Completed testing 74182N chips, diagnosing the M1000 card reader moves forward

 BUILDING BUFFERS ON A BREADBOARD TO COMPLETE CHIP TESTING

Since the chips in question draw well in excess of the 8ma max that the Retro Chip Tester Pro will sink from the input when they are set to logic low state, I made use of two very high capacity buffer chips from my inventory. They were wired up on a breadboard and hooked to the chip under test making use of the Breakout Adapter from the chip tester design files. 

Failure due to inadequate sink current capability

The 7407 chip is a TTL hex buffer, open collector, that can sink up to 40ma per pin and tolerates up to 30V logic high levels, definitely in excess of the needs for this test but they were handy. I put two on a breadboard since I have a total of eight input pins to drive, four propagate carry and four generate carry signals. 

I wired the chip to +5V, tied the ground together to the tester pin that fed pin 7 of the 74181N. Pulling the jumpers from the eight input pins disconnected the chip under test pins from the tester pins allowing me to wire the chip under test to eight of the buffer outputs and the tester pins to the corresponding inputs of the buffers. The outputs were pulled to +5V by 4.7K resistors since this was an open collector chip. 

The tests ran to successful completion for all of my 74182N chips on hand, now that the test can drive the chip adequately. 

Buffer chips in circuit to the eight inputs being tested

DIAGNOSING THE M1000 CARD READER

Once the logic analyzer was fully connected and the signals all verified, it was time to run tests again. Previously the reader was balking at reading, immediately throwing a Read Check and Stacker Check when I attempted a pick. That was not present today - but I worry about erratic behavior. 

I saw the signals registering properly on some runs with the analyzer capturing data, so I hooked up to the laptop and read a small sample deck, flawlessly. I was very pleased and moved over to my COBOL compiler deck to begin archiving that. 

Alas, the gremlins came back. Every card read properly according to the eighty columns of data I captured, but the reader threw up a Read Check error at the end. This comes from the reader checking at the time for column 81 to verify that no lights are present, and again at the time for column 84 to verify that no lights are blocked. This tests that the right edge of the card is in the correct position at the end of a read. 

This flaky behavior is going to be a challenge to troubleshoot. I need to swap the card extender from the Clock card position where it was to the Control Card position which is where the Read Check is generated, so that I have ready access to the pins on the card as well as the backplane signals that are wired to the logic analyzer. 

I can already see that the magnetic pickup is working properly and registering the card column counts properly, based on the data I was capturing, so the error must occur past column 80. I will be prepared to detect that, assuming the reader has not moved on to some other behavior when I next power it up.