Communication Breakdown (Part 3)

Sorry for the delay, I've been busy and I've been waiting for replacement parts to arrive.  I'll spare you most of the gory details and keep today's post short, since a lot of this is the same old schtick (just a lot of faulty chips that were fortunately easy to trace down) -- but to give you a general idea of what I did over the past week or so, here's a list of the chips I replaced:

5 74H61s
2 74H52s
1 7408

At this point both RS232 interfaces (primary TTY and secondary TKA) are working properly at 300 and 9600 baud respectively, and the Keyboard input (and the Keyboard itself) are also working correctly.  There are no stuck bits left in the Accumulator & Input Mixer boards so I'm going to declare victory here -- the Imlac is now able to speak to the world, and the world can talk back to it.

Next on my list of tasks is to deal with the memory problems.  As you may recall, there are two known issues with the core memory:

1.  A few randomly dead bits (stuck off) across the address space.
2. The address space wraps around at 2K, indicating faulty addressing logic.

Issue #2 can be fixed, it's just a matter of tracing down the addressing logic on the core memory control board.  Issue #1 is more problematic, as the randomess of the dead bits makes it pretty clear that it's an issue with the core memory itself, probably defective cores.  While these issues aren't impossible to fix, they are extremely difficult given the very small scale at which work needs to be done.  Let's just say I'm not optimistic.

My plan is to swap core planes with the other 8KW boardset to see if it works any better.  If the cores from that set test out OK, I will likely live with 8KW for the time being -- it's enough to run most Imlac software -- and I'll fix the addressing logic. If the other plane has errors as well, then there's not much point in fixing the addressing and I'll look into building a replacement using more modern static RAM chips.

Until next time, let a smile be your umbrella!

Update:
I swapped in the other core plane and it too has stuck bits.  Many fewer than the current core plane, but even one is enough to make it unusable.  I managed to partially solve the addressing issue -- I can access memory above 2K properly from the front panel but indirect accesses under program control read the wrong address; this looks like a CPU problem.  To add to the mystery, I can address the full 8K with the original core plane, but only 4K works with the other swapped in --  this may explain why the bootstrap loader was rejiggered for a 12K machine...

So, in summary:  Looks like I'll be building some manner of replacement for the core memory.