Applies To:
  • CitectSCADA 1.00 1.01 1.10 1.11 1.20 2.00 2.01 3.00
Summary:
If the standby device is declared before the primary device in the IO Device form and the primary device never comes on line then the standby won't come on line either. If the primary device is declared before the standby then the standby will come on line if the primary is offline.  

Solution:
To make sure that IO server behaves correctly regardless of the ordering of how the devices are declared in the IO Device form the following two rules must be followed when configuring the Citect.
  1. All IO devices must have unique name even if they are primary/standby devices. For example if you name your primary I/O Device PLC1, you should name the standby PLC1S. This also makes it easier to identify which I/O Device is which, for example when using the IODeviceControl() function. The primary and the standby must have the same Number, this is how Citect knows they are a primary/standby pair.
  2. All references to PLC variables must be via the primary device name. So if you have named your primary I/O Device PLC1 and standby PLC1S, you should associate all variable tags with PLC1, don't associate any with PLC1S.
 
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