The Good, the Bad and the Ugly

The next thing I got obsessed with was the keyboard. At first I had planned to get an old keyboard off eBay and reverse engineer it. So I bought an old VT-260 keyboard and removed its circuit board to expose the raw key matrix connections and I spent a fair bit of time with a multimeter working out how that matrix was organized. I got as far as having my own microcontroller reading the keyboard and displaying simple scan codes. But by the time I had done that, I had decided that the keyboard itself was not really what I wanted LEO-1 to have. I wanted something special.

You may recall my admiration of the ICL 2900 series mainframes. Well, the keyboards those things had were absolutely gorgeous and nothing like anything I’ve seen before or since. I wanted LEO-1 to have a keyboard like that, but sadly, not everything is available on eBay. The closest I could come to that would be to make my own keyboard with a similar layout and similar keycaps. So I started reading about custom keyboards and found that there is a whole subculture of people who are really into keyboards. I found a great YouTube channel by this funny bloke who calls himself Chyrosran22 and watched all his videos. After a month or two of this I think I knew more about keyboards than I know about my own mother so it was time to take a step back and regroup.

ICL 2900 keyboard

ICL 2900 keyboard

While looking for keyboards on eBay, I had discovered a cool device called a Rubidium Frequency Standard which piqued my clock-obsession — and so I spent a few months building my own atomic clock! I already have enough clocks in this room and didn’t need another one, but I couldn’t resist. I had lost my way. It seemed that I was just finding excuses to postpone finishing LEO-1 and looking back I can now see that I was terrified that I wouldn’t be able to make it work. If it didn’t work, it would be the biggest waste of time and money and I would look like such an idiot for even attempting something as absurd as trying to build a contraption as complicated as this.

Even so, I had been working on the PCB design for the Memory Board and the Control Board during the autumn and they were pretty much finished. Eventually after obsessively checking it over and over again, I had the Memory Board manufactured and I populated it. My initial tests with switches were promising, but then something terrible happened. In 2015 I had seen a very weird problem when testing the ROMs where the data bus would go into continual oscillation for no apparent reason. I thought I’d solved it with bus termination resistors, but now this problem came back to bite me in the arse once again, this time on my nicely made PCB. I couldn’t understand why all these people out there could build entire CPUs on breadboards with loads of untidy wires and have it all work perfectly, but I couldn’t even get a ROM to work on a neat PCB. At one point I just considered picking the whole thing up and placing it into the trash, but of course that was only a fleeting thought. I tried all kinds of hacks and guesses but really had no idea what was going on. This problem only happened on ROM not on RAM or any of the other devices. I blamed the ROM chips; they must be buggy. I managed to crowbar an old-school EPROM into the socket and would you believe it, the problem happened on those too. I checked the data sheet over and over again and couldn’t see anything that could explain it. Then I realized what I was doing. I was using switches to change the address bus for testing, and I was doing it while the ROM was chip-enabled and also output-enabled. My VDU does that and doesn’t have any trouble, but I wondered what would happen if I followed the process that the real CPU would. The address bus would only change when the ROM was not enabled, and the output-enable would only be on for a moment. Well, when I started testing it like that, the problem went away. I visualized the problem as this: when you change the address, the internal circuitry of the ROM will ‘rattle’ as the device finds the correct cell to output. If the device is output-enabled, this rattling will appear on the data bus as a randomly changing mess lasting some tens of nanoseconds. This is high-frequency business; you don’t want that noise getting onto the data bus in the first place. Possibly, the length and layout of the PCB traces could cause frequencies like that to start an oscillation, and if the changes are ‘evil’ enough, it could create power supply glitches, ground bounce or such, which would upset the ROM, cause it to rattle some more… and around you go, ad infinitum. This is all speculation. The real cause is still somewhat of a mystery, but the important thing was that I found that as long as the rattling doesn’t get onto the data bus, you’re safe. Unfortunately, I also found that this happens if the ROM’s chip-enable and output-enable are asserted at the same moment, and my CPU design actually did that. The only way it would work properly would be if the chip-enable came first, then the output-enable, on a different tick. I was going to have to change the design of the CPU’s sequencer. It was an easy fix, and a lucky catch at the eleventh hour, especially since I had not ordered the PCB yet.

Oscillation

Ugly things on the data bus

 

You may recall that I have a terrible habit of going off at a tangent and ordering things that I hope to be able to use in the future, even when I’m not sure that any of it will work. Well, I had ordered a small thermal receipt printer a month before, so with the memory board working, I decided to try it out. This also enabled me to test the device expansion ports. I hooked up a UART to the memory board and the printer, and manipulated it as a device using the test switches. I was able to manually make the printer print a single character. So this means LEO-1 will almost certainly be able to use that printer too. Finally happy with these results, I decided to press on and finish the control board.

