hmmm... not mandatory
First: indeed there is a big error on WPC-S schematic, the nibbles D0..D3/D4..D7 are inverted on U24. Of course, this is only a pin number error, as D0 goes to D0's CPU. No bus inverted here, only an error on pins numerotation.
The RD wires are the "Read Bus" for the SECURITY PIC. It as two purposes : read the commands/datas from CPU, and read the rows returns from matrix.
If you have RD5..RD7 grounded, you should first suspect U13 74LS240 that is the matrix return (part U13A for the MSB).
Of course, if the PIC (U22) is unable to read commands from CPU, the outputs can have strange behaviour.
First, fix the inputs problem. Note that:
- Deffective U24 has no effect on RD bus, so it is unlikely that this is the case.
- Deffective U23 has also no effect on RD bus.
Check if you see activity on RD0 (U14 pin 9) ~ using a scope, or by default, a voltmeter (any voltage different from 5V or ground):
If yes, this means that the PIC periodicaly read the inputs from U14. Thus, U22 is not dead. It detect command latched (both U14 pin 11 - for the latch - and U22 pin 1 - commands detection), then it read the latch using RA1 port (OC, U14 pin 1).
Usually, when a SECURITY PIC is dead... it is fully dead ! barelly partially. So, if the command detection works... U22 is alive !
Note that... if you really need to replace U22, I well know some French guys that can provide, a personnalized SECURITY CHIP with the serial number you want (and not 999999 or something like)