Weekly Classic Computer Trivia Question (20141226)

[Doug Ingraham]

I said this one is seeming easy and I meant it.  If you look at the photo
on Wikipedia

Desktop Straight 8
<http://upload.wikimedia.org/wikipedia/commons/6/6d/PDP-8.jpg>

You can count on the horizontal lines of bulbs labeled:

Program Counter 18
Memory Address 12
Memory Buffer 12
Accumulator 13
Multiplier Quotient 12

On the right side there are two vertical rows of bulbs labeled as follows:

AND    FETCH
TAD    EXECUTE
ISZ    DEFER
DCA    BREAK
JMS
JMP    ION
IOT    PAUSE
OPR    RUN

This gives 82.  However there is a secret bulb in the space between the
BREAK lamp and the ION lamp below it.  There is a hole in the fiberboard
mask for the bulb, the driver transistor and resistor are on the lamp
board, the bulb was in place on my machine and there is a wire running to
the backplane connected to it.  But there is no way to see it if it ever
turned on with the glass face plate in place as the silkscreen over the
bulb position is opaque.  Based on this I would say that the actual count
is 83 but 82 are useful.  One of these days I will trace the wire to see
where it is cabled into the back plane.  It does not show on the schematics
I have looked at.

I stole that bulb at one point to use as a replacement for a burned out
one.  At the moment I have three early vintage white LED's in the front
panel of my machine.  These LED's are a cool blue white light and look out
of place for a couple of reasons.  In addition to the color, they are on
dimly whenever the machine is running.  Cross talk in the wires is enough
to trigger an LED.  They also turn on and off too quickly.  I calibrated
the brightness at just under a ma as being the correct brightness.  The
original bulbs draw about 30 ma when on.  Anyone who has ever taken apart
the front panel of a straight 8 knows that it is such a pain to replace
bulbs that I have decided the next time to just replace all of them with an
LED, two resistors and a capacitor to somewhat simulate the delays and
persistence of a filament bulb.  I may even use a soft white LED and a
yellow LED in order to get better color temperatures but this will take too
much room.  One of my friends says this would be sacrilege and I should
only use filament bulbs.  Of course he also says it would be OK to use a
tri-color LED and a microprocessor for each bulb to generate the correct
color temps and delays.  Seems like too much work to me.

The next trivia question will be next year so Happy New Year everyone!

Doug Ingraham
PDP-8 SN 1175 (Running OS/8 from DECTape)


On Fri, Dec 26, 2014 at 2:57 PM, Doug Ingraham <dpi at dustyoldcomputers.com>
wrote:

> This one is seemingly easy.
>
>
> On the front panel of the original PDP-8 (straight 8), how many lights are
> there?
>
>
> There is a pretty good picture of one on Wikipedia if you feel like
> sneaking a peek.
>
>
>
> Doug Ingraham
> PDP-8 S/N 1175
>