The Series 90 TCP/IP Triplex Communications enabler can be configured for PLC groups of up to 3 PLCs. The PLCs are ranked in order of importance Primary (PLCA)ÞSecondary (PLCB)ÞThird (PLCC).
The enabler reads and writes data to the highest-ranking PLC. Since the CIMPLICITY data points for this PLC group are only defined once, it is a requirement that the PLCs use the same memory references.
To determine the status of each of the PLCs in the PLC group, the enabler uses:
The network connection status (up/down),
The PLC status (running/stopped) and
Two mode bits per PLC.
The two mode bits designated Health and Master are controlled by the PLCs and have the following meanings:
Health, the PLC is healthy - passes internal checks and is operating correctly.
Master, the PLC has performed all initialization actions and data can be read from this device.
The enabler uses the highest ranked running PLC with a network path that has both the Health and Master bits set.
The write and read behavior can be modified by configuration options as follows:
PLC writes can be modified by a configuration radio button to cause writes to be performed to all PLCs
PLC reads can be modified by a configuration check box to ignore the Master bit in which case data will be read from the highest ranking running healthy PLC with a network connection.
Series 90 TCP/IP redundancy overview.