自定义读取接口&自定义写入接口

Reviewed-on: #4

client.go

@@ -793,7 +793,7 @@ func (mc *ModbusClient) WriteRegisterWithRes(addr uint16, value uint16) (bytes [
 	req.payload = append(req.payload, uint16ToBytes(mc.endianness, value)...)
 
 	// run the request across the transport and wait for a response
-	res, err = mc.executeRequest(req)
+	res, err = mc.executeRequestWithRes(req)
 	if err != nil {
 		return
 	}
@@ -1194,12 +1194,6 @@ func (mc *ModbusClient) readRegistersWithFunctionCode(addr uint16, quantity uint
 		return
 	}
 
-	if functionCode == 0 {
-		err = ErrUnexpectedParameters
-		mc.logger.Errorf("unexpected register type (%v)", functionCode)
-		return
-	}
-
 	req.functionCode = functionCode
 
 	if quantity == 0 {
@@ -1208,9 +1202,10 @@ func (mc *ModbusClient) readRegistersWithFunctionCode(addr uint16, quantity uint
 		return
 	}
 
-	if quantity > 1024 {
+	// 16 * 16 * 40
+	if quantity > 10240 {
 		err = ErrUnexpectedParameters
-		mc.logger.Error("quantity of registers exceeds 1024")
+		mc.logger.Error("quantity of registers exceeds 10240")
 		return
 	}
 
@@ -1232,21 +1227,53 @@ func (mc *ModbusClient) readRegistersWithFunctionCode(addr uint16, quantity uint
 	}
 
 	// validate the response code
+	// switch {
+	// case res.functionCode == req.functionCode:
+	// 	// make sure the payload length is what we expect
+	// 	// (1 byte of length + 2 bytes per register)
+	// 	if len(res.payload) != 1+2*int(quantity) {
+	// 		err = ErrProtocolError
+	// 		return
+	// 	}
+
+	// 	// validate the byte count field
+	// 	// (2 bytes per register * number of registers)
+	// 	if uint(res.payload[0]) != 2*uint(quantity) {
+	// 		err = ErrProtocolError
+	// 		return
+	// 	}
+
+	// 	// remove the byte count field from the returned slice
+	// 	bytes = res.payload[1:]
+
+	// case res.functionCode == (req.functionCode | 0x80):
+	// 	if len(res.payload) != 1 {
+	// 		err = ErrProtocolError
+	// 		return
+	// 	}
+
+	// 	err = mapExceptionCodeToError(res.payload[0])
+
+	// default:
+	// 	err = ErrProtocolError
+	// 	mc.logger.Warningf("unexpected response code (%v)", res.functionCode)
+	// }
+
 	switch {
 	case res.functionCode == req.functionCode:
 		// make sure the payload length is what we expect
 		// (1 byte of length + 2 bytes per register)
-		if len(res.payload) != 1+2*int(quantity) {
+		// if len(res.payload) != 1+2*int(quantity) {
-			err = ErrProtocolError
+		// 	err = ErrProtocolError
-			return
+		// 	return
-		}
+		// }
 
 		// validate the byte count field
 		// (2 bytes per register * number of registers)
-		if uint(res.payload[0]) != 2*uint(quantity) {
+		// if uint(res.payload[0]) != 2*uint(quantity) {
-			err = ErrProtocolError
+		// 	err = ErrProtocolError
-			return
+		// 	return
-		}
+		// }
 
