<I've got the 8/e frontpanel prints here. From what I remember the 8/f is
<a little different.
The displays are leds (minor change around that part), otherwise it's
identical to the 8e.
<THe frontpanel display logic is mostly built from diode transmission
<gates - this circuit
<
<In---->|-----+-----|<------+--- Lamp driver
< | |
<Sel-\/\/-----+ |
< |
<>From other gates-----------+
<for that lamp
Does not compute based on the chip part numbers.
<The MD inputs have their own set of gates. I'd start by tracing from MD1
<(pin AL1) through the diodes to the appropriate 380 lamp driver
<(E26/6,7,2 on the 8/e). My guess is an open-circuit diode...
Ther are very few diodes and the drivers appear to be 7404s with 74l54s
used as selectors.
Restate the problem... The display is the variable 12bit on off the
rotary switch and only occures when MD is selected. This suggests the
data selector logic or it's particular source on the board as MD on the bus
has the correct data.
<Oh, these old machines are quite easy to work on (famous last words...)
Generally the PDP-8 series is fairly straightforward.
Allison
Received on Sun Apr 04 1999 - 18:20:31 BST