From: Charles: Tuesday, September 08, 2015 4:07 PM
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!
When we did the 32K Omnibus memory boards, we initially went
with ABT parts, but discovered they were way too fast, and creating
phenomenal ground bounce. Switching to LS parts reduced the
problem to where the boards are useable. Maybe an LS244 will
do the same here?
Vince
