Ugh, PC0 + PDP-8I problem.

From: Gary Oliver <go_at_ao.com>
Date: Sun Dec 5 15:33:40 1999

At 10:11 PM 11/24/99 -0500, you wrote:
>I have just started to look at this problem but I don't like it already.
>
>Here we go:
>
>If I load a RIM paper tape into my 8I using (single step) it loads in fine.
>
>If I load in a RIM paper tape into my 8I running (no manual steps).. the
>data loaded is garbage , and may not even be in the right place.



>..snip


Just a guess: are the "working" tapes the opaque grey DEC (un-oiled) paper,
while the tapes that fail were punched on oiled (probably yellow or green)
tape? We had problems with illumination on our 300cpm reader when reading
the more "transparent" oiled yellow and green tape (the kind you punched on
your ASR33.) Seems as the bulb aged, the filament got smaller as the
tungsten boiled off and the brightness went up until it failed. I guess
as the resistance went up a constant current source just kept cranking
up the voltage, and the color temp went up... Anyway, for whatever reason...

The reader eventually "saw right through" the tape.

The source of illumination was an "automotive" style end-pin clip-in bulb
with a single in-line filament and its aging characteristics, even at
approximately 1/2 intensity, were not good. The reader needed tweaking
every few months. A good design would have had a feedback cell controlling
the light source... But I digress...

We diagnosed the problem by creating a loop of tape with the pattern

   10101010 01010101 10101010 01010101 ...

and running a dumb "reader loop" program to just cycle the bytes into
(and ignored by) the computer. Then you adjust the illumination and
gain of the photo detectors until the pulses are shaped correctly.
I suppose you could also check the results on the computer and update
a counter in the AC (I just love having real control panels...) so
you had some additional feedback the problem was being solved.

This wasn't on a DEC machine, but the principles may be the same.

Just a suggestion.

-Gary


>Any suggestions????????????????
>
>john
Received on Sun Dec 05 1999 - 15:33:40 GMT