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The Modbus registers of a device are organized around the four basic data reference types noted above and this data type is further identified by the leading number of the reference address as follows:
Modbus Addresses
Modbus devices have 4 types of addresses:
- Coil
- Discrete Input
- Input Register
- Holding Register
Coils are 1-bit (boolean) read/write devices. Each coil can be in either on or off state. Discrete inputs are similar to coils, but they are read-only - it’s impossible to set the value of a discrete input. You can think of Coils as outputs of a PLC and Discrete Inputs as input of a PLC.
Holding registers are like PLC memory. They are 16-bit words which you can both read and write via Modbus protocol. Input Registers are also 16-bit words, but they are read-only, like readings of a sensor.
A Modbus address is a 16-bit unsigned integer that is transmitted with every request to indicate which data should be read or written. Address occupies two characters in the Modbus message and the most significant byte is sent first (big-endian).
Supported function codes
| Functions code | Functions | Spesifics | |
|---|---|---|---|
| FC01 | Read Coils | This function code allows the master to query the state of slave’s coils. | A read coils request is always 8 bytes long. A read coils response is at least 6 bytes long. |
| FC02 | Read Discrete Inputs | This function code allows the master to query the state of slave’s discrete inputs. | A read discrete inputs request is always 8 bytes long. A read discrete inputs response is at least 6 bytes long. |
| FC03 | Read Holding Registers | This function code allows the master to query the state of slave’s holding registers. | A read holding registers request is always 8 bytes long. A read holding registers response is at least 7 bytes long. |
| FC04 | Read Input Registers | With this function code, master queries the state of slave’s input registers. | A read input registers request is always 8 bytes long. A read holding registers response is at least 7 bytes long. |
| FC05 | Write Single Coil | Sets the value of a single slave’s coil. | A write single coil request is always 8 bytes long. response? |
| FC15 | Write Multiple Coils | Sets the value of a single slave’s holding register. | |
| FC06 | Write Single Register | Sets the value of a consecutive range of slave’s coils. | |
| FC16 | Write Multiple Registers | Sets the value of a consecutive range of slave’s holding registers. | |
| FC23 | Read/Write Multiple Registers | It combines reading and writing functions, allowing the master to read and write values of multiple holding registers in a single request |
Start configuration with a client/master Modbus TCP Client/Master
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