Applies To:
  • CitectSCADA 1.00 1.01 1.10 1.11 1.20 2.00 2.01 3.00

Question: For some reason the PLCs are going on and off line and I am getting very slow response time on my screens. It may take up to 1 minute for the data to be displayed on the screen. I have also noticed that if I shutdown the second I/O server the response time will come back to 1 second. What is the cause of this problem. 

This is caused when you have configured two primary Units with the same Unit Number. Each unit will fight to be the primary causing each Citect Client to swap back and forth between the two units. This will have the effect of very slow communications for the clients. You most likely have used the same unit number on each of your I/O Servers. That is why the response time is OK when you shutdown one of the Servers.

You must reconfigure the Units so they all have unique Unit Numbers and restart all the Clients and I/O Servers.

This error is now checked in Citect version 3.0. If you have configured two primary units with the same unit number on startup you get one of the following error message:

IO Device <name> configured twice as primary.
Delete second IO Device or change to STANDBY.


IO Device <name> and IO Device <name>
Have the same IO Device Number and are both primary.
Use unique IO Device Numbers.