PDP-8E diagnostic help needed
Mark G. Thomas
Mark at Misty.com
Fri May 9 10:36:04 CDT 2008
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
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
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 G. Thomas (Mark at Misty.com)
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