Modbus address arrays
Modbus defines access functions for different data areas. These data areas are declared as variables in a TwinCAT PLC program, e.g. as word arrays, and transferred to the Modbus slave function block as input parameters. Each area has a different Modbus start address, so that the areas can be distinguished unambiguously. This offset has to be taken account of for addressing.
Inputs
The Inputs data area usually describes the physical input data with read-only access. They can be digital inputs (bit) or analog inputs (word). The PLC programmer can decide whether or not to grant the communication partner direct access to the physical inputs. It is also possible to define an input area for Modbus communication that is not identical with the physical inputs:
Definition of the Modbus input data as direct image of the physical inputs. Start and size of the data area can be specified freely. They are limited by the actual size of the input process image of the controller used.
VAR
Inputs AT%IW0 : ARRAY[0..255] OF WORD;
END_VAR
Definition of the Modbus input data as a separate Modbus data area independent of the physical inputs
VAR
Inputs : ARRAY[0..255] OF WORD;
END_VAR
The maximum size of the Input data area is 2048 words (ARRAY[0..2047] OF WORD).
Access to the Input area via a Modbus master is possible with the following Modbus functions:
2 : Read Input Status
4 : Read Input Registers
Addressing
The Input area is addressed with a 0 offset, i.e. address 0 as transferred in the telegram addresses the first element in the Input data area.
Examples:
PLC variable | Access type | Address in the | Address in the end device |
|---|---|---|---|
Inputs[0] | Word | 16#0 | 30001 |
Inputs[1] | Word | 16#1 | 30002 |
Inputs[0], Bit 0 | Bit | 16#0 | 10001 |
Inputs[1], Bit 14 | Bit | 16#1E | 1001F |
Outputs
The Outputs data area usually describes the physical output data with read and write access. Outputs can be digital outputs (coils) or analog outputs (output registers). Like for the Inputs, the area can be declared as a physical output variable or as a simple variable.
Definition of the Modbus output data as direct image of the physical outputs. Start and size of the data area can be specified freely. They are limited by the actual size of the output process image of the controller used.
VAR
Outputs AT%QW0 : ARRAY[0..255] OF WORD;
END_VAR
Definition of the Modbus output data as a separate Modbus data area independent of the physical outputs
VAR
Outputs : ARRAY[0..255] OF WORD;
END_VAR
The maximum size of the Output data area is 14336 words (ARRAY[0..14335] OF WORD).
Access to the Output area via a Modbus master is possible with the following Modbus functions:
1 : Read Coil Status
3 : Read Holding Registers
5 : Force Single Coil
6 : Preset Single Register
15 : Force Multiple Coils
16 : Preset Multiple Registers
Addressing
The Output area is addressed with a 16#800 offset, i.e. address 16#800 as transferred in the telegram addresses the first element in the Output data area.
Examples:
PLC variable | Access type | Address in the | Address in the end device |
|---|---|---|---|
Outputs[0] | Word | 16#800 | 40801 |
Outputs[1] | Word | 16#801 | 40802 |
Outputs[0], Bit 0 | Bit | 16#800 | 00801 |
Outputs[1], Bit 14 | Bit | 16#81E | 0081F |
Memory
The Memory data area describes a PLC variable area without physical I/O assignment.
Definition of the Modbus memory data as PLC flags. Start and size of the data area can be specified freely.
VAR
Memory AT%MW0 : ARRAY[0..255] OF WORD;
END_VAR
Definition of the Modbus memory data as variable without flag address
VAR
Memory : ARRAY[0..255] OF WORD;
END_VAR
The maximum size of the Memory data area is 16384 words (ARRAY[0..16383] OF WORD).
Access to the Memory area via a Modbus master is possible with the following Modbus functions:
3 : Read Holding Registers
6 : Preset Single Register
16 : Preset Multiple Registers
Addressing
The Memory area is addressed with a 16#4000 offset, i.e. address 16#4000 as transferred in the telegram addresses the first word in the Memory data area.
Examples:
PLC variable | Access type | Address in the | Address in the end device |
|---|---|---|---|
Memory[0] | Word | 16#4000 | 44001 |
Memory[1] | Word | 16#4001 | 44002 |