Reproduction DEC 144-lamp indicator panels

Noel Chiappa jnc at
Thu Dec 9 10:47:08 CST 2021

    > From: Jay Jaeger

    > Also, if someone (else, presumably) does up a replica of the indicator
    > panel board (perhaps with the option to use LEDs, with some resistor
    > packs that could be bypassed for lamps

Two points.

First,there's the question 'are you trying to produce something that just
_looks_ alike, or do you want something that's electrically compatible (i.e.
can be swapped in in place of an original'?

If the latter, it might be going to take a little work; it might not be just
'wire up some LEDs and go'. If you look at the indicator panel prints (pg.
190 of the RF11 prints), the incoming lines are tied to the base of a
transistor; its emitter is tied to ground, and the light bulb is wired
between the collector and +6.5V. This means, I think (I'm basicaly a software
guy :-), that one turns the bulb on by putting a voltage on the input, which
turns the transistor on, and current flows through the bulb to ground. (If I
have that wrong, will someone please orrect me?)

Given that the way TTL works is that 'logic' outputs actually sink currect
when their output is '0' (i.e. current goes _in_ the 'output' pin), it might
take a little work to make the right thing happen. Although maybe not;
looking at the RK11-C prints, it seem to drive the indicator panel straight
from the output of normal gates. I _think_ what happens is that when a gate's
output is '1', the output's voltage floats high, and that's enough to turn on
the transistor (above) driving the bulb. (Ditto the request for correction!)

But the real issue with 'electrically compatible indicator panels' is the
wiring. In the originals, the flat cables that drive them are soldered directly
to the indicator panel board, and also to the paddle boards. So the _only_
standard interface location is the paddle boards.

I suppose one could put Berg headers on both the indicator panel board and
the paddle boards, and use a standard IDC cable betweenthe two...

Mention of the paddle board interface brings me to the second point: even if
one did produce electrically-compatible indicator panels - where are you
going to use them in a PDP-11 system? Not in the CPUs - those all had their
own front panels. The only PDP-11 devices which used indicator panels which I
know of were:

- the DX11 (I don't think anyone's got one of those)
- the RF11 (ditto - although Guy was discussing emulating one at one point)
- the RP11 (but the indicator panel is built into the controller rack there,
  so if one has an RP11, one already has the indicator panel)
- the RK11-C (and several people who have those already have indicator panels)

I agree, the indicator panels look cool - but where are you going to use one
in a historical PDP-11 system?

Sure, one could use either a electrically-compatible or
non-electrically-compatible indicator panel anywhere you want, plugging into
some non-historical hardware, but.. (The non-electrically-compatible
indicator panel Dave did for the QSIC is initially being used in something
which emulates an RK11 and/or RP11, so there's some rationale for it.)


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