Tuesday, October 28, 2014

Keypunch interface stubborn issue, SAC receiver circuit working, and testing the CHI interface/disk


I wired diodes across the relay contacts in my box in order to absorb any induced back EMF from the opening of the coils. Electrically it would be more ideal to install these at or close to the punch solenoids in the 029, but we are trying to keep the keypunch modifications to a minimum. However, the small gauge wire connecting to the interface box will have to absorb the current spike flowing through the diodes.

Diodes added across relay contacts in order to quench back EMF from keypunch coils
With the diodes installed, I hooked up the box but found that the machine was not recognizing registration of a card even initially, with no attempts to punch. I will have to troubleshoot this to understand what is blocking registration.

I made up the dummy plug and verified that the keypunch works properly with the interface disconnected. Therefore, there is no damaged component in the keypunch. I hooked up the punch cable to my box, leaving all other cables including power disconnected. The keypunch worked properly one time with cards but after a clear cycle, it was again failing to recognize when the next card was registered.

Dummy plug for punch cable when box not attached to keypunch
This tells me that something in my static wiring is causing this problem - the wiring that is linked through relay contacts is somehow causing a problem - the relay contacts being open. It may be a wiring issue or a short in the connectors or something else, but I have to diagnose this before the box can be tested further.

I tested every wire going into the box from the punch cable and they were all infinite resistance to each other, except for the pair for multipunch pole 2 which are normally closed. I ran the keypunch with the dummy plug for a while, then plugged in the punch cable but otherwise had nothing connected to the interface box. This worked well for about five or six cycles of registering, spacing and clearing cards.

I then plugged in the power brick connector (but no AC was hooked to the brick), which locked up the keypunch on the second cycle. Very odd. At this point I have to suspect that a connection is intermittently shorting somewhere in the SMS block, so that small movements of the cables introduce the error.


I powered up the CHI interface electronics and they came up cleanly. Looking over the cards, I came to the conclusion that it appeared likely to be working properly. As I result, I will cable this to the SAC interface and give it a try later tonight or tomorrow.

CHI interface looking good, decided to begin testing it
The interface came up fine and the drive loaded a pack I was willing to risk. I found a slight darker mark where the head was riding on the disk. I will inspect it in the sunlight to see whether it damaged the pack, but the heads must be cleaned at a minimum before I put any other cartridges into the drive.

Superficially clean drive, but some marking on a cartridge where the heads flew


I picked up the solder this morning and rewired the demo receiver circuit, which is hooked through a transmission line (I twisted wires together from the driver output and from the receiver input, then connected those twisted pairs temporarily). This is intended to verify that TTL input signals to the driver will be properly reflected as TTL outputs from the receiver, as well as my verification that the voltages are at appropriate levels in both states.

The circuit worked perfectly. I varied resistances to ground on the TTL input to the driver to see at what voltage it would switch between on and off. It will recognize a logic level 1 down to .1V but is off somewhere above .066V, well within the specifications for the logic I will use to drive signals towards the 1131.

Driver circuit on top and receiver circuit at bottom - working perfectly
Now that the circuits work with each other, I will temporarily hook the receiver up to one of the SAC output lines and the driver up to one of the SAC input signals, in order to prove these circuits work with the IBM SLT logic cards. I chose interrupt request and the corresponding interrupt level status as the signals on the SAC interface, as I should be able to see the request mirrored on the 1131 console and the status of the interrupt level on the panel mirrored at the TTL output.

I lieu of testing this, I powered up the CHI interface and disk drive (see above). I may do some XIO tests to determine how the CHI interface is responding (and to what addresses), but the important testing will be with my circuits once I move on to try them with the SAC.

I went ahead and ordered the transistors, diodes and resistors I need to implement the full interface which will give me TTL logic on my side and for the other side it will deal with the 1131 SLT transmission lines coming out of the SAC interface. I expect to have all the parts by the end of tomorrow, thus doing the assembly on Thursday and Friday.

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