Applies To:
  • CitectSCADA

Some PLCs, using the MODBUS and MODNET protocol, can only accept function codes 15 and 16 for writing to coils and registers, rather the function codes 15, 16, 5 and 6 which the MODBUS and MODNET drivers currently use selectively depending on the request size. 

The MODBUS (V2.4.6.0) and MODNET ( V2.4.4.2) drivers have been enhanced to optionally use only function codes 15 and 16 for writes. To use function code 15 rather than 15 and 5 for coil writes set ForceMultiCoilsOnly = 1 in the Citect.ini file. To use Function code 16 rather than 16 and 6 for coil writes set PresetMultiRegistersOnly = 1 in the Citect.ini file.


The following Citect INI file is an example to show how the different devices attached to the same port can be set to use different write function codes. (These two parameters can be set at a Global, Channel or Unit level as outlined in KB article Q2848).

Say PORT1 has two IODevices attached, DEV1A and DEV1B. The required result will be that DEV1B only will use the feature described above.


ForceMultiCoilsOnly = 1

PresetMultiRegistersOnly = 1