The Serial Card uses the following Modbus Communications Protocol function codes to read and write values to and from the Modbus Device when acting as a Modbus master device. These same Modbus function codes are supported in slave mode to allow the master device to read and write data to and from the Serial Card.
Code |
Meaning |
Description |
|---|---|---|
1 |
Read Coil Status |
Obtains current status (on/off) of a group of logic coils |
2 |
Read Input Status |
Obtains current status (on/off) of a group of discrete inputs |
3 |
Read holding registers |
Obtains current binary value in one or more holding registers |
4 |
Read input registers |
Obtains current binary value in one or more input registers |
5 |
Force Single Coil |
Forces logic coil to a state of ON or OFF |
6 |
Preset Single Register |
Writes a single binary value into a holding register |
8 |
Diagnostics |
In master mode, Subfunction 2 is used to retrieve the Diagnostic Register of a PLC. In slave mode, Subfunction 0 is supported to allow remote masters to check the Communications link. |
15 |
Force Multiple Coils |
Forces a series of consecutive logic coils to defined ON or OFF states |
16 |
Preset Multiple Registers |
Writes specific binary values into a series of consecutive holding registers |
17 |
Report Slave ID |
Obtains the Run (on/off) state of a PLC |