 		// remove the byte count field from the returned slice
 		bytes = res.payload[1:]
@@ -1374,3 +1401,29 @@ func (mc *ModbusClient) executeRequest(req *pdu) (res *pdu, err error) {
 
 	return
 }
+
+func (mc *ModbusClient) executeRequestWithRes(req *pdu) (res *pdu, err error) {
+	// send the request over the wire, wait for and decode the response
+	res, err = mc.transport.ExecuteRequestWithRes(req)
+	if err != nil {
+		// map i/o timeouts to ErrRequestTimedOut
+		if os.IsTimeout(err) {
+			err = ErrRequestTimedOut
+		}
+		return
+	}
+
+	// make sure the source unit id matches that of the request
+	if (res.functionCode&0x80) == 0x00 && res.unitId != req.unitId {
+		err = ErrBadUnitId
+		return
+	}
+	// accept errors from gateway devices (using special unit id #255)
+	if (res.functionCode&0x80) == 0x80 &&
+		(res.unitId != req.unitId && res.unitId != 0xff) {
+		err = ErrBadUnitId
+		return
+	}
+
+	return
+}

modbus.go

@@ -41,7 +41,7 @@ const (
 	fcWriteFileRecord uint8 = 0x15
 
 	// customize
-	fcCustomize uint8 = 0x29
+	fcCustomize uint8 = 0x41
 
 	// exception codes
 	exIllegalFunction         uint8 = 0x01

rtu_transport.go

@@ -8,7 +8,7 @@ import (
 )
 
 const (
-	maxRTUFrameLength	int = 256
+	maxRTUFrameLength int = 256
 )
 
 type rtuTransport struct {
@@ -21,10 +21,10 @@ type rtuTransport struct {
 }
 
 type rtuLink interface {
-	Close()		(error)
+	Close() error
-	Read([]byte)	(int, error)
+	Read([]byte) (int, error)
-	Write([]byte)	(int, error)
+	Write([]byte) (int, error)
-	SetDeadline(time.Time)	(error)
+	SetDeadline(time.Time) error
 }
 
 // Returns a new RTU transport.
@@ -58,11 +58,11 @@ func (rt *rtuTransport) Close() (err error) {
 // Runs a request across the rtu link and returns a response.
 func (rt *rtuTransport) ExecuteRequest(req *pdu) (res *pdu, err error) {
 	var ts time.Time
-	var t  time.Duration
+	var t time.Duration
-	var n  int
+	var n int
 
 	// set an i/o deadline on the link
-	err	= rt.link.SetDeadline(time.Now().Add(rt.timeout))
+	err = rt.link.SetDeadline(time.Now().Add(rt.timeout))
 	if err != nil {
 		return
 	}
@@ -78,7 +78,7 @@ func (rt *rtuTransport) ExecuteRequest(req *pdu) (res *pdu, err error) {
 
 	// build an RTU ADU out of the request object and
 	// send the final ADU+CRC on the wire
-	n, err	= rt.link.Write(rt.assembleRTUFrame(req))
+	n, err = rt.link.Write(rt.assembleRTUFrame(req))
 	if err != nil {
 		return
 	}
@@ -109,10 +109,63 @@ func (rt *rtuTransport) ExecuteRequest(req *pdu) (res *pdu, err error) {
 	return
 }
 
+func (rt *rtuTransport) ExecuteRequestWithRes(req *pdu) (res *pdu, err error) {
+	var ts time.Time
+	var t time.Duration
+	var n int
+
+	// set an i/o deadline on the link
+	err = rt.link.SetDeadline(time.Now().Add(rt.timeout))
+	if err != nil {
+		return
+	}
+
+	// if the line was active less than 3.5 char times ago,
+	// let t3.5 expire before transmitting
+	t = time.Since(rt.lastActivity.Add(rt.t35))
+	if t < 0 {
+		time.Sleep(t * (-1))
+	}
+
+	ts = time.Now()
+
+	// build an RTU ADU out of the request object and
+	// send the final ADU+CRC on the wire
+	n, err = rt.link.Write(rt.assembleRTUFrame(req))
+	if err != nil {
+		return
+	}
+
+	// estimate how long the serial line was busy for.
+	// note that on most platforms, Write() will be buffered and return
+	// immediately rather than block until the buffer is drained
+	rt.lastActivity = ts.Add(time.Duration(n) * rt.t1)
+
+	// observe inter-frame delays
+	time.Sleep(rt.lastActivity.Add(rt.t35).Sub(time.Now()))
+
+	// read the response back from the wire
+	res, err = rt.readRTUFrameWithRes()
+
+	if err == ErrBadCRC || err == ErrProtocolError || err == ErrShortFrame {
+		// wait for and flush any data coming off the link to allow
+		// devices to re-sync
+		time.Sleep(time.Duration(maxRTUFrameLength) * rt.t1)
+		discard(rt.link)
+	}
+
+	// mark the time if we heard anything back
+	if err != ErrRequestTimedOut {
+		rt.lastActivity = time.Now()
+	}
+
+	return
+}
+
 // Reads a request from the rtu link.
 func (rt *rtuTransport) ReadRequest() (req *pdu, err error) {
 	// reading requests from RTU links is currently unsupported
-	err	= fmt.Errorf("unimplemented")
+	err = fmt.Errorf("unimplemented")
 
