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 | ||||
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Modbus | Modbus | Local Variables | Modbus | Modbus | Local Variables |
3 read | 0x800…807 | %IB128...%IB143 | 3 read | 0x000...007 | %QB128...%QB143 |
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| 4 read | 0x000...007 | %QB128...%QB143 |
6 write | 0x800...807 | %IB128...%IB143 |
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16 write | 0x800...807 | %IB128...%IB143 |
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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.
- Entry of all the byte-oriented Bus Terminals (such as the analog Bus Terminals)
- Entry of all the digital Bus Terminals (rounded up to 16 bits)
- Entry of the PLC variables