Applies To:
  • CitectSCADA 1.00, 1.01, 1.10, 1.11, 2.00, 2.01

The response time of Citect will be dependent on the speed of the slowest PLC. This is because Citect will not run Alarms, Trends, Reports or display animation on the Screen until all the PLC data has been gathered from the PLCs. Because of this, the performance will be effected by the slowest PLC in the system. 

If you have some fast and some slow PLCs, you may tune your system to effectively operate at the speed of the faster PLCs - using this method, the data will be read from the fastest PLCs at their rate and at a slower rate from the slower PLCs.

For example, a display page needs data from two PLCs, PLC 1 takes 500ms to respond and PLC 2 takes 3000ms to respond. Normally, the display page will update every 3000ms. You can improve the response time by setting the cache time on the slower PLC to 2500ms (a bit less that the total time to read the data). This will cause Citect to read the slower PLC data at a period around 3000ms, which is its maximum data rate, however data will be returned (from the cache) that will allow the page to be updated at around the 500ms rate. Note that the data on the page from the slower PLC will only be updated at the 3000ms rate - while the data from the faster PLC will update at 500ms.

Note: If you set the cache time too high you may generate high network loading as Citect will re-request data it has already read. See Q1070