The 74f parts may be too fast. Try adding damping resistors (10 to 30 ohms) in series with the outputs to slow things down or just switch to 74ls
Joe > On Sep 8, 2015, at 7:07 PM, "Charles" <charlesmorris...@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 >
