Applies To:
  • CitectSCADA 2.x 3.x 4.x

We have an installed system that comprises of 2 Modbus Plus Networks linked by a BP85 Bridge. On this second Modbus Plus network are Modbus PLC's connected serially via a BM85 Bridge. The problem is that when we power down one of the PLC's serially connected to the BM85 bridge, a bottleneck occurs which causes all communications to the PLC's on the second Modbus Plus network to cease.

What seems to be happening is that when the PLC is powered down, Citect tries to establish communications again and a new channel on the BP85 is opened. These channels do not close and when all eight channels have been opened no further communications is possible from one Modbus Plus network to the other. Upon restoration of power to the offending PLC the BP85 channels close and the communications return to normal. We are not sure where the problem lies whether it is Citect or the Bridges. We are running on Windows NT 3.51 and Citect V4.10.


Upon further investigation of the problem we found that the timeout in Citect was still set to the default setting. i.e

Timeout=2 (Seconds)

Further to this timeout in Citect there is also a timeout setting in the BM85 bridge connected to the offending PLC which was set to 100 seconds. Because of this large timeout on the bridge Citect continued to timeout and retry before any error response from the bridge causing a bottle neck. In addition, because of the complexity involved and the distances in the Citect server and the PLC, a timeout of 2 seconds did not leave much room for error.

The final solution to the problem was to set the timeout in Citect to 6 seconds and the Bridge timeout to a value lower than this of around 4 seconds. Since making these changes no further problems have been experienced but more extensive testing on this site will be done in the near future.