Chapter 11.1: Headwinds -- 68701
I looked up at the tap at the open door.
"Pat. Hi. What brings you to the Electrical and Electronics Building? Slumming?"
"The profs were talking about your success yesterday with raw I/O. We wanted to see for ourselves."
"We? Ah, Mike. George. Hi guys."
"Yo."
"Hey."
"Well, come in and join the crowd."
They came into the electronics lab and joined the group of students gathered around the lab table where my lab partners, Jeff and Mark, were sitting in front of the Micro Chroma 68.
Dr. Brown hadn't wanted to take more class time, so he had suggested that I take time after class to let interested students watch me reproduce my results from the day before. I could chase them off after, if they distracted my progress too much.
Jeff and Mark were by this time familiar with the Micro Chroma 68, and had volunteered to help as soon as they saw me coming in with the computer again.
We had the video output connected through a splitter to both my TV and one of the lab's large-screen TVs so the other students could see what was showing on my TV screen. (A 40 inch screen was fairly large at the time.)
Jeff and Mark helped me walk through my work of the day before, operating the computer and setting up the circuits from my notes while I drew diagrams on the chalkboard and explained about the circuits and the timing issues as I understood them, starting with bit-banging and working up to the FM encoding. Dr. Brown added comments when he thought they were necessary.
Part way through the demonstration, I noticed Julia watching from the doorway. When I raised my hand to wave her in, Pat, George, and Mike turned and looked, and something passed between them that I didn't follow. Julia entered the room and sat in my seat at the lab table.
When we'd finished demonstrating recording and playing back test data in FM, Jeff looked up from the screen. "Is there some way we can look at a track before it gets written?"
I looked at Dr. Brown and he just grinned back at me. So I thought for a moment. I reached over to the breadboard and patched a line over the FM decode unit, effectively removing it from the circuit.
"We haven't written anything to track 30, yet, Jeff. Bump the heads up to track thirty."
He typed in the commands that moved the head.
"Run the read routine and let's look at the read buffer."
He did so, and we watched him list the results on the screen -- patches of mostly zeroes, of short repeating sequences, and of apparently random numbers.
"Does it look pretty random?"
Everyone thought it did.
"There actually isn't any track until you write one, although, I think, if we looked hard enough, we should find twelve cycles of line noise in there. Should we take the time?"
Julia frowned. "I hate to ask a stupid question, but what's line noise?"
Mike said, "Thank you for asking."
I gave her a nod I hoped was reassuring. "There are no stupid questions, right, Dr. Brown?"
"Except the ones you don't ask." He nodded sagely.
Dr. Brown's students all nodded, too. They'd heard it before.
"The sixty Hertz hum in the power line," Mark answered for me.
"Sorry?" Julia looked puzzled.
Jerry held up a power plug from one of the instruments on his lab table. "Wall power. If I'm following this right, 300 revolutions per minute means that a track takes a fifth of a second to read." He looked at Dr. Brown for confirmation.
"Right."
"And a fifth of a second," he continued, "is enough time for twelve alternating current cycles, from plus 120 volts RMS to minus 120 volts RMS and back again."
Julia tilted her head. "Volts I know. RMS, I don't. But is the noise by any chance related to the hum my stereo makes when I turn the volume way up and nothing is playing?"
"Yep, that's it."
"Oh, I get it!" Greg laughed. "Thank you for asking that question."
Several other students murmured agreement.
I added, "RMS is root mean square, a way of trying to talk about effective voltage when the voltage varies regularly."
Julia pursed her lips in thought at my explanation.
Winston shook his head. "Finding that line noise sounds to me like an exercise in advanced statistics."
Nobody seemed anxious to spend time on the required analysis.
"Oh, well," I said, "I'll just have to try this in the statistics class, I guess."
"So what about yesterday's data?" Mark asked. "Is it still there?"
"It should be. Jeff, can you bump the heads back down to track 5 and run the read routine?"
He did so, and listed the buffer again.
"Mathematical sequences," Julia commented in a low voice. Then Giselle's and Julia's names showed up.
"Hey, hey, hey, who are Giselle and Julia?" Mike wisecracked.
