Sunday, June 12, 2022

Digging into failure to reset request for interrupt


The circuitry has a flipflop called KB Response that is set when a key is pressed and it is reset when a XIO Sense with Reset Bit 15 is received for Area 1, the device code for the keyboard and console printer. The AND gate in the reset path below is triggered when the KB Resp flipflop is set, so that the notQ output is low, and a pulse arrives from the bottom left AND gate when XIO Sense Reset 15 and Area 1 are both true. 

Logic that resets the keyboard trigger to request IL4

I will monitor the KB Resp line as well as the XIO Sense Reset and Area 1 signals with the oscilloscope. If the XIO and Area 1 lines are true, but the FF doesn't reset, then the issue is on the SLT card itself with the flipflop circuitry. If the lines don't change appropriately the issue is somewhere before the left lower AND gate. 

I hope to discover what is going wrong. Most likely one of the signals is  not getting to the AND gate which resets the response flipflop, but we shall see. It could also be a hot signal setting the flipflop repeatedly, but one that is only becoming hot after the first keypress is registered. 

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