Knowledge Base

The  CIMWAY  protocols  Open  Modbus  Master  and  Open  Modbus  Slave  support block  size  lengths  that  may  exceed  the  maximum  number  of  blocks  defined  in  the various equipment manufacturer dependent standards.

The online help list all the maximum values for the different data types: 

The maximum amount of variables per block varies between the different equipments
types  implementing  the  Modbus  protocol  but  hardly  any  type  uses  the  theoretically
maximum block length.

Example:
Here is a table that exemplarily illustrates these differences for communicating with
Schneider Electric PLCs:

Symptom: 

CIMWAY equipment error 0003 – Illegal data value, wrong size.

Solution:
Generally, the block length limitation can be reduced by some specific PLC limitation.
Therefore,  it  is  the  responsibility  of  the  application  engineer  to  make  sure  that  the
PLC  supports  the  specified  block  length.  To  find  out  about  the  maximal  supported
block length it is either advisable to refer to the equipment’s documentation or to start
with small frame size lengths that can be increased by trial and error.