Go modbus stack

Description

This package is a go implementation of the modbus protocol. It aims to provide a simple-to-use, high-level API to interact with modbus devices using native Go types.

Both client and server components are available. The client can be used over both TCP and RTU (serial), and supports an RTU over TCP mode to allow the use of remote serial ports or cheap TCP to serial bridges.

The server is TCP-only for now.

A CLI client is available in cmd/modbus-cli.go and can be built with

$ go build -o modbus-cli cmd/modbus-cli.go
$ ./modbus-cli --help

Getting started

$ go get github.com/simonvetter/modbus

Using the client

import (
    "github.com/simonvetter/modbus"
)

func main() {
    var client  *modbus.ModbusClient
    var err      error

    // for a TCP endpoint
    client, err = modbus.NewClient(&modbus.ClientConfiguration{
        URL:      "tcp://hostname-or-ip-address:502",
        Timeout:  1 * time.Second,
    })

    // for an RTU (serial) device/bus
    client, err = modbus.NewClient(&modbus.ClientConfiguration{
        URL:      "rtu:///dev/ttyUSB0",
        Speed:    9600,                    // default
        DataBits: 8,                       // default, optional
        Parity:   modbus.PARITY_NONE,      // default, optional
        StopBits: 2,                       // default if no parity, optional
        Timeout:  300 * time.Millisecond,
    })

    // for an RTU over TCP device/bus (remote serial port or
    // simple TCP-to-serial bridge)
    client, err = modbus.NewClient(&modbus.ClientConfiguration{
        URL:      "rtuovertcp://hostname-or-ip-address:502",
        Timeout:  1 * time.Second,
    })

    // read a single 16-bit holding register at address 100
    var reg16   uint16
    reg16, err  = client.ReadRegister(100, modbus.HOLDING_REGISTER)
    if err != nil {
      // error out
    } else {
      // use value
      fmt.Printf("value: %v", reg16)        // as unsigned integer
      fmt.Printf("value: %v", int16(reg16)) // as signed integer
    }

    // read 4 consecutive 16-bit input registers starting at address 100
    var reg16s  []uint16
    reg16s, err = client.ReadRegisters(100, 4, modbus.INPUT_REGISTER)

    // read the same 4 consecutive 16-bit input registers as 2 32-bit registers
    var reg32s  []uint32
    reg32s, err = client.ReadUint32s(100, 2, modbus.INPUT_REGISTER)

    // by default, 16-bit registers are decoded as big-endian and 32-bit registers
    // as big-endian with the high word first.
    // change the byte/word ordering of subsequent requests to little endian, with
    // the low word first (note that the second argument only affects 32-bit registers)
    client.SetEncoding(modbus.LITTLE_ENDIAN, modbus.LOW_WORD_FIRST)

    // read the same 4 consecutive 16-bit input registers as 2 32-bit floats
    var fl32s   []float32
    fl32s, err  = client.ReadFloat32s(100, 2, modbus.INPUT_REGISTER)

    // write -200 to 16-bit (holding) register 100, as a signed integer
    var s int16 = -200
    err         = client.WriteRegister(100, uint16(s))

    // Switch to unit ID (a.k.a. slave ID) #4
    client.SetUnitId(4)

    // write 3 floats to registers 100 to 105
    err         = client.WriteFloat32s(100, []float32{
	3.14,
        1.1,
	-783.22,
    })

    // close the TCP connection/serial port
    client.Close()
}

Using the server component

See examples/tcp_server.go for an example.

Supported function codes, golang object types and endianness/word ordering

Function codes:

• Read coils (0x01)
• Read discrete inputs (0x02)
• Read holding registers (0x03)
• Read input registers (0x04)
• Write single coil (0x05)
• Write single register (0x06)
• Write multiple coils (0x0f)
• Write multiple registers (0x10)

Go object types:

• Booleans (coils and discrete inputs)
• Signed/Unisgned 16-bit integers (input and holding registers)
• Signed/Unsigned 32-bit integers (input and holding registers)
• 32-bit floating point numbers (input and holding registers)

Byte encoding/endianness/word ordering:

• Little and Big endian for 16-bit integers
• Little and Big endian, with and without word swap for 32-bit integers and 32-bit floating point numbers.

TODO (in no particular order)

• Add RTU (serial) support to the server
• Add more tests
• Add diagnostics register support
• Add fifo register support
• Add file register support

Dependencies

• github.com/goburrow/serial for access to the serial port (thanks!)

License

MIT.