Applies To:
  • CitectSCADA 5.xx

The Softhub server used by the SQDNET driver does not support multiple outstanding requests to a single device. Therefore, this has been prevented by the Citect driver by ensuring that the MaxPending driver parameter is 1 (one) by default for a port, and multiple logical Citect devices with the same physical address are on the same port.

This can lead to some performance problems in the situation where multiple devices are placed on the same logical port. Firstly data throughput is reduced because the one outstanding request allowed for the port must be shared between several devices. Secondly, if one of the devices times out, then the data requests to the other devices are delayed considerably because of the combination of the timeout handling and the restriction of MaxPending being set to 1 (one). 

Multiple devices can be used with the SQDNET driver, without the two problems listed if, when the Citect project is being configured, there is one device allocated per port.