modbus

README

go modbus

Fault-tolerant, fail-fast implementation of Modbus protocol in Go.

Supported functions

Bit access:

• Read Discrete Inputs
• Read Coils
• Write Single Coil
• Write Multiple Coils

16-bit access:

• Read Input Registers
• Read Holding Registers
• Write Single Register
• Write Multiple Registers
• Read/Write Multiple Registers
• Mask Write Register
• Read FIFO Queue

Supported formats

• TCP
• Serial (RTU, ASCII)

Usage

Basic usage:

// Modbus TCP
client := modbus.TCPClient("localhost:502")
// Read input register 9
results, err := client.ReadInputRegisters(8, 1)

// Modbus RTU/ASCII
// Default configuration is 19200, 8, 1, even
client = modbus.RTUClient("/dev/ttyS0")
results, err = client.ReadCoils(2, 1)

Advanced usage:

// Modbus TCP
handler := modbus.NewTCPClientHandler("localhost:502")
handler.Timeout = 10 * time.Second
handler.SlaveId = 0xFF
handler.Logger = log.New(os.Stdout, "test: ", log.LstdFlags)
// Connect manually so that multiple requests are handled in one connection session
err := handler.Connect()
defer handler.Close()

client := modbus.NewClient(handler)
results, err := client.ReadDiscreteInputs(15, 2)
results, err = client.WriteMultipleRegisters(1, 2, []byte{0, 3, 0, 4})
results, err = client.WriteMultipleCoils(5, 10, []byte{4, 3})

// Modbus RTU/ASCII
handler := modbus.NewRTUClientHandler("/dev/ttyUSB0")
handler.BaudRate = 115200
handler.DataBits = 8
handler.Parity = "N"
handler.StopBits = 1
handler.SlaveId = 1
handler.Timeout = 5 * time.Second

err := handler.Connect()
defer handler.Close()

client := modbus.NewClient(handler)
results, err := client.ReadDiscreteInputs(15, 2)

References

• Modbus Specifications and Implementation Guides

Documentation

Overview

Package modbus provides a client for MODBUS TCP and RTU/ASCII.

Index

• Constants
• type Client
  • func NewClient(handler ClientHandler) Client
  • func NewClient2(packager Packager, transporter Transporter) Client
  • func TCPClient(address string) Client
• type ClientHandler
• type ModbusError
  • func (e *ModbusError) Error() string
• type Packager
• type ProtocolDataUnit
• type TCPClientHandler
  • func NewTCPClientHandler(address string) *TCPClientHandler
  • func TCPHandlerFromConnection(conn net.Conn) *TCPClientHandler
  • func (mb *TCPClientHandler) Close() error
  • func (mb *TCPClientHandler) Connect() error
  • func (mb *TCPClientHandler) Decode(adu []byte) (pdu *ProtocolDataUnit, err error)
  • func (mb *TCPClientHandler) Encode(pdu *ProtocolDataUnit) (adu []byte, err error)
  • func (mb *TCPClientHandler) Send(aduRequest []byte) (aduResponse []byte, err error)
  • func (mb *TCPClientHandler) Verify(aduRequest []byte, aduResponse []byte) (err error)
• type Transporter

Constants

const (
	// Bit access
	FuncCodeReadDiscreteInputs = 2
	FuncCodeReadCoils          = 1
	FuncCodeWriteSingleCoil    = 5
	FuncCodeWriteMultipleCoils = 15

	// 16-bit access
	FuncCodeReadInputRegisters         = 4
	FuncCodeReadHoldingRegisters       = 3
	FuncCodeWriteSingleRegister        = 6
	FuncCodeWriteMultipleRegisters     = 16
	FuncCodeReadWriteMultipleRegisters = 23
	FuncCodeMaskWriteRegister          = 22
	FuncCodeReadFIFOQueue              = 24
)

const (
	ExceptionCodeIllegalFunction                    = 1
	ExceptionCodeIllegalDataAddress                 = 2
	ExceptionCodeIllegalDataValue                   = 3
	ExceptionCodeServerDeviceFailure                = 4
	ExceptionCodeAcknowledge                        = 5
	ExceptionCodeServerDeviceBusy                   = 6
	ExceptionCodeMemoryParityError                  = 8
	ExceptionCodeGatewayPathUnavailable             = 10
	ExceptionCodeGatewayTargetDeviceFailedToRespond = 11
)

Types

type Client

type Client interface {

	// ReadCoils reads from 1 to 2000 contiguous status of coils in a
	// remote device and returns coil status.
	ReadCoils(address, quantity uint16) (results []byte, err error)
	// ReadDiscreteInputs reads from 1 to 2000 contiguous status of
	// discrete inputs in a remote device and returns input status.
	ReadDiscreteInputs(address, quantity uint16) (results []byte, err error)
	// WriteSingleCoil write a single output to either ON or OFF in a
	// remote device and returns output value.
	WriteSingleCoil(address, value uint16) (results []byte, err error)
	// WriteMultipleCoils forces each coil in a sequence of coils to either
	// ON or OFF in a remote device and returns quantity of outputs.
	WriteMultipleCoils(address, quantity uint16, value []byte) (results []byte, err error)

