Applies To:
  • CitectSCADA
Summary:
Some Citect protocols support digitals within word types. When a disk or memory PLC is chosen instead of the protocol driver, it is found the behaviour of disk and memory PLC differs in regard to support of digitals within word types.

For instance, the Modbus driver supports the two sample addresses: integer at 30001 and digital at 30001.7. If these two addresses are used with a disk PLC, it will be seen that changes to the bit within a word tag (30001.7) do update the associated word tag (30001). If these two addresses are used with a memory PLC, it will be seen that changes to the bit within a word do not update the associated word. Why is this so?

 

Solution:
The DISKDRV driver will store in the same area, tags having the same address and UnitType value. The MEMORY driver will store in the same area, tags having the same address, UnitType, and BitWidth value. UnitType and Bitwidth are fields in the specification dbf for a protocol driver. In the previous example 30001 and 30001.7 have the same word address (30001), and UnitType, but not BitWidth. This explains the difference in behaviour of a disk PLC compared to a memory PLC. 
Keywords:
 

Attachments