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.
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.
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.
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.