Comunication Problem with ABB PLC


Thread Starter


I have this problem:

I try to setup a comunication between a modbus master simulator ( and a real ABB PLC slave (this PLC use the comunication modbus interface CI867).

When the master send a request (for example 03 Read Holding Registers) the PLC respondes with an exception: "04 Illegal Response Length". I try with other function codes and it's the same and I try with another simulator (Triangle MicroWorks) and it's the same...

Anyone have suggestions to solve this problem ?

Patrick Lansdorf

Hi there,

Are you sure that the exception, 04, is Illegal Response Length. The modbus spec. says that 04 is: An unrecoverable error occurred while the slave was attempting to perform the requested action.

That Exception is sometimes called "Illegal Response Length" and sometimes "Slave Device Failure". Anyway I'm near to solve the problem, but ABB documentation is very confusing. Now I try to configure the communication interface for modbus and there are some function block like MBConnect, MBRead ext anyone knows this function?
Quoting from the ABB Control Builder M On-Line Help Documentation ......

<b>Usage of ModBus</b>

ModBus can be used for Point-to-Point or Multidrop communication. Communication takes place serially and asynchronously, based on the Master/Slave principle, and in half duplex.

Controllers only support master communication.