 	return
 }
@@ -123,7 +176,7 @@ func (rt *rtuTransport) WriteResponse(res *pdu) (err error) {
 
 	// build an RTU ADU out of the request object and
 	// send the final ADU+CRC on the wire
-	n, err	= rt.link.Write(rt.assembleRTUFrame(res))
+	n, err = rt.link.Write(rt.assembleRTUFrame(res))
 	if err != nil {
 		return
 	}
@@ -135,16 +188,16 @@ func (rt *rtuTransport) WriteResponse(res *pdu) (err error) {
 
 // Waits for, reads and decodes a frame from the rtu link.
 func (rt *rtuTransport) readRTUFrame() (res *pdu, err error) {
-	var rxbuf	[]byte
+	var rxbuf []byte
-	var byteCount	int
+	var byteCount int
-	var bytesNeeded	int
+	var bytesNeeded int
-	var crc		crc
+	var crc crc
 
-	rxbuf		= make([]byte, maxRTUFrameLength)
+	rxbuf = make([]byte, maxRTUFrameLength)
 
 	// read the serial ADU header: unit id (1 byte), function code (1 byte) and
 	// PDU length/exception code (1 byte)
-	byteCount, err	= io.ReadFull(rt.link, rxbuf[0:3])
+	byteCount, err = io.ReadFull(rt.link, rxbuf[0:3])
 	if (byteCount > 0 || err == nil) && byteCount != 3 {
 		err = ErrShortFrame
 		return
@@ -160,15 +213,15 @@ func (rt *rtuTransport) readRTUFrame() (res *pdu, err error) {
 	}
 
 	// we need to read 2 additional bytes of CRC after the payload
-	bytesNeeded	+= 2
+	bytesNeeded += 2
 
 	// never read more than the max allowed frame length
-	if byteCount + bytesNeeded > maxRTUFrameLength {
+	if byteCount+bytesNeeded > maxRTUFrameLength {
-		err	= ErrProtocolError
+		err = ErrProtocolError
 		return
 	}
 
-	byteCount, err	= io.ReadFull(rt.link, rxbuf[3:3 + bytesNeeded])
+	byteCount, err = io.ReadFull(rt.link, rxbuf[3:3+bytesNeeded])
 	if err != nil && err != io.ErrUnexpectedEOF {
 		return
 	}
@@ -180,19 +233,88 @@ func (rt *rtuTransport) readRTUFrame() (res *pdu, err error) {
 
 	// compute the CRC on the entire frame, excluding the CRC
 	crc.init()
-	crc.add(rxbuf[0:3 + bytesNeeded - 2])
+	crc.add(rxbuf[0 : 3+bytesNeeded-2])
 
 	// compare CRC values
-	if !crc.isEqual(rxbuf[3 + bytesNeeded - 2], rxbuf[3 + bytesNeeded - 1]) {
+	if !crc.isEqual(rxbuf[3+bytesNeeded-2], rxbuf[3+bytesNeeded-1]) {
 		err = ErrBadCRC
 		return
 	}
 
