Monitor connectivity status with the PLC

The built-in discrete item, $SYS$Status, can be used to monitor the status of communications with the PLC. This item is set to:

• 0 (zero) when communications with the PLC fails.

• 1 (one) when communications is successful.

  Note: For DDE/SuiteLink clients, $SYS$Status always comes from the leaf level of a Communication Driver hierarchy branch, which is the destination PLC node. For OPC clients, $SYS$Status can be accessed at all hierarchy levels. $SYS$Status at the root level of the whole hierarchy tree is always good, as it represents the quality status of the local computer itself. Hence, for practical application, OPC clients should reference $SYS$Status at any hierarchy levels other than the root.

Enter the following DDE reference formula in the appropriate place in your client:

=DASABTCP|ABPLC!$SYS$Status

where:

DASABTCP is the name of the Communication Driver application.

ABPLC is the exact device group defined in the Communication Driver for the PLC.

$SYS$Status is the discrete item used to monitor the status of connectivity with the PLC.

Enter the following OPC item reference syntax when adding the item in your OPC client:

YourOPCAccessPath.$SYS$Status

where:

YourOPCAccessPath is the assembly of hierarchy node names leading to a specific controller device.

$SYS$Status is the discrete item used to monitor the status of connectivity with the controller device.

Note: In the case of a PLC disconnect, the Communication Driver will retry three times before entering into slow poll mode. In the case of reply time-out, the Communication Driver will go into slow poll mode immediately.