Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 103)

Programming Techniques
Chapter 5
5-22
another station on the Data Highway is
initiating a message
possibly neither of the processors will respond to the
message. You must program the other station to
respond to this condition.
another station is communicating with the
primary processor
the other station will receive no indication that
switchover has occurred. You can, however, program a
message instruction to execute a message upon
switchover (Figure 5.7) or attempt to send commands
to the backup processor. If you can communicate with
the backup processor, you know that switchover has
not occurred.
The programming considerations for PLC-3 backup systems using memory
communication modules include:
ladder programming for a PLC-3 backup system using memory
communication modules
disallowed instructions
Ladder Programming
When developing programs for a PLC-3 backup system equipped with
memory communication modules, certain instructions cannot be executed
in the backup processor. To program your PLC-3 backup system, you can
program around the disallowed instructions.
CAUTION: To guard against damage to equipment, do not
attempt to execute any of the instructions listed below. If you
do, the backup processor may shut down due to a fault (bad
address).
The following instructions are disallowed in a backup processor.
shift register - BSL, BSR
file arithmetic - ADF, SBF, MLF, DVF, NGF, SQF
file comparison - SEQ, SNE, SLS, SLE, SGR, SGE
file logical - ANF, ORF, XOF, NTF
file transfer - MVF, MMF
first-in-first-out (FIFO) register - FFL, FFU
diagnostic - DDT, FBC
communications - MSG, BTR, BTW
In a PLC-3 backup system, the backup processor cannot execute some of
the PLC-3 instructions. When programming a PLC-3 backup system, we
suggest you insert the rung shown in Figure 5.8 into the ladder programs
Programming Considerations
for Memory Communication
Modules