Applies To:
  • CitectSCADA 1.xx 2.xx 3.xx 4.xx 5.xx

Question: How many Standby Units can I have on my Citect system? 

The redundancy with Citect I/O Servers is very flexible so that you can have a virtually unlimited number of Standby units. The alarms, trends and report servers operate in a primary Standby mode so that you have one primary and one Standby computer. The redundancy on the I/O Server operates at the Unit level and allows you to have up to 255 I/O Servers with a total of 255 units (these limits may be greater in later Citect versions). So you could have 1 Primary Unit path and 10 Standby Unit paths. You normally don't need more than 1 Standby Path to the PLC, however Citect does not restricted you to this.

If the Primary path fails, the next path defined is checked. The order that Standby paths are checked, strictly follows the order in the Units.dbf file in the project. If the first Standby path fails, then the Primary is checked again and if it still fails then the next Standby path is checked and so on down the list of Standby paths.

To configure more that 1 Standby Unit you add extra records to the units database in the same way as the first Standby unit with the same information as the primary unit and with a Startup Mode of STANDBY.

For more information on how redundancy of I/O Server operates see Q1138.


multiple I/O Devices; hot standby