First commit

client.go

@@ -0,0 +1,109 @@
+package mbtcpclient
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"net"
+	"time"
+)
+
+type Mbclient struct {
+	transactionCounter uint16
+	conn               net.Conn
+	header             [7]byte
+	unit               uint8
+}
+
+func New(address string, unit uint8) (*Mbclient, error) {
+	var err error
+
+	c := new(Mbclient)
+	c.conn, err = net.Dial("tcp", address)
+	if err != nil {
+		return nil, err
+	}
+	c.unit = unit
+	return c, nil
+}
+
+func (m *Mbclient) ReadRegisters(first uint16, numRegs uint16) ([]uint16, error) {
+	const requestLength = 12
+	m.transactionCounter++
+	req := make([]byte, requestLength)
+	var responseHeader mbapHeader
+	var mbpayload mbPDU
+
+	binary.BigEndian.PutUint16(req[0:2], m.transactionCounter)
+	binary.BigEndian.PutUint16(req[2:4], 0)
+	binary.BigEndian.PutUint16(req[4:6], 6) // Length
+	req[6] = m.unit
+	req[7] = 3 //FunctionCode
+	binary.BigEndian.PutUint16(req[8:10], first-1)
+	binary.BigEndian.PutUint16(req[10:12], numRegs)
+	byteswritten, err := m.conn.Write(req)
+	if err != nil {
+		return nil, err
+	}
+	if byteswritten != requestLength {
+		return nil, fmt.Errorf("Failed to send request")
+	}
+	m.conn.SetDeadline(time.Now().Add(10 * time.Second))
+	_, err = io.ReadFull(m.conn, m.header[:])
+	responseHeader.unMarshal(m.header)
+	expectedDataLength := responseHeader.length - 1
+
+	response := make([]byte, expectedDataLength)
+	_, err = m.conn.Read(response)
+
+	err = mbpayload.unMarshal(response)
+	if mbpayload.functionCode != 3 {
+		return nil, fmt.Errorf("modbus exception %v", mbpayload.functionCode&0x7F)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	return mbpayload.registers, nil
+}
+
+type mbapHeader struct {
+	transactionID      uint16
+	protocolIdentifier uint16
+	length             uint16
+	unitIdentifier     byte
+}
+
+// func (m *mbapHeader) marshalBinary() ([]byte, error) {
+
+// }
+
+func (m *mbapHeader) unMarshal(data [7]byte) error {
+	m.transactionID = binary.BigEndian.Uint16(data[0:2])
+	m.protocolIdentifier = binary.BigEndian.Uint16(data[2:4])
+	m.length = binary.BigEndian.Uint16(data[4:6])
+	m.unitIdentifier = data[6]
+
+	return nil
+}
+
+type mbPDU struct {
+	functionCode uint8
+	length       uint8
+	registers    []uint16
+}
+
+func (d *mbPDU) unMarshal(data []byte) error {
+	d.functionCode = data[0]
+	d.length = data[1]
+	if d.length+2 != uint8(len(data)) {
+		return fmt.Errorf("Lenght mismatch in modbus payload")
+	}
+	d.registers = make([]uint16, d.length/2)
+	var n uint8
+	for n < d.length/2 {
+		d.registers[n] = binary.BigEndian.Uint16(data[n*2+2 : n*2+4])
+		n++
+	}
+	return nil
+}