ModbusTCP Process Image

The ModbusTCP process image makes a fundamental distinction between digital and byte-oriented (word-oriented) signals. This will be clarified by examples.

Example 1 (default setting)

All the connected Bus Terminals are assigned to the local PLC. The 16 bytes of PLC variables are available for communication with the ModbusTCP master. PLC variables are part of the byte-oriented data packets, and can therefore not be accessed by the digital Modbus functions, even though the variables are digitally mapped in the BCxxxx. Difficulties are most often associated with whether signals are considered as inputs or outputs. PLC variables that are outputs for the BCxxxx are inputs for the Modbus master, and vice versa. The default setting can be changed with the KS2000 configuration software (register description table 1).

Local PLC inputs

Local PLC outputs

Modbus
function

Modbus
address/offset

Local Variables

Modbus
function

Modbus
address/offset

Local Variables

3 read

0x800…807

%IB128...%IB143

3 read

0x000...007

%QB128...%QB143

 

 

 

4 read

0x000...007

%QB128...%QB143

6 write

0x800...807

%IB128...%IB143

 

 

 

16 write

0x800...807

%IB128...%IB143

 

 

 

23 read/write

0x800...807

%IB128...%IB143

23 read/write

0x000..007

%QB128...%QB143

Example 2

In addition, the Bus Terminals can be assigned once to the BC or to the local PLC. This creates additional complications in the process image. The rule governing how the process image is assembled is, however, simple.