-	res	= &pdu{
+	res = &pdu{
-		unitId:		rxbuf[0],
+		unitId:       rxbuf[0],
-		functionCode:	rxbuf[1],
+		functionCode: rxbuf[1],
 		// pass the byte count + trailing data as payload, withtout the CRC
-		payload:	rxbuf[2:3 + bytesNeeded  - 2],
+		payload: rxbuf[2 : 3+bytesNeeded-2],
+	}
+
+	return
+}
+
+func (rt *rtuTransport) readRTUFrameWithRes() (res *pdu, err error) {
+	var rxbuf []byte
+	var byteCount int
+	var bytesNeeded int
+	var crc crc
+
+	rxbuf = make([]byte, 4096)
+
+	// read the serial ADU header: unit id (1 byte), function code (1 byte) and
+	// PDU length/exception code (1 byte)
+	byteCount, err = io.ReadFull(rt.link, rxbuf[0:3])
+	if (byteCount > 0 || err == nil) && byteCount != 3 {
+		err = ErrShortFrame
+		return
+	}
+	if err != nil && err != io.ErrUnexpectedEOF {
+		return
+	}
+
+	// figure out how many further bytes to read
+	bytesNeeded, err = expectedResponseLenth(uint8(rxbuf[1]), uint8(rxbuf[2]))
+	if err != nil {
+		return
+	}
+
+	// we need to read 2 additional bytes of CRC after the payload
+	bytesNeeded += 2
+
+	// never read more than the max allowed frame length
+	if byteCount+bytesNeeded > maxRTUFrameLength {
+		err = ErrProtocolError
+		return
+	}
+
+	byteCount, err = io.ReadFull(rt.link, rxbuf[3:3+bytesNeeded])
+	if err != nil && err != io.ErrUnexpectedEOF {
+		return
+	}
+	if byteCount != bytesNeeded {
+		rt.logger.Warningf("expected %v bytes, received %v", bytesNeeded, byteCount)
+		err = ErrShortFrame
+		return
+	}
+
+	// compute the CRC on the entire frame, excluding the CRC
+	crc.init()
+	crc.add(rxbuf[0 : 3+bytesNeeded-2])
+
+	// compare CRC values
+	if !crc.isEqual(rxbuf[3+bytesNeeded-2], rxbuf[3+bytesNeeded-1]) {
+		err = ErrBadCRC
+		return
+	}
+
+	_, err = io.ReadFull(rt.link, rxbuf[3+bytesNeeded:])
+	if err != nil && err != io.ErrUnexpectedEOF {
+		return
+	}
+
+	res = &pdu{
+		unitId:       rxbuf[0],
+		functionCode: rxbuf[1],
+		// pass the byte count + trailing data as payload, withtout the CRC
+		payload: rxbuf[3+bytesNeeded:],
 	}
 
 	return
@@ -200,18 +322,18 @@ func (rt *rtuTransport) readRTUFrame() (res *pdu, err error) {
 
 // Turns a PDU object into bytes.
 func (rt *rtuTransport) assembleRTUFrame(p *pdu) (adu []byte) {
-	var crc		crc
+	var crc crc
 
-	adu	= append(adu, p.unitId)
+	adu = append(adu, p.unitId)
-	adu	= append(adu, p.functionCode)
+	adu = append(adu, p.functionCode)
-	adu	= append(adu, p.payload...)
+	adu = append(adu, p.payload...)
 
 	// run the ADU through the CRC generator
 	crc.init()
 	crc.add(adu)
 
 	// append the CRC to the ADU
-	adu	= append(adu, crc.value()...)
+	adu = append(adu, crc.value()...)
 
