Thursday, March 26, 2015

Issue with SAC Interface driver circuits diagnosed, developing the fairly easy resolution


I huddled with other antique computer restorers at CHM yesterday to think through my options to repair the punch feed wheel whose ceramic rim has fractured leaving a missing section of the 'tire'. Materials like hard rubber or plastics are going to be easier to fabricate but with repeated operation they will 'polish' away any gripping features I mill into them. The original wheels have a gritty ceramic, reminiscent of a grinding wheel, because of the challenge of wear over time.

No perfect solution but at least I have some clearer thinking about what sorts of repairs will be suitable and what the requirements are for the repaired part.


I really wish I had card schematics for the SLT cards that drive and receive over the SLT cable - given the debugging situation I face. There is also a manual IBM printed with diagrams and relevant facts, but I only know of one physical copy, on the other coast, and no scanned version. I am not seeing the behavior I expected on the ends of the signal lines, but they are pretty consistent across large numbers of these signals going into the 1131. Either I have the same failure on all of them or my interpolated circuits are way off.

In order to check further, I looked at the resistance on the cable pairs which were to be driven - they were effectively open circuits, which meant they looked like the standard SLT input which has a diode isolating it. When you pull the line to ground, current flows through the diode, changes the biasing of the  transistor in the receiver in the 1131 causing it to switch states. However, unlike the circuits I found for other drivers which had the 1131 pulling the signal up to +3V with a 100 ohm terminating resistor, these do not!

To check out my theory, I found and unhooked six driver lines - interrupt request for levels 2, 3, 4 and 5, plus cycle steal request and channel gate write. I found an old TTL open collector hex chip, the 7406, hooked these six up to the outputs and drove the inputs from a TTL breadboarding console I had. Immediate success with all the interrupt levels no longer triggered when I ran the 1130.

After a minute or two, I began to get triggering on IL3 but no other - this could be a flaky gate in the junk box chip I used or it could be the lack of a pull up resistor and good impedance matching. I really wish I knew the exact circuit inside the receiver circuits, e.g SLT card 7196, but will some experimenting and scoping of a fast enough signal to ensure I have decent impedance matching.

Once I am sure I have a properly working driver circuit, I will whip up a card that will drive the 41 output signals. The wiring will be detached from the four existing boards, which work fine as receivers of signals coming from the 1131 to me, then put on the new board that will drive signals out to the 1131.

My test using a 100 ohm resistor pulling the line up to 3.0 V, which is what I though was being done on the 1131 side, produced the results I expected and stopped the errant interrupt on level 3. The working plan is to select a chip that provides open collector output, so I can pull the line up to SLT levels (+3), with input specs that allow it work with the FPGA operating at LVCMOS 3.3 voltage levels, with a fast enough operation that it doesn't delay my signal changes too much, and ideally supporting a 3.3V supply since that is readily available in the current SAC Interface chassis. Finally, it should be able to sink the max defined for SLT, 30ma per channel.

I have found quite a few hex inverters or hex buffers but they either require TTL input levels, 5V supply, or are tristate with a very very long delay in popping up to high impedance mode. A DIP chip with hex gates is also pretty handy since once board will let me handle all 41 signals. A bit of research and then on to order parts or drive over to Anchor Electronics. Since Anchor closes at 4PM most days (earlier other days) and isn't open Sunday, it doesn't match the most natural times when I might be heading out to pick up parts.

I have the paperwork for the field engineering change that installed the SAC onto my 1130 system - I am hoping it is detailed enough that I can verify that there isn't some simple problem, like a loose wire from a string of 100 ohm resistors, that could be fixed inside the 1131. Once satisfied that the pullup to 3V is needed at my end, I will move on to parts ordering and construction.

After reviewing everything, I went ahead and selected the 74LS06 or 74LS16 chip because it meets all my requirements other than requiring +5V supply. The PC power supply I used in the SAC Interface produces that voltage, it just has to be connected to the new board. I will run over to Anchor Electronics and pick up what I need.


  1. 10 minutes to get to Anchor? I'll try to keep out of your way;-)

  2. I infer that you saw the posting time of this entry and thought I was leaving at that moment, further that Anchor would be open for another ten minutes? My intent was to go the next morning (today) which is what I am preparing to do, once rush hour traffic dies down.

    However, since I live near highway 85, the mapquest projection for travel time, using 85 and 101, is 12 minutes at legal speeds. Therefore, 10 minutes isn't far off, although it would indeed require some extralegal motion to complete the trip in just 10 minutes. Traffic lights being what they are, the real trip is more like 14 or 15 minutes each way with no unusual traffic.

  3. When I drove to the store at 9:50AM, I encountered no less than three separate accidents on the highways, thus 85, 101 and 237 were all creeping along. I hopped over to Central Expressway but my trip time was nowhere near 10 or 15 minutes.