rtu.go

package modbus

import (
	"encoding/binary"
	"fmt"
	"io"
	"time"
)

const (
	rtuMinSize = 4
	rtuMaxSize = 256

	rtuExceptionSize = 5
)

// RTUPackager implements Packager interface.
type RTUPackager struct {
}

// Encode encodes PDU in a RTU frame:
//  Slave Address   : 1 byte
//  Function        : 1 byte
//  Data            : 0 up to 252 bytes
//  CRC             : 2 byte
func (rtu *RTUPackager) Encode(slaveID byte, pdu *ProtocolDataUnit) (adu []byte, err error) {
	length := len(pdu.Data) + 4
	if length > rtuMaxSize {
		err = fmt.Errorf("modbus: length of data '%v' must not be bigger than '%v'", length, rtuMaxSize)
		return
	}
	adu = make([]byte, length)

	adu[0] = slaveID
	adu[1] = pdu.FunctionCode
	copy(adu[2:], pdu.Data)

	// Append crc
	var crc crc
	crc.reset().pushBytes(adu[0 : length-2])
	checksum := crc.value()

	adu[length-1] = byte(checksum >> 8)
	adu[length-2] = byte(checksum)
	return
}

// Verify verifies response length and slave id.
func (rtu *RTUPackager) Verify(aduRequest []byte, aduResponse []byte) (err error) {
	length := len(aduResponse)
	// Minimum size (including address, function and CRC)
	if length < rtuMinSize {
		err = fmt.Errorf("modbus: response length '%v' does not meet minimum '%v'", length, rtuMinSize)
		return
	}
	// Slave address must match
	if aduResponse[0] != aduRequest[0] {
		err = fmt.Errorf("modbus: response slave id '%v' does not match request '%v'", aduResponse[0], aduRequest[0])
		return
	}
	return
}

// Decode extracts PDU from RTU frame and verify CRC.
func (rtu *RTUPackager) Decode(adu []byte) (pdu *ProtocolDataUnit, err error) {
	length := len(adu)
	// Calculate checksum
	var crc crc
	crc.reset().pushBytes(adu[0 : length-2])
	checksum := uint16(adu[length-1])<<8 | uint16(adu[length-2])
	if checksum != crc.value() {
		err = fmt.Errorf("modbus: response crc '%v' does not match expected '%v'", checksum, crc.value())
		return
	}
	// Function code & data
	pdu = &ProtocolDataUnit{}
	pdu.FunctionCode = adu[1]
	pdu.Data = adu[2 : length-2]
	return
}

func (rtu *RTUPackager) transceive(transporter Transporter, logger Logger, aduRequest []byte, timeout time.Duration) (aduResponse []byte, err error) {
	// make sure port is connected
	err = transporter.Connect()
	if err != nil {
		return
	}

	// Set write and read timeout
	if timeout > 0 {
		if err = transporter.SetReadTimeout(timeout); err != nil {
			return
		}
	}

	// Send the request
	log(logger, "modbus: sending % x\n", aduRequest)
	if _, err = transporter.Write(aduRequest); err != nil {
		return
	}
	function := aduRequest[1]
	functionFail := aduRequest[1] & 0x80
	bytesToRead := calculateResponseLength(aduRequest)

	var n int
	var n1 int
	var data [rtuMaxSize]byte
	//We first read the minimum length and then read either the full package
	//or the error package, depending on the error status (byte 2 of the response)
	n, err = io.ReadAtLeast(transporter, data[:], rtuMinSize)
	if err != nil {
		return
	}
	//if the function is correct
	if data[1] == function {
		//we read the rest of the bytes
		if n < bytesToRead {
			if bytesToRead > rtuMinSize && bytesToRead <= rtuMaxSize {
				if bytesToRead > n {
					n1, err = io.ReadFull(transporter, data[n:bytesToRead])
					n += n1
				}
			}
		}
	} else if data[1] == functionFail {
		//for error we need to read 5 bytes
		if n < rtuExceptionSize {
			n1, err = io.ReadFull(transporter, data[n:rtuExceptionSize])
		}
		n += n1
	}

	if err != nil {
		return
	}
	aduResponse = data[:n]
	log(logger, "modbus: received % x\n", aduResponse)
	return
}

func calculateResponseLength(adu []byte) int {
	length := rtuMinSize
	switch adu[1] {
	case FuncCodeReadDiscreteInputs,
		FuncCodeReadCoils:
		count := int(binary.BigEndian.Uint16(adu[4:]))
		length += 1 + count/8
		if count%8 != 0 {
			length++
		}
	case FuncCodeReadInputRegisters,
		FuncCodeReadHoldingRegisters,
		FuncCodeReadWriteMultipleRegisters:
		count := int(binary.BigEndian.Uint16(adu[4:]))
		length += 1 + count*2
	case FuncCodeWriteSingleCoil,
		FuncCodeWriteMultipleCoils,
		FuncCodeWriteSingleRegister,
		FuncCodeWriteMultipleRegisters:
		length += 4
	case FuncCodeMaskWriteRegister:
		length += 6
	case FuncCodeReadFIFOQueue:
		// undetermined
	default:
	}
	return length
}