Testing the plotter controller card in the IBM 1130, without a plotter hooked up - part 1

TESTING IN THE 1130

Since it did everything properly on the bench setup, it was time to verify that it works correctly with the IBM 1130 while inserted into the logic compartment. The newly constructed card was inserted again into slots M4 and M5 in compartment A-C1 of the 1130 system, which is set up for the IBM card that controls the plotter. 

I did basic verification that the slots were indeed wired up to N5 where the connection to the plotter exits the backplane. I then set up some simple tests to see how it behaved without the -24 volts from the plotter applied to pin B08.

It should produce a Device Status Word (DSW) with bit 15 set to 1, indicating that the plotter is either not attached or not powered on. I issued an XIO instruction to an IOCC specifying a Sense Device operation on area code 5 (the 1627). The resulting DSW was all zeroes. This was not what I expected.

I then changed the IOCC to specify a write operation on area code 5 pointing at a word where I had both bits 0 and 5 set to 1. When I executed the XIO, I saw the light for interrupt level 3 come on, which is exactly what I would expect. Having turned on the Interrupt Delay switch under the top cover of the 1130, it didn't jump into the interrupt handler code, so I could more easily debug the card. 

I reset the IOCC to a Sense Device operation and executed it. I saw a DSW with bit 0 turned on (operation complete) which is consistent with the interrupt request. However, I put the oscilloscope on the output pin and never saw it dip down from 12V to 0, as it should have for 50 milliseconds. 

That is consistent with the board behavior if the plotter is not turned on - immediate completion but no command signal sent to the plotter. Everything was correct with the behavior except for the lack of DSW bit 15. 

I then tried to load some memory locations but wasn't able to set bit 15 to a 1 value. Locations that previously had that value would display the 1 in bit 15, but I couldn't load any new values using that bit. The console entry switch (CES) that I used to load memory values shows every switch that is in the 1 position in a register EXCEPT the switch for bit 15 didn't turn on the register bit.

Before I proceed to more elaborate testing, I need to investigate this relatively small issue - bit 15 not appearing in the DSW nor available to load into memory. This could be a short inside my connector on the card, for example, or some other issue in the 1130 that only manifests when the card is plugged in.