pdp-8/e address won't increment

[chapmro]

I am trying to get a pdp-8/e working. I have replaced the core memory with a single, new semiconductor memory board (http://tronola.com/html/ram_for_pdp-8e.html). I can load an address, deposit something in it, examine it, but the address does not increment when I use the deposit or examine keys; the address goes to one (000000000001) and stays there. The same happens if I execute a simple operate (opcode 7) instruction like increment accumulator (7001), from any address, after that the address goes to one. I have not tried an instruction like TAD that accesses memory -- that is for tomorrow. I can single step over and over with 7001 in address 0001 and watch the accumulator count up, but the address never increments. Looking at the maintenance manual and print set, I see that EN0,EN1,EN2 control which major register gets routed into the adders. From what I have done so far, it looks like I cannot gate CPMA into there, but I can gate AC. Is this thinking reasonable? So I am inclined to look at the logic on the M8310 board that generates EN0, EN1, EN2, which are mercifully present on the top card edges E and F. Any other thoughts about what the trouble might be? Thanks very much.

 

[vrs42]

Yes, I'd say that somehow the "old value" isn't getting to the adder, so all you ever get is the carry-in (1+0==1).

I'm out for the next several hours, but I'll try to make some time to look at the drawings this afternoon.

Vince

 

[vrs42]

Since it seems no part of MA is getting routed to the adder, that suggests to me a problem in the M8310, though I suppose it could be the inverter from pin 5 to pin 6 of E64 in the M8300.

I'd probably start with the "MAC" flop at E17 on the M8310. That should set on every rising TP4, which is to say that MAC\ should go low.

It looks like the control signals do go across the top connectors, so it is also possible that a problem there is keeping them from getting from one board to the other.

OTOH, that may also be your easiest shot at getting a look at MAC\, unless you have the right extender card and the flexible top connectors to go with it.

 

[chapmro]

So today I tried a TAD instruction at address 1, and it worked. In fact I could execute it over and over and watch the AC go up by the amount in the memory address. But this tells me a good bit, in particular that all the adders are working, and the CPMA itself is working, and that the multiplexors that route things from major registers into the adders are at least somewhat working, since MB must be going into the adders even if CPMA is not. Also, clear that EN0 is not simply stuck.

 

[chapmro]

Since it seems no part of MA is getting routed to the adder, that suggests to me a problem in the M8310, though I suppose it could be the inverter from pin 5 to pin 6 of E64 in the M8300.

I'd probably start with the "MAC" flop at E17 on the M8310. That should set on every rising TP4, which is to say that MAC\ should go low.

It looks like the control signals do go across the top connectors, so it is also possible that a problem there is keeping them from getting from one board to the other.

OTOH, that may also be your easiest shot at getting a look at MAC\, unless you have the right extender card and the flexible top connectors to go with it.

That is a good suggestion, that the problem may lie in the top connectors themselves. A lot of this machines troubles have been fixed by applying Deoxit to connectors. Maybe this one also, but if not I will certainly start with MAC on E17.