pdp-8/e fault

Brent Hilpert hilpert at cs.ubc.ca
Mon Mar 23 23:52:33 CDT 2009

Rod Smallwood wrote:
>     My pdp-8/e has developed a nasty fault.
> I have all the manuals and a scope but before delving in too deep I thought
> I'd see what the 'Hive mind' knew.
> Its straight forward enough of a fault. I can no longer toggle in values via
> the switch register.
> The address sets up ok and the switch register value appears to transfer to
> core on raising DEP.
> The location counter increments normally.
> However going back and checking the locations shows nothing there.
> It does not appear to be a problem with the core as replacing the low
> address 4k with a higher one makes no difference.
> Experienced PDP-8 fixers please comment.

When you say you replaced the core did that include the address and inhibit
drivers, as well as the actual core and sense amps? I'm not familiar with the
8/e specifically but web info indicates the 4K mem is composed of 3 distinct
boards, probably distributed as core and sense amps on one board, address
drivers, and inhibit drivers.

Drivers are a common point of failure in core mem systems. All the bits in a
word failing would suggest an address line driver, a single bit across words
could be an inhibit driver.

If it's an address line driver, either a block of sequential words, or a
sequence of individual words separated by the same offset, in either case
coincident with the core matrix dimensions, will be faulty. Determining the
failure range by examination can lead you to the specific driver.

If it's not the drivers what about the core power supplies, as distinct from
the logic supplies?

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