9 May
2008
9 May
'08
8:36 a.m.
Hi,
I've got a PDP-8E which I've almost got working.
Can anyone here help me figure out this remaining problem?
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.
I can manually load an address 1000000 or 10000000, and hit examine to
see 1000001, 1000010, 1000011, etc..., but once I reach 1001111, it's
back to 1000000.
It was recommended to me that it might be the carry between E52 and E37,
or the E38 input multiplexer for bit 7 (on M8300), so last night I
socketed and replaced all three of those ICs, but I still see the same
symptoms.
I have extender boards, so can access M8300 during operation. I
measured the carry line between E52 and E37 go low when I reach 1111, but
the light for line 10000 doesn't light on the front panel on the next
address, and I see the data from memory location 0000, 0001, etc.
repeated, displayed as I continue to step through memory locations,
as described above.
Of course, if someone has a spare M8300 they would be willing to sell me,
that's another option.
Mark
--
Mark G. Thomas (Mark at Misty.com)
voice: 215-591-3695
http://mail-cleaner.com/
9 May
9 May
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.-
4:09 p.m.
Hi,
I've got a PDP-8E which I've almost got working.
Can anyone here help me figure out this remaining problem?
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.
I can manually load an address 1000000 or 10000000, and hit examine to
see 1000001, 1000010, 1000011, etc..., but once I reach 1001111, it's
back to 1000000.
It was recommended to me that it might be the carry between E52 and E37,
It's been soem time since I've been inside a PDP8/e, but I would have
thought that a carry prolem would have caused all the its to 'stick' (or
latch), but only the bottom <n> to increment.
or the E38 input multiplexer for bit 7 (on M8300), so
last night I
socketed and replaced all three of those ICs, but I still see the same
symptoms.
I repeiared a PDP9/e with a similar-ish problem years ago, and it was one
of the latch chips I think.
I have extender boards, so can access M8300 during operation. I
measured the carry line between E52 and E37 go low when I reach 1111, but
the light for line 10000 doesn't light on the front panel on the next
OK, can you find where the 10000 bit comes from? Look at the inputs to
that chip -- should the appropriate output be being asseted? If not,
which of the input(s) is wrong? Trace that back, and so on.
This is not a complciated processor from what I remember, so it shouldn't
be too ahrd t odo that.
-tony
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