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.