Problems with 8/A Programmer's Panel

Charles charlesmorris800 at
Tue Sep 8 18:07:07 CDT 2015

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? ;)

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