 	return
 }
@@ -220,24 +342,31 @@ func (rt *rtuTransport) assembleRTUFrame(p *pdu) (adu []byte) {
 func expectedResponseLenth(responseCode uint8, responseLength uint8) (byteCount int, err error) {
 	switch responseCode {
 	case fcReadHoldingRegisters,
-	     fcReadInputRegisters,
+		fcReadInputRegisters,
-	     fcReadCoils,
+		fcReadCoils,
-	     fcReadDiscreteInputs:            byteCount = int(responseLength)
+		fcReadDiscreteInputs,
+		0x41:
+		byteCount = int(responseLength)
 	case fcWriteSingleRegister,
-	     fcWriteMultipleRegisters,
+		fcWriteMultipleRegisters,
-	     fcWriteSingleCoil,
+		fcWriteSingleCoil,
-	     fcWriteMultipleCoils:            byteCount = 3
+		fcWriteMultipleCoils:
-	case fcMaskWriteRegister:             byteCount = 5
+		byteCount = 3
+	case fcMaskWriteRegister:
+		byteCount = 5
 	case fcReadHoldingRegisters | 0x80,
-	     fcReadInputRegisters | 0x80,
+		fcReadInputRegisters | 0x80,
-	     fcReadCoils | 0x80,
+		fcReadCoils | 0x80,
-	     fcReadDiscreteInputs | 0x80,
+		fcReadDiscreteInputs | 0x80,
-	     fcWriteSingleRegister | 0x80,
+		fcWriteSingleRegister | 0x80,
-	     fcWriteMultipleRegisters | 0x80,
+		fcWriteMultipleRegisters | 0x80,
-	     fcWriteSingleCoil | 0x80,
+		fcWriteSingleCoil | 0x80,
-	     fcWriteMultipleCoils | 0x80,
+		fcWriteMultipleCoils | 0x80,
-	     fcMaskWriteRegister | 0x80:      byteCount = 0
+		fcMaskWriteRegister | 0x80,
-	default: err = ErrProtocolError
+		0x41 | 0x80:
+		byteCount = 0
+	default:
+		err = ErrProtocolError
 	}
 
 	return

tcp_transport.go

@@ -9,23 +9,23 @@ import (
 )
 
 const (
-	maxTCPFrameLength	int = 260
+	maxTCPFrameLength int = 260
-	mbapHeaderLength	int = 7
+	mbapHeaderLength  int = 7
 )
 
 type tcpTransport struct {
-	logger		*logger
+	logger    *logger
-	socket		net.Conn
+	socket    net.Conn
-	timeout		time.Duration
+	timeout   time.Duration
-	lastTxnId	uint16
+	lastTxnId uint16
 }
 
 // Returns a new TCP transport.
 func newTCPTransport(socket net.Conn, timeout time.Duration, customLogger *log.Logger) (tt *tcpTransport) {
 	tt = &tcpTransport{
-		socket:		socket,
+		socket:  socket,
-		timeout:	timeout,
+		timeout: timeout,
-		logger:		newLogger(fmt.Sprintf("tcp-transport(%s)", socket.RemoteAddr()), customLogger),
+		logger:  newLogger(fmt.Sprintf("tcp-transport(%s)", socket.RemoteAddr()), customLogger),
 	}
 
 	return
@@ -33,7 +33,7 @@ func newTCPTransport(socket net.Conn, timeout time.Duration, customLogger *log.L
 
 // Closes the underlying tcp socket.
 func (tt *tcpTransport) Close() (err error) {
-	err  = tt.socket.Close()
+	err = tt.socket.Close()
 
 	return
 }
@@ -41,7 +41,7 @@ func (tt *tcpTransport) Close() (err error) {
 // Runs a request across the socket and returns a response.
 func (tt *tcpTransport) ExecuteRequest(req *pdu) (res *pdu, err error) {
 	// set an i/o deadline on the socket (read and write)
-	err	= tt.socket.SetDeadline(time.Now().Add(tt.timeout))
+	err = tt.socket.SetDeadline(time.Now().Add(tt.timeout))
 	if err != nil {
 		return
 	}
@@ -49,7 +49,7 @@ func (tt *tcpTransport) ExecuteRequest(req *pdu) (res *pdu, err error) {
 	// increase the transaction ID counter
 	tt.lastTxnId++
 
-	_, err	= tt.socket.Write(tt.assembleMBAPFrame(tt.lastTxnId, req))
+	_, err = tt.socket.Write(tt.assembleMBAPFrame(tt.lastTxnId, req))
 	if err != nil {
 		return
 	}
@@ -59,30 +59,34 @@ func (tt *tcpTransport) ExecuteRequest(req *pdu) (res *pdu, err error) {
 	return
 }
 
