette circuit's tone generator. Dr. Brown filled in the gaps in my explanations and guided me a bit when I wasn't sure how to proceed.
I set up reference signals from the frequency generator on one channel on the 'scope, then the corresponding signals from the data output circuit on the other, and adjusted a potentiometer until the signal frequencies matched. I used that to record a set of test tones, then used those test tones to test the input.
Several times, when I had to reset it, it came up in an uncontrolled state, and I explained why.
"Would it sometimes come up running a game or a programming language?"
"Only something that is already in ROM, and that is never stable."
"Infinite monkeys?" Dr. Brown asked.
"I don't particularly want to destroy this baby trying random reboots an infinite number of times."
"Good point. Let's check the power supply."
We shifted the oscilloscope probes around, and Dr. Brown showed me where the instability occurred because the reset push-button switch contacts bounced a lot. We tried a larger filter capacitor on the reset circuit, and that cleaned up the reset signal a bit.
"I need to get a better reset button."
That cleared, I tried loading the assembler, and it loaded and ran.
"Known good tapes are an important test tool," Dr. Brown explained. I typed in the input-and-loop hack that I had shown my dad, and assembled it to show what the output looked like, then ran it to show how typing on the screen could look.
Then Dr. Brown had me sketch the bit recovery circuit for the cassette input out on the blackboard, and we used the 'scope to see where the bits from the test tones were getting latched and shifted into the serial port. We also did some calculations of recording speed and tape capacity, and I typed in a short BASIC program and saved it, powered the Micro Chroma 68 down and back up, and demonstrated the process of loading the program back in.
That all took about half the class, after which Dr. Brown tied some of what I had shown into what we were studying, and proceeded with the lecture for the day.
After class, several of the students wanted to know how they could get one of the computers, and I explained that it was a prototyping kit, and only cheap because my brother got me parts.
I also explained how Radio Shack's TRS-80 Color Computer had the same video generator, but a better microprocessor, and that it would probably cost more to build the Micro Chroma 68, buying the parts at regular price than to just buy the Color Computer. Even so, there was something of a consensus that building your own from scratch was more interesting.
Over lunch break, Dr. Brown looked at my list of parts for the labs from the textbook.
"Looks usable. But remember I'll have to ask you to show your work. I'll probably ask you to do some hand-assembling of programs on the tests, to make sure you understand that. And I'll ask you to remove the BASIC ROM when you pass things off. This all assumes you can find a lab partner willing to work on the 6800, of course."
"No problem. I'm thinking now that I'll be doing both the 8080 and the 6800 anyway."
"Suit yourself on that." He was still grinning. "I can only give you credit for one time through the class, howsomever."
After lunch, we repeated the demonstration for the AC circuits class, with Dr. Brown prompting me to focus more on the AC parts than the digital parts.
(The real me never got up the ambition to tune the cassette interface with a 'scope and a frequency reference, much less demonstrate the process in class. Kansas City tapes were somewhat notorious for being hard to read, although the reputation was probably more because people didn't bother to find 'scopes and tune them than for anything else. And I was more interested in the high speed cassette interface and the novel I was trying to read -- and daydreaming about how to use the various ICs. And the real me never got an assembler for the Micro Chroma 68. I even assembled the fig-Forth source by hand instead, but I'm getting ahead of myself.)
Before heading home, I made copies of my sketches of the box on the library's copier, to send to Denny.
After delivering the newspapers, I reviewed my sketches and went out to the garage to see if we had materials for the computer case. We had sheet metal leftover from the flashing when we shingled the roof before my mission, but I wanted plywood with a better finish than we had.
Standoffs to mount the board and a fan to keep the power supply cool were also things I needed to buy. So I went to Radio Shack to get standoffs, and see if they had a fan. They had a 5 inch AC powered fan, and I got it, even though I thought I could get a better price somewhere else.
While there, I looked at the software and hardware for their computers.
Returning home, I made another test tape, and put it in an envelope with copies of my box sketches, to send to Denny. And I wrote a check and an order to TSC, for my own official copies of the assembler, the debug package, the text editor, and the text processor.
And called it a long Wednesday.
(Lots of things on this day that the real me did differently and at different times, if at all. Lots of things that were different all week, and all semester.)
*****
Thursday, in the BASIC class, I discovered that rumors of the Micro Chroma 68 had been circulating, and some of the students wanted me to bring it to that class, too. Professor Crane said it would be okay, so I promised to bring it once it was in a box.
On the way home, I went around to the post-office behind our house and sent Denny the test cassette and my sketches. Also sent the software order to TSC.
After delivering newspapers, I went to church to help out with the youth program, then ran by Gibson's Hardware on the way home to get the plywood and a length of one-by-one for the inside corners of the box. When I got home, I sketched the measurements onto the wood, but held off cutting because I didn't want to bother the neighbors.
Instead of continuing with the box, I made sure I was ready for Friday's classes.
Before reading my nightly scriptures, I studied the Micro Chroma 68's memory map and spent fifteen minutes sketching out possible circuits for the dynamic RAM and extra ROM.
[Backed up at https://joel-rees-economics.blogspot.com/2020/02/bk-33209-marion-had-micro-chroma.html.]
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