Modbus is a serial communication protocol used to exchange information between electronic devices. Typically, only the modbus master in a modbus network is the one initiating communication and slaves respond to these requests. In this case, the modbus-rest acts as the master and communication (writing and reading registers) is initated through the REST API.
npm install -g modbus-rest
/usr/local/bin/modbus-rest --help
/usr/local/bin/modbus-rest --serial=/dev/ttyUSB0
Common path parameters in the API are: unit - the slave id, address - the starting address of a read or write, quantity - the number of registers/coils to read or write.
| URL | Method | Description |
|---|---|---|
| /{unit}/coil/{address} | GET | Reads the state of a coil. |
| /{unit}/coil/{address}/{quantity} | GET | Reads the states of multiple coils. |
| /{unit}/coil/{address} | POST | Writes a coil state. POST body should contain true or false |
| /{unit}/coil/{address}/{quantity} | POST | Writes multiple coil states. POST body should contain a JSON array of true/false |
| /{unit}/discrete/{address} | GET | Reads the state of a discrete input. |
| /{unit}/discrete/{address}/{quantity} | GET | Reads the states of multiple discrete inputs. |
| /{unit}/holding/{address} | GET | Reads the value of a holding register. |
| /{unit}/holding/{address}/{quantity} | GET | Reads the values of multiple holding registers. |
| /{unit}/holding/{address} | POST | Writes a value to a holding register. POST body should contain the value to write. |
| /{unit}/holding/{address}/{quantity} | POST | Writes values to multiple holding registers. POST body should contain the raw buffer (2 bytes per register) to write, base64 encoded. |
| /{unit}/input/{address} | GET | Reads the value of an input register. |
| /{unit}/input/{address}/{quantity} | GET | Reads the values of multiple input registers. |
There are also a number of query parameters available:
length defines the raw register length of a value. This can be useful when a device uses more than one register to store a value.
interpreters defines transformation methods to use when reading registers, see below. Multiple interpreters can be chained by comma separation.
The following interpreters are supported:
raw - Do not default to any interpreter, always return raw response
integer (default for registers) - Returns an integer representation of the register value
tenths - Divide value by 10.
float32 - Interpret the input as a floating value according to IEEE-754.
float32le - Interpret the input as floating value according to IEEE-754 with little endian.