PLC Redundancy
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.