9 May
2008
9 May
'08
12:03 p.m.
On Fri, 9 May 2008, Mark G. Thomas wrote of a PDP-8E thusly:
[...] As I examine memory, the address lights count up
to 01111, then
go back to 00000, instead of 10000. I can manually enter an address
higher than 1111, but the 10000 and 100000 bits don't stick -- they
go low, as soon as I hit the examine switch to step to the next
memory location. [...]
My first impression is that those two bits of the MA are toast. When two
adjacent bits of a register simultaneously get up and die, that sounds
like a flip-flop which holds both - a 7474, or 74H74, or DEC 8271 - has
gone south. I've never had an 8/E, but... <shuffling through online module
drawings> ...would the 8271 chip at M8300, position E25, be the one
controlling MA bits 6 and 7?
I'm not up on my 8/Es, so please: grab drawings and see if that's the
right spot. :) But I'd look at testing that chip before I went after
any others.
-O.-