How far apart are they?
No need for ASCII. If they are within 50 feet (15m) then the serial port ( null-modem cable) with DF1 protocol using a message instruction will work just fine.
If they are more then 15m apart then configure the SLC5/04's serial ports for DH485 and connect to the SLC5/03's DH485 port, some extra hardware may be required AIC+,AIC and again use a message instruction.