+func (tt *tcpTransport) ExecuteRequestWithRes(req *pdu) (res *pdu, err error) {
+	return nil, nil
+}
+
 // Reads a request from the socket.
 func (tt *tcpTransport) ReadRequest() (req *pdu, err error) {
-	var txnId	uint16
+	var txnId uint16
 
 	// set an i/o deadline on the socket (read and write)
-	err	= tt.socket.SetDeadline(time.Now().Add(tt.timeout))
+	err = tt.socket.SetDeadline(time.Now().Add(tt.timeout))
 	if err != nil {
 		return
 	}
 
-	req, txnId, err	= tt.readMBAPFrame()
+	req, txnId, err = tt.readMBAPFrame()
 	if err != nil {
 		return
 	}
 
 	// store the incoming transaction id
-	tt.lastTxnId	= txnId
+	tt.lastTxnId = txnId
 
 	return
 }
 
 // Writes a response to the socket.
 func (tt *tcpTransport) WriteResponse(res *pdu) (err error) {
-	_, err	= tt.socket.Write(tt.assembleMBAPFrame(tt.lastTxnId, res))
+	_, err = tt.socket.Write(tt.assembleMBAPFrame(tt.lastTxnId, res))
 	if err != nil {
 		return
 	}
@@ -93,11 +97,11 @@ func (tt *tcpTransport) WriteResponse(res *pdu) (err error) {
 // Reads as many MBAP+modbus frames as necessary until either the response
 // matching tt.lastTxnId is received or an error occurs.
 func (tt *tcpTransport) readResponse() (res *pdu, err error) {
-	var txnId	uint16
+	var txnId uint16
 
 	for {
 		// grab a frame
-		res, txnId, err	= tt.readMBAPFrame()
+		res, txnId, err = tt.readMBAPFrame()
 
 		// ignore unknown protocol identifiers
 		if err == ErrUnknownProtocolId {
@@ -111,9 +115,9 @@ func (tt *tcpTransport) readResponse() (res *pdu, err error) {
 
 		// ignore unknown transaction identifiers
 		if tt.lastTxnId != txnId {
-			tt.logger.Warningf("received unexpected transaction id " +
+			tt.logger.Warningf("received unexpected transaction id "+
-					   "(expected 0x%04x, received 0x%04x)",
+				"(expected 0x%04x, received 0x%04x)",
-					   tt.lastTxnId, txnId)
+				tt.lastTxnId, txnId)
 			continue
 		}
 
@@ -125,33 +129,33 @@ func (tt *tcpTransport) readResponse() (res *pdu, err error) {
 
 // Reads an entire frame (MBAP header + modbus PDU) from the socket.
 func (tt *tcpTransport) readMBAPFrame() (p *pdu, txnId uint16, err error) {
-	var rxbuf	[]byte
+	var rxbuf []byte
-	var bytesNeeded	int
+	var bytesNeeded int
-	var protocolId	uint16
+	var protocolId uint16
-	var unitId	uint8
+	var unitId uint8
 
 	// read the MBAP header
-	rxbuf		= make([]byte, mbapHeaderLength)
+	rxbuf = make([]byte, mbapHeaderLength)
-	_, err		= io.ReadFull(tt.socket, rxbuf)
+	_, err = io.ReadFull(tt.socket, rxbuf)
 	if err != nil {
 		return
 	}
 
 	// decode the transaction identifier
-	txnId		= bytesToUint16(BIG_ENDIAN, rxbuf[0:2])
+	txnId = bytesToUint16(BIG_ENDIAN, rxbuf[0:2])
 	// decode the protocol identifier
-	protocolId	= bytesToUint16(BIG_ENDIAN, rxbuf[2:4])
+	protocolId = bytesToUint16(BIG_ENDIAN, rxbuf[2:4])
 	// store the source unit id
-	unitId		= rxbuf[6]
+	unitId = rxbuf[6]
 
