Problems with 8/A Programmer's Panel

[Paul Anderson]

I should have a few around here somewhere. I'll try to look this weekend.

Paul

On Tue, Sep 8, 2015 at 6:07 PM, Charles <charlesmorris800 at centurytel.net>
wrote:

> Several years ago, Vince Slyngstad and I "cloned" the rare DKC8-AA
> Programmer's Panel for PDP-8 with some improvements (0.6 vs. 0.3" LED
> displays, real "click" buttons instead of that membrane keypad, and fixed a
> couple of bugs in the original design). Also, since 8235's were scarce even
> then, we used 74F244 buffers instead to gate the input vs. output of the
> switch register onto the bus back to the M8316 Option 1 board.
>
> That may have caused a lurking problem, since occasionally the SR will
> change bits when an address is loaded. Turns out an oscillation starts on
> the ground lines at about 20 MHz with certain entries at the keypad to LSR
> and LA (I can't see an obvious pattern). This parasitic is large enough (up
> to 2V p-p with respect to the chassis/rack) that it's clocking the D-flops
> used as the physical switch register! It can be seen in the 7-segment
> displays as a "flicker" which will change when I touch the ground on the
> board... attempts to add more grounds actually made the problem worse!
>
> I'm thinking of redoing the board layout with better attention to ground
> planes/power buses. There's a .01 uf decoupling cap at every IC and 12 out
> of 40 pins on each of the two connecting ribbon cables are dedicated to
> ground. 7 on one, 5 on the other are for +5 volts.
>
> On the other hand, I am tired of tinkering and considering just buying a
> real DKC8-AA if there's one out there. Anyone? ;)
> -Charles
>
>