Modbus Data Representation (DSC Module or Real-Time Module)
- Updated2023-02-21
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Modbus Data Representation (DSC Module or Real-Time Module)
This topic introduces how Modbus represents and references data.
Modbus Data Table
A data table is an array or a block of memory that stores data. Modbus bases its data model on data tables, as described in the following table:
| Data Table | Data Type | Access Type | Use Case | Comment |
|---|---|---|---|---|
| Discrete Input | Boolean | Read-Only | Representing sensor inputs and other Boolean values to read. | Both Modbus masters and slaves can read discrete inputs. Only Modbus slaves can write discrete inputs. |
| Coil | Boolean | Read or Write | Representing outputs and internal bits to read and write. | Both Modbus masters and slaves can read and write coils. |
| Input Register | Unsigned 16-Bit Integers | Read-Only | Representing analog input values and other integer values to read. | Both Modbus masters and slaves can read input registers. Only Modbus slaves can write input registers. |
| Holding Register | Unsigned 16-Bit Integers | Read or Write | Representing analog outputs and internal numbers to read and write. | Both Modbus masters and slaves can read and write holding registers. |
Modbus Data Table Address
Modbus references data using data table addresses. Each data table has its own assigned address. In read and write operations, Modbus allows masters and slaves to select up to 65,536 data items for each data table. However, the maximum number of contiguous data items that Modbus masters can select depends on the data table type and the operation to perform. For example, the maximum number of coils a Modbus master can write is 1,968. The maximum number of discrete inputs a Modbus master can read is 2,000.
Modbus data table addresses are integer numbers. The addresses are 0-based, meaning that the starting address for each data table is 0. However, some devices use 1-based addresses. Verify the starting address in each device to ensure that you select the appropriate data items.