"Giselle is Joe's sister," Julia said with barbs in her voice.
"Yep." My eyes met Julia's.
She shook her head, barely perceptibly.
"My sister. Shall we type in everyone's names, and write them to track 30?"
There was some laughter, but Jeff started editing the buffer contents, grinning as he worked. He added Julia's and Giselle's names without prompting, and asked for Pat, Mike, and George's names, to add, too. He ran the write routine, cleared the buffer, and then ran the read routine, and it read back perfectly.
Julia clapped a slow clap, and others joined in, with some cheering and laughter.
"I stlll don't really follow what you're doing," she said when the cheers died down, "and your presentation could use some work, but now I believe you when you say it puts stuff on the disk."
"That wasn't the presentation for the board of directors, 'though, thank you." I chuckled, and Julia laughed. Some of the other students laughed nervously, and Dr. Brown chuckled.
"Okay, everybody. I now need to dive deep into some arcane math, so I can use these drives to share data with other computers. The fun stuff is over for the day, unless you're into really arcane math."
Mark and Jeff exchanged glances.
"So have you figured out yet how we can get computers like these?" Mark asked.
"There is Radio Shack's Color Computer, as I've mentioned before, and it has a better CPU."
Jeff shook his head. "We want to build our own."
"I can ask Denny if Motorola has any more of the Micro Chroma 68 PC boards that they would be willing to sell to students, but wouldn't you rather build something with a more modern processor?"
"Waiting for you to build something better has its own costs," Jeff groused. "This obviously works now."
There were murmurs of agreement.
"I'll ask. But I really think we could design something cool if we worked together."
There were murmurs of approval.
Jeff was not to be satisfied. "But when?"
"What do you think, Dr. Brown?"
Dr. Brown grinned. "I'm not going to tell anyone what they can't do with their own time."
"Okay." I rubbed my forehead and checked my watch. "But I want everyone to understand a few things. The reason I am using Motorola parts is that my brother Denny can get me lots of those. I don't think he can do that for non-family, so if you decide to build these with me, you'll be buying the parts yourselves."
Jerry gave two thumbs up. "No problems here."
Winston added, "All the more reason to get parts ordered now."
"And I'm not promising anything, especially if we start now. Don't blame me if you decide you wanted a 6502 or Z-80 or 1802."
"No problem."
"We've got this."
"What's a Z-80 got to do with it?"
(I wonder if anyone recognizes that this expression was common idiom in Texas and many other parts of the English speaking world well before Tina Turner borrowed it into her lyrics.)
"What indeed? But," I added, "no guarantees of anything. Strictly participate at your own risk."
"If you're saying we're going to see something today, I'm definitely in," Mark said, to general nods and murmurs of agreement.
"See something?"
"A parts list would be good."
"Okay, but what I have in mind for myself is replicating this box using the 6801, then the 6809, and then the 68000. But I'm sure most of you don't want to end up with three or four computers."
Jennifer shook her head. "Let us decide when to get off the train and where?"
I thought for a full minute before proceeding with a sigh. "How many of you want trainers like the 8080 trainers that the lab has?"
All the electronics students raised their hands.
I looked at Dr. Brown.
He grinned back. "What do you suggest?"
I tilted my head left and right, stretching the muscles in my neck while I thought.
"For a trainer, I've been thinking I want to start with an EPROM 6805 SOC, just enough to decode a keypad and a keyboard and send address and data to seven segment displays. A 6805 is powerful enough to build a calculator with, but it will quickly become uninteresting if you want to hang a graphical display on it and run a full OS and programming languages."
Lots of blank looks.
I went to the chalkboard and erased a panel while I thought.
"Starting with a 6801 will be one step quicker, but it will cost more money."
Winston spoke up. "Time is money for me, for what it's worth."
"Okay." I drew a square for the CPU. "68701 here." I wrote the number in.
"Should I take notes?" Julia offered, to my surprise.
"If you would, sure. Thanks."
She got a pencil and a pad of paper out of her backpack. "68 .. 68701, right?" She copied what I was drawing.
