PDP-8m Console Switch Problems

Agreed! [... snip more ...] I can't be of much help, because shorting a TTL output to force a transition in the inside logic of the D flipflop to flip the other output, is not my cup of tea. However, perhaps this will work. It keeps the 175, but you might need some changes in the traces/wriring, Don. +5V +5V +5V | | | | | | |~| |~| |~| | | | | | | |_| |_| |_| 1/6*7404 | | | 1/4*74S175 |\ | | | +-----------+ | \ to | | +---| Dn Qn |------| >O-----9318 | | | | | / priority | +-----+----|>Clk | |/ encoder | | | | +----------------o| MR~ Qn* |o--- | | +-----------+ | | __O ADDR | O-/ LOAD | | O------------+ | (active position) GND The switch is drawn in its rest position. The flipflop is held in reset thru MR* at GND. The D input is already high. When you push the switch, MR* goes high at a moment later CLK goes low. When you release the switch Clk goes high again, and the flip flop loads the D input to the output. A moment after that MR* goes low again and resets the flip flop. This is theory ... test in practice :-) success, Henk PA8PDP This message and attachment(s) are intended solely for the use of the addressee and may contain information that is privileged, confidential or otherwise exempt from disclosure under applicable law. If you are not the intended recipient or agent thereof responsible for delivering this message to the intended recipient, you are hereby notified that any dissemination, distribution, or copying of this communication is strictly prohibited. If you have received this communication in error, please notify the sender immediately by telephone and with a "reply" message. Thank you for your cooperation.