Applies To:
  • CitectSCADA 1.00, 1.01, 1.10, 1.11, 1.20, 2.00, 2.01
Summary:
With some configurations of your Units database, you may find that the Citect for Windows I/O Server will not respond to PLC requests from Citect for DOS. This will occur when you have the same unit configured twice in the Units database, and the Online unit is the second one. This is a normal configuration when you have a primary and standby unit as the same unit number is used for the primary and standby unit. For example, if your units database looks like this:
 
Rec No: I/O Server Unit Name Unit No Mode
1 Server1 UnitOne 1 Standby
2 Server1 UnitTwo 2 Standby
3 Server2 UnitOne 1 Primary
4 Server2 UnitTwo 2 Primary

I/O Server2 will not respond to request for units 1 or 2 from Citect for DOS clients. I/O Server 1 will respond to requests for Citect for DOS clients, but if those units go offline or I/O Server 1 is shutdown, I/O Server2 will still not respond. 

Solution:
You can work around this problem by reordering the Units database so that all units on each I/O Server are before other units. If you have 2 I/O Servers, you must use two different databases. Each database will put the units associated with that I/O Server before the other: For example

Use this database order for I/O Server 1

Rec No: I/O Server Unit Name Unit No Mode
1 Server1 UnitOne 1 Standby
2 Server1 UnitTwo 2 Standby
3 Server2 UnitOne 1 Primary
4 Server2 UnitTwo 2 Primary

Use this database order for I/O Server 2

Rec No: I/O Server Unit Name Unit No Mode
1 Server2 UnitOne 1 Primary
2 Server2 UnitTwo 2 Primary
3 Server1 UnitOne 1 Standby
4 Server1 UnitTwo 2 Standby

This problem does not effect any Citect for Windows Clients or Servers.

CIT are researching this problem and will post new information here as it becomes available. 
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