"Right. 68701. We'll probably have to find a source for buying that. The keyboard and keypad don't need exact timing so I think we can use an RC circuit for the clock. I'll work those out later." I skipped the clock circuit in the diagram. "Latches for de-multiplexing address and data can found at Radio Shack, too. Don't need full 6821s for most of the required ports, so we can use latches for some of those, too. We can talk about specifics next week." I drew them in.
"Hexadecimal keypad and six, no, seven seven-segment LED displays." And I drew those in. "We'll have to find part numbers for displays that can be persuaded to show hexadecimal codes. We may be able to find those at Radio Shack, or maybe not. The 7400 series logic to glue it all together can definitely be found there."
I continued filling in blanks, and most of the students had paper out and were copying the diagrams. Julia was checking the notes she was taking for me with Mark.
"Boy, I wish you guys had this much motivation in my classes." Dr. Brown deadpanned.
Laughter erupted.
"It's because of your classes that we can follow what this genius is doing."
"Winston," I shook my head. "We are all geniuses."
"Whatever, Joe Genius."
More laughter.
"Yeah. No. Anyway. This should be enough for a trainer that can be attached to a breadboard, like the 8080 trainers, and I'll find a source for the monitor program. The monitor for the Micro Chroma won't work because the hardware will be different, even though the CPU will be similar.
"And then maybe we can turn it into a keyboard controller, and from there turn it into a useful computer. Let's get together after class next week to make real schematics. What you guys should be doing over the weekend is sketching out your own ideas about what the schematic should look like, and looking around in stores, in magazines, and in the library or whatever, for information and sources for buying the parts. And we can share what we find next week. Probably want to hold off actually ordering anything until we've had some more time to get together and plan things out."
I stopped to think for a moment. "And I guess, if Motorola does have enough leftover Micro Chroma 68s, it should be possible to shoehorn a 6801 on a daughterboard into it, and still use the TVBUG monitor ROM. That would be another option."
George looked at Mike and Pat. "I think I'm in on this."
Mike responded with, "Me, too. Pat?"
Pat tilted her head. "I'm thinking about it. Julia, what do you think?"
"I'm here for my own reasons."
There were scattered chuckles.
Julia's and my eyes met, and something passed between us that I wasn't sure I understood.
Dr. Brown cleared his throat. "Was anyone hoping to build something with the 68000?"
Several hands went up.
"For what it's worth, you should start small. I do not plan on approving the 68000 for the microprocessors class, at least, not this time around, anyway. It's a bit too powerful and too expensive, and looks like a nice big rabbit hole to dive into. When the cost comes down, I'll rethink that, but for now, I very much approve of the decision to start with the 6801."
There were some groans, but mostly there were nods of agreement.
He continued, "Which isn't to say I'll try to stop you from doing something with the 68000. Just plan ahead, be sure you do your own work, take notes, make diagrams. Leave something you can show me for each step, and we can probably come up with something we can use in the microprocessors class."
More murmurs of enthusiasm.
I sighed. "Okay, I've got to get these disks running. Thanks for coming to encourage me, and let's get together again next week."
"When?"
"I'll tell Dr. Brown when I'm ready, and he'll let you know."
Most of the students stopped on their way out to say thanks. Pat, Julia, Mike, and George left together after Julia had me look over the notes she'd taken.
Mark and Jeff insisted on staying, said they'd work on homework while waiting to see what come up out of the rabbit hole.
Dr. Brown hummed an Eddie Rabbit tune for a minute, then stopped. "I've been talking with the school board."
I gave him a sharp look.
"You bet I have. I have permission, if there's interest, to turn this project of yours into a multi-semester project class, if things go well. You were going to be the pilot, but it looks like you'll have a lot of company."
"Now I really feel pressure." I chuckled wryly. "First the manager over at the local IBM, now the college administration breathing down my neck."
He just grinned.
I still had time, before I had to go home to deliver newspapers, to put an hour into working out math, turning the math into circuitry, and testing my results with the oscilloscope, with Mark and Jeff watching and helping.
[Backed up at https://joel-rees-economics.blogspot.com/2020/03/bk-33209-headwinds-68701.html.]
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