Why does my READ_VAR lock up on a M580 HSBY switchover?

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Issue
Communication function lockup on HSBY

Product Line
Modicon M580

Environment
EcoStruxure Control Expert
Unity Pro

Resolution
If your READ_VAR is not communicating after the M580 goes through a switchover you may not be following the proper procedure for the HSBY system. Follow procedure below.

Asynchronous Communication Procedures

During a Switchover event, asynchronous communication procedures: READ_VAR, WRITE_VAR, DATA_EXCH, INPUT_CHAR, INPUT_BYTE, PRINT_CHAR, do not automatically resume operation on the new Primary PLC without special care.

The following procedure should be used to allow asynchronous communication EFs to automatically resume operation after a Switchover:

Program your application so that all EFs management parameters are not exchanged with Standby PLC. To do this, de-select the Exchange on STBY attribute for the management parameter.
Initialize the Length parameter each time the function is called.
Set the Timeout parameter accordingly to your application:
 If the communication function is send through the CPU, the typical timeout value is 500 ms.
 If the communication function is send through a NOC module, the typical timeout value is 2 s.

NOTE: If for some reason you are unable to follow this procedure, and a Switchover renders your communication function inoperative, write your application program so that it sets the function activity bit to 0 before restarting the function in the new Primary CPU.