Applies To:
  • CitectSCADA 3.40 4.20

In Citect version 4.10 / 3.30 and earlier the I/O Server would cache I/O device data even if the protocol driver stated that the data should not be cached. This problem has been fixed in versions 4.20 and 3.40 and later. This change may cause a degrading of performance for some protocols drivers which have stated that the data should not be cached. 

Caching is disabled by only a very few protocol drivers so it is unlikely that this will cause a problem with a standard PLC protocol drivers. If you have upgraded to these versions and you have noticed a degrading of your I/O device protocol performance, check if the cache is still enabled. You can do this by displaying the Page Unit in the Citect Kernel and checking if Cached: has YES or NO value. If the cache is disabled on Page Unit when and it is enabled the your Citect Project then the protocol has specified that the data should not be cached. If this is causing a performance problem then contact the developer of the I/O device protocol or Citect support.

The DISK PLC driver does specify that the I/O device data will not be cached, so you may see a very slight degrading of performance when you use these versions. If this is causing a major problem you can disable the default delay in the DISK PLC driver by setting the following CITECT.INI option:


Warning: Setting the delay=0 for the DISK PLC driver will cause it to reply immediately and may cause overloading of the CPU and network. This could occur if you have cicode which has a lot of ReRead() or have adjusted the [Page]ScanTime=0. Most project will not do this and adjusting the delay=0 will be OK. If you do set the Delay=0 you should check that Citect CPU, or Network loading has not greatly increased.