After reset typically ones have to be written into the input and output enable registers (IEN & OEN).

No command exist to set the result register RR to a known state.

The OEN and IEN commands can just write the Data signal to the IEN and OEN registers. In most MC14500 designs the result register RR is externally to the MC14500 fed to an input and can therefore be read and used in commands. This means both operands of a logical operation can be the result register RR. To set the result register RR to 1 and then set OEN and IEN to 1 do:


RR is the IO address where the RR signal is fed to an MC14500 external input.

ANDC RR would set RR to zero.

A minimalistic init.asm file looks as follows:

RR     EQU 000  Pin RR is wired to input 0
IN1    EQU 001
IN2    EQU 002
OUT0   EQU 000
ORC    RR       Set RR to 1   
IEN    RR       Enable inputs 
OEN    RR       Enable outputs

Linurs Hosttech startpage