PDP-11/70 progress (and a cry for help)

Hi Josh, In the situation you describe, I guess I would first chip clip '174s for a slice of both PCA and PBC on the LA, run the troublesome instruction sequence, and look at the trace. Check that CLKPCA H and CLKPCB H are happening when and only when expected, and that all the timing there looks okay. You would also be able to see good-data-clocked-in-but-bad-data-presented on that trace, indicating more failed '174, or see any bad data arriving from the ALU upstream. I'll take a look through the flows after dinner. The "0002" might be one operand used to increment the PC, but the other operand shows up as all zeros because of a bad ALU setup? I guess I'd trace to definitively rule out PCA and PCB first, and then move backward around the chain from there verifying the ALU, ALU muxs, mux sources, etc.? --FritzM.