I have broken out the /RESET and SHIFTLOCK circuits as a separate small PCB, in case anyone cares to add the functionality to a Superboard, or an older 542 keyboard.
There are other ways to implement these circuits, including the original OSI RESET circuit, or using a flip flop or using 555 timers, but this circuit has some advantages:
- Both functions are implemented using only 2 chips, and relatively few discrete parts--about the same number of discretes as the 542C reset circuit.
- Timing of RESET delay, RESET duration, Power-on RESET duration, and switch debounce time are easy to set separately, without any complicated interactions.
- The soft-shiftlock can be configured for ON or OFF at power up.
- LED to indicate the Shiftlock status for both soft- and hard- shiftlock
- Can switch between hard- and soft-shiftlock functions with a single jumper setting.
[Edited 5/25/20 to replace schematic with fixed version which includes additional diode D5 to prevent discharge of C5 into the 5V rail; and also change jumper to allow selecting default ON or OFF for the soft Shift Lock.]