Modbus Server supported access
This page describes the data elements and data types supported by the Modbus Server.
Variables
The Modbus Server supports the following Modbus registers as variables:
| Variable | Description |
|---|---|
| Coils | Table region of read only single Bit variables. |
| Discrete Inputs | Table region of readable and writeable single Bit variables. |
| Input Registers | Table region of 16 bit (Word) read only variables. |
| Holding Registers | Table region of 16 bit (Word) read and write variables. |
Data types, data conversion and addressing
The Modbus Server supports the Modbus data types and the runtime data types as follows:
| Modbus data type | Size | Runtime data type |
|---|---|---|
| Bit | 1 Byte | BOOL |
| Word | 2 Bytes | INT2 |
| Unsigned Word | 2 Bytes | UINT2 |
| Double | 4 Bytes (Two Contiguous Words) | INT4 |
| Unsigned Double | 4 Bytes (Two Contiguous Words) | UINT4 |
| Floating Point | 4 Bytes (Two Contiguous Words) | FLOAT4 |
| String | Multiple Contiguous Words of a predetermined length | STRING |
The Modbus data types are supported in the runtime
environment wherever a variable is accessed and the
variable is associated with a Modbus Server. Some examples
are:
- Transport map data type parameters
- Trigger data type parameters
- Trigger action variable data type parameters
- From the Workbench > Devices >
Variables tab Read and Write
functions.
The data representations in the Modbus Server, runtime and the Workbench are supported by the necessary data conversion functions (such as Words to Float4).
The variable addressing supports the assignment of a
variable of one data type into a variable with a different
data type at the correct starting address location. One way
to see this variable addressing support is to use the
Workbench > Devices > Variables tab
Read and Write functions by right
clicking on a data element and reading or writing as a type
different than the data element.
The Variable panel can also display values in hexadecimal
format. This may be a preferable way to view the data in
the Content array in the Message variables.
Limitations
- The Modbus Server does not support the reading or writing of individual bits within a 16 bit data element.
- Reading and writing String data into a contiguous group of word based data elements is not supported.