Applies To:
  • CitectSCADA 4.xx, 5.xx

When using the TITCPIP driver, Citect cannot re-connect to the I/O Device after a 'unit offline error'.  This happens only with TCP (not UDP). 

In the TITCPIP driver re-connection is controlled by a timeout.  After a number of unsuccessful retries, the TITCPIP driver will return a unit 'offline error' to Citect, which triggers Citect to re-initialise this unit after 30 seconds.  However, re-initialising a unit will not cause the TITCPIP driver to abort the previous connection and establish a new one.  If using UDP, then every thing is fine because UDP is connection-less, and re-establishing a connection is not necessary.  In the case of TCP, which is connection-oriented, it will not be able to restore the normal connection after the communication is broken - due to power down of the PLC, physical cable broken etc.

The following changes address this problem:

  1. The error code has been changed from unit offline to channel offline, the driver will attempt to re-establish a new connection 30 seconds after the problem occurs. there will be no side affects since only one unit is allowed in one channel (Citect port).
  2. A new Citect.INI parameter has been added to separate the number of retries from number of timeouts the driver can have before it declare a channel if offline.

Retry=value where value >= 0, default is 3 (standard parameter)
where value >= 0, default is 3 (special parameter)

Released in TITCPIP driver version 2.34 or above