PDP-8m Console Switch Problems

[Lyle Bickley]

Hi Don,

On Wednesday 06 September 2006 01:11, Don North wrote:
--snip--

> In ASCII art, here is a synopsis of the circuit:
>
>      +5V                   +5V         1/6*7404
>
>       |       1/4*74S175   |            |\
>
>       /              +-----+-----+      | \      to
>   470 \         GND--| Dn     Qn |------|  >O-----9318
>   ohm /              |           |      | /       priority
>       \         GND--|>Clk   QBn |O--+  |/        encoder
>
>       +-------------o| MR~       |   |
>
>       |              +-----+-----+   |
>
>     __O  ADDR             GND        |
>  O-/     LOAD                        |
>
>  |    O------------------------------+
>
>  +----+
>
>     __O  XTND
>  O-/     LOAD
>
>  |    O---------->QBn...
>
>  +----+
>    ...
>
>     __O
>  O-/     DEPOSIT
>
>  |    O---------->QBn...
>
> GND
>

--snip--

> Anybody have any other ideas on what to look for or at?

The method of tying a switch (or OC pulldown) to the output of a FF in order 
to "forcibly" switch it is not particularly uncommon. (If you think that's 
bad, you ought to see some of the PDP-1 logic ;-)

At any rate, the circuit is described in detail in the PDP 8/E/F/M maintenance 
manual Volume I on pages 3-108 and 3-109 - and IMHO - it's a good write-up.

I, too, had to hack my way though that circuitry to get my PDP-8/F working. I 
had very similar symptoms to what you are experiencing (some control switches 
working, some not) but the indications were quite different.

Here's a couple of suggestions:

1) Since the circuit, as designed, works perfectly for the several PDP-8/E/Fs 
I've worked on (and thousands of others that were in production), I wouldn't 
"redesign" it.

2) I'd check the inverters on the output of the 74175 to see if they are 
pulling the FF the "other way".

3) If the inverters are O.K., I'd start tracing out the circuit to make sure 
there's not a short and/or open between/in the related traces.

Cheers,
Lyle
-- 
Lyle Bickley
Bickley Consulting West Inc.
Mountain View, CA
http://bickleywest.com

"Black holes are where God is dividing by zero"