PDP 11 Line Time Clock (LW11L) M787 and EIS board, (KE11E) M7238

To understand this symptom, one needs to understand how the EIS interacts with the main CPU. Both include microcode, and what is supposed to happen is that when an EIS instruction happens, control is passed to the microcode on the EIS board (the actual microcode words being fed back to the main CPU through those three over-the-back jumper cables). The microcode on the EIS board can then control the data paths, etc in the main CPU, to feed the EIS data, and take back the results of the computation performed on the EIS card. I'm trying to understand what W1 does, but I'm not there yet. It's shown on the KD11-A print K3-8 (pg. 48), in the lower left corner, but its effects are somewhat obscure. To start with, the array of odd chips E6-E7 (74H60's) and E17 (74H53) are expandable AND-OR gates. I'd never seen these before, but the lines running to and from pins 11 and 12 on the 'H53 join the other three gates below it into it - i.e. that whole array of AND gates all feed into one NOR gate (output on pin 8 of the 'H53). So far, so good, but from there I'm still lost. When W1 is inserted (no EIS) it grounds the signal ECIN00, which comes in from off-board (as shown by the "A05S2", which is the pin it arrives on). The output of that giant NOR gate is CIN00, which is immediately sent off-board (pin 'A05P1'). I have yet to try and chase these signals down, and work out what they do; the KD11-A Tech Manual is fairly cryptic on the subject. Note also that, IIRC, the front console operates under control of microcode. So I'm _guessing_ that what is happening is that somehow the EIS is, when enabled, messing up the operation of the microcode in the main CPU, causing it to freeze.