Debugging continues for new Documation card reader controller

 INVESTIGATING THE MISSING ROW 9 DATA

To divide up the set of suspected failures I tacked a wire onto the output of the gates driving the D9 (Row 9 data) signal, just before it exits the Control card and flows over the connectors into my controller box. The scope showed me valid D9 data which eliminated the internal Documation circuitry from the list of suspects.

I then did a continuity test between the shield connector on the Arduino and my tacked wire to see if the signals are making it from the reader into my box. I found an open circuit. I then removed the wire from the shield and verified there was a proper connection. The flaw was a poorly connected wire onto the shield.

With that repaired, I read some cards to prove that I am receiving all the data that is punched on the cards, without dropout or glitches. 

INVESTIGATING THE FALSE ERROR STATUS

At the same time as I had tacked on the wire to the Control board for the data line, I tacked a wire to the Error card to check out the error signal. I found that it was improperly reporting an error condition when none existed. It appeared that the buffer gate was bad - input said no error but output flipped to on. 

This buffer chip is not fully used. I first cut the leads from the package for the failing gate (pins 10 and 11), added jumpers to the unused gate at pins 8 and 9, then retested. Whatever went wrong with the one gate was still wrong with the substitute.

Jumpers substitute unused gate for failed on

I pulled out my Hakko desoldering gun, removed the bad chip and soldered in a replacement 7417 open collector buffer chip. The ERROR signal was now properly generated, but the interface still saw it as active.

Next up was the continuity testing, which proved that the ERROR line got to the shield connector for pin 35 of the Arduino. This pin is read by bit 2 of Port C of the Atmel Mega2560 processor on the Arduino (PC2). All eight bits of Port C are inputs accessed by the same method and the other seven are correctly read. However, I am not seeing PC2 reflecting the signal coming from the reader. 

I may have a mechanical issue with the shield to Arduino pin for 35, or the Arduino processor may have been damaged by whatever was wrong with the original buffer chip, leading to PC2 being unusable. I will have to investigate this further on my next session.