“You’re all clear kid, now let’s blow this thing and go home!” — Han Solo

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Toil and trouble

My worst fears have come to pass and for a while I thought the LEO-1 was not going to make it. I had made a test board to act as a fake control board so that I could test the memory board. I had tested it with the Real Time Clock chip and everything was working great. So I carried on building the memory board and connected up the two ZIF sockets. I burned some test data to one of the EEPROMs and put it in one of the sockets, connected up a couple of digits to one half the data bus and used the test board to address the ROM. Lo and behold, the digits showed the correct data coming out of the ROM. It was working as expected. Here’s a picture of the set-up. The digits on the protoboard are showing 3 4 which is the first byte in the ROM.

Memory Board working

Memory board displaying data

I switched various addresses and watched the data come out and everything seemed fine, when suddenly the digits went all weird, like a square 8 which is an impossible display. It was pretty clear that the display was oscillating between two values at such a high speed that it looked like all the dots were on at the same time.

And so began several weeks of pure hell trying to figure out what was going on and what caused it. I was able to reliably repeat the problem by setting a certain address and then changing to the next address so that the data changed from 0 0 to  0 1. Often, but not always, the thing would start oscillating. I put my scope on it and found an 8Mhz oscillation on the data bus. I suspected the ROM was faulty but all the ROMs had the same behaviour. I suspected the test board because I had forgotten to buffer the address bus lines. That was a dumb mistake which I decided to fix, so I spent a week adding 74244 buffers to the test board. That didn’t fix the problem. Then I realised I had forgotten to buffer the control signals too, so I used a few spare gates on the board’s 7400 to do that. Didn’t fix it. Weeks had gone by and this random oscillation was still happening. I remembered that you can get this kind of problem if you forget to connect an unused input on a gate, so I checked the schematics and scoured the board for disconnected pins. No luck. Everything was as it should be. One night in early December as I sat there with my head in my hands, I realised that if I couldn’t solve this, there would be no chance of the LEO-1 working at all. My fairly trivial memory board wasn’t even stable at manual switching speeds! I thought about giving up — all that time, effort and money down the drain. By the time I went to bed, I was pretty depressed.

Diagnostics

Trying to figure out the oscillation problem

The next day I took one last crack at solving the problem. I had been studying all the data signals on my scope’s logic analyzer and had not had any ideas beyond unconnected pins and faulty chips.

You can see in the picture above that the test board’s ribbon cable was sticking up in the air. It was like that because I was using a short cable for the address bus and the long data bus cable had to be bent in order to be plugged in. Well, for some reason, while the oscillation problem was happening, I just happened to push the cable a bit flatter — and the oscillation stopped. That didn’t seem like a coincidence, so I tried changing the address with the cable flat — no oscillation. I bent the cable again and then next time I switched addresses, the oscillation started again. I could control whether the oscillation would happen or not by bending and straightening the cable. What’s more, the scope showed that the oscillation frequency was changing between about 7 and 8 MHz depending on how the cable was folded.

Then it hit me. Was the ribbon cable suffering from transmission line problems? Those problems that I had been obsessing over right back at the start? Things like reflections and ringing and all that annoying stuff? And did folding the cable do something random like perhaps change the impedance or cause cross-talk? And could that make the bus transceiver chip go into some kind of oscillation? Well, I was not sure and I’m still not sure now, but I have a theory. When reading data out of the ROM, the test-board end of the 12″ cable was not connected to anything. That meant that when the data bus switched from 00 to 01, that 1 bit went down to the other end of the 12″ and reflected back. Ordinarily I would have expected the reflection to die out pretty quickly and just cause a bit of ringing. But something about the folded cable caused it to keep bouncing, perhaps amplified by the bus driver itself. After reading up a bit about line termination, I did an experiment. I added a bunch of 82 ohm resistors at the memory board end of the cable, just before the 74245 transceiver. After that, I couldn’t repeat the problem any more. No amount of switching addresses or folding the cable in any way caused the oscillation and I haven’t seen the problem since. I don’t really understand why it works except that possibly the resistors absorb the reflection and damp it so that it doesn’t turn into an oscillation, but I still don’t understand why I got full permanent oscillation in the first place. I’d love to hear from any experts out there who can explain what the heck was going on.

As if that wasn’t enough trouble, the next thing that went wrong was that I discovered that I had not left nearly enough room between the address bus header and the first bunch of chips. There was no room left to solder any more wires and I still had a couple of busses to connect there. I found I had no option but to move the address bus header to the other side of the board. I had to disconnect all the wires, desolder the header, re-solder it and redo all the work of connecting the 24 address lines to the ZIF sockets. While I was at it, I bit the bullet and removed the majority of the thick annoying wires and replaced them all with magnet wire. I learned a valuable lesson there. You have to leave plenty of room between headers and the chips they connect to so that there’s room to solder multiple busses to the same header pins. From now on, I’m only going to use regular wire for power connections. Everything else will be done with thin magnet wire.

Address Bus

Address Bus connections

So, the LEO-1 lives on after all and right now I’m in the process of wiring up the main RAM and ROM sockets. After that it’s the I/O chips and then board number 1 of 4 is complete.

Memory Board

Memory board as of New Year 2016