Applies To:
  • CitectSCADA 4.xx, 5.xx

When writes to DIGITALs within the same word address are programmed in Cicode to occur almost simultaneously, using either the ABKT2, ABKT500 or  ABRSLINX500 protocols, some of the writes may not work. 

The driver for the ABKT2, ABKT500 or  ABRSLINX500 protocols writes to DIGITALs using a Read-Modify-Write technique. This means when a DIGITAL tag is written to, first the corresponding word is read in, then it is modified with the DIGITAL tag's current value and the word is written out. This technique can cause the aforementioned problems with the ABKT2, ABKT500 or  ABRSLINX500 protocols when multiple physical requests to a device are possible. The solution is to prevent multiple physical requests by setting the driver's MaxPending parameter to 1 in the Citect.ini file:



Allen-Bradley, SLC500, PLC2, bits of words