 	// determine how many more bytes we need to read
-	bytesNeeded	= int(bytesToUint16(BIG_ENDIAN, rxbuf[4:6]))
+	bytesNeeded = int(bytesToUint16(BIG_ENDIAN, rxbuf[4:6]))
 
 	// the byte count includes the unit ID field, which we already have
 	bytesNeeded--
 
 	// never read more than the max allowed frame length
-	if bytesNeeded + mbapHeaderLength > maxTCPFrameLength {
+	if bytesNeeded+mbapHeaderLength > maxTCPFrameLength {
 		err = ErrProtocolError
 		return
 	}
@@ -163,8 +167,8 @@ func (tt *tcpTransport) readMBAPFrame() (p *pdu, txnId uint16, err error) {
 	}
 
 	// read the PDU
-	rxbuf		= make([]byte, bytesNeeded)
+	rxbuf = make([]byte, bytesNeeded)
-	_, err		= io.ReadFull(tt.socket, rxbuf)
+	_, err = io.ReadFull(tt.socket, rxbuf)
 	if err != nil {
 		return
 	}
@@ -178,9 +182,9 @@ func (tt *tcpTransport) readMBAPFrame() (p *pdu, txnId uint16, err error) {
 
 	// store unit id, function code and payload in the PDU object
 	p = &pdu{
-		unitId:		unitId,
+		unitId:       unitId,
-		functionCode:	rxbuf[0],
+		functionCode: rxbuf[0],
-		payload:	rxbuf[1:],
+		payload:      rxbuf[1:],
 	}
 
 	return
@@ -189,17 +193,17 @@ func (tt *tcpTransport) readMBAPFrame() (p *pdu, txnId uint16, err error) {
 // Turns a PDU into an MBAP frame (MBAP header + PDU) and returns it as bytes.
 func (tt *tcpTransport) assembleMBAPFrame(txnId uint16, p *pdu) (payload []byte) {
 	// transaction identifier
-	payload	= uint16ToBytes(BIG_ENDIAN, txnId)
+	payload = uint16ToBytes(BIG_ENDIAN, txnId)
 	// protocol identifier (always 0x0000)
-	payload	= append(payload, 0x00, 0x00)
+	payload = append(payload, 0x00, 0x00)
 	// length (covers unit identifier + function code + payload fields)
-	payload = append(payload, uint16ToBytes(BIG_ENDIAN, uint16(2 + len(p.payload)))...)
+	payload = append(payload, uint16ToBytes(BIG_ENDIAN, uint16(2+len(p.payload)))...)
 	// unit identifier
-	payload	= append(payload, p.unitId)
+	payload = append(payload, p.unitId)
 	// function code
-	payload	= append(payload, p.functionCode)
+	payload = append(payload, p.functionCode)
 	// payload
-	payload	= append(payload, p.payload...)
+	payload = append(payload, p.payload...)
 
 	return
 }

transport.go

@@ -1,18 +1,20 @@
 package modbus
 
 type transportType uint
+
 const (
-	modbusRTU        transportType   = 1
+	modbusRTU        transportType = 1
-	modbusRTUOverTCP transportType   = 2
+	modbusRTUOverTCP transportType = 2
-	modbusRTUOverUDP transportType   = 3
+	modbusRTUOverUDP transportType = 3
-	modbusTCP        transportType   = 4
+	modbusTCP        transportType = 4
-	modbusTCPOverTLS transportType   = 5
+	modbusTCPOverTLS transportType = 5
-	modbusTCPOverUDP transportType   = 6
+	modbusTCPOverUDP transportType = 6
 )
 
 type transport interface {
-	Close()              (error)
+	Close() error
 	ExecuteRequest(*pdu) (*pdu, error)
-	ReadRequest()        (*pdu, error)
+	ExecuteRequestWithRes(*pdu) (*pdu, error)
-	WriteResponse(*pdu)  (error)
+	ReadRequest() (*pdu, error)
+	WriteResponse(*pdu) error
 }