30 September 2021
The Standby IO Device reports as Standby in the IO Server Kernel even with the primary IO Server shutdown.
Citect SCADA All Versions
The usage of "STANDBY" for Unit Status, when the standby IODevice is online but not active, can be misleading.
This field is essentially only indicating if the device is online or not.
An additional field "Unit State" was added in v7.00 for pub/sub, which indicates if the device is active or not. This provides the information that is missing from "Unit Status". The two fields together provide the entire story.
Given that an IO Device can have n level redundancy with a Primary and multiple Standbys (on one or more IO Servers), the Unit State can be one of the following values:
> "Inactive": The unit is either offline or online with a lower priority than the redundant local active unit
> "Local active": The unit is the highest priority redundant online unit on that IO Server
> "Global active": The unit is the highest priority redundant online unit across all IO Servers
There is also an edge case state of "Force active" which occurs if a Local Active unit is forced into operation when receiving subscription requests. However, typically these subscription requests are sent to the Global Active redundant unit.