	// ReadInputRegisters reads from 1 to 125 contiguous input registers in
	// a remote device and returns input registers.
	ReadInputRegisters(address, quantity uint16) (results []byte, err error)
	// ReadHoldingRegisters reads the contents of a contiguous block of
	// holding registers in a remote device and returns register value.
	ReadHoldingRegisters(address, quantity uint16) (results []byte, err error)
	// WriteSingleRegister writes a single holding register in a remote
	// device and returns register value.
	WriteSingleRegister(address, value uint16) (results []byte, err error)
	// WriteMultipleRegisters writes a block of contiguous registers
	// (1 to 123 registers) in a remote device and returns quantity of
	// registers.
	WriteMultipleRegisters(address, quantity uint16, value []byte) (results []byte, err error)
	// ReadWriteMultipleRegisters performs a combination of one read
	// operation and one write operation. It returns read registers value.
	ReadWriteMultipleRegisters(readAddress, readQuantity, writeAddress, writeQuantity uint16, value []byte) (results []byte, err error)
	// MaskWriteRegister modify the contents of a specified holding
	// register using a combination of an AND mask, an OR mask, and the
	// register's current contents. The function returns
	// AND-mask and OR-mask.
	MaskWriteRegister(address, andMask, orMask uint16) (results []byte, err error)
	//ReadFIFOQueue reads the contents of a First-In-First-Out (FIFO) queue
	// of register in a remote device and returns FIFO value register.
	ReadFIFOQueue(address uint16) (results []byte, err error)
}

func NewClient

func NewClient(handler ClientHandler) Client

NewClient creates a new modbus client with given backend handler.

func NewClient2

func NewClient2(packager Packager, transporter Transporter) Client

NewClient2 creates a new modbus client with given backend packager and transporter.

func TCPClient

func TCPClient(address string) Client

TCPClient creates TCP client with default handler and given connect string.

type ClientHandler

type ClientHandler interface {
	Packager
	Transporter
}

ClientHandler is the interface that groups the Packager and Transporter methods.

type ModbusError

type ModbusError struct {
	FunctionCode  byte
	ExceptionCode byte
}

ModbusError implements error interface.

func (*ModbusError) Error

func (e *ModbusError) Error() string

Error converts known modbus exception code to error message.

type Packager

type Packager interface {
	Encode(pdu *ProtocolDataUnit) (adu []byte, err error)
	Decode(adu []byte) (pdu *ProtocolDataUnit, err error)
	Verify(aduRequest []byte, aduResponse []byte) (err error)
}

Packager specifies the communication layer.

type ProtocolDataUnit

type ProtocolDataUnit struct {
	FunctionCode byte
	Data         []byte
}

ProtocolDataUnit (PDU) is independent of underlying communication layers.

type TCPClientHandler

type TCPClientHandler struct {
	// contains filtered or unexported fields
}

TCPClientHandler implements Packager and Transporter interface.

func NewTCPClientHandler

func NewTCPClientHandler(address string) *TCPClientHandler

NewTCPClientHandler allocates a new TCPClientHandler.

func TCPHandlerFromConnection

func TCPHandlerFromConnection(conn net.Conn) *TCPClientHandler

TCPHandlerFromConnection creates a TCP handler from an existing connection.

func (*TCPClientHandler) Close

func (mb *TCPClientHandler) Close() error

Close closes current connection.

func (*TCPClientHandler) Connect

func (mb *TCPClientHandler) Connect() error

Connect establishes a new connection to the address in Address. Connect and Close are exported so that multiple requests can be done with one session

func (*TCPClientHandler) Decode

func (mb *TCPClientHandler) Decode(adu []byte) (pdu *ProtocolDataUnit, err error)

Decode extracts PDU from TCP frame:

Transaction identifier: 2 bytes
Protocol identifier: 2 bytes
Length: 2 bytes
Unit identifier: 1 byte

func (*TCPClientHandler) Encode

func (mb *TCPClientHandler) Encode(pdu *ProtocolDataUnit) (adu []byte, err error)

Encode adds modbus application protocol header:

Transaction identifier: 2 bytes
Protocol identifier: 2 bytes
Length: 2 bytes
Unit identifier: 1 byte
Function code: 1 byte
Data: n bytes

func (*TCPClientHandler) Send

func (mb *TCPClientHandler) Send(aduRequest []byte) (aduResponse []byte, err error)

Send sends data to server and ensures response length is greater than header length.

func (*TCPClientHandler) Verify

func (mb *TCPClientHandler) Verify(aduRequest []byte, aduResponse []byte) (err error)

Verify confirms transaction, protocol and unit id.

type Transporter

type Transporter interface {
	Send(aduRequest []byte) (aduResponse []byte, err error)
}

Transporter specifies the transport layer.