Add support for reading input registers

client.go

@@ -19,9 +19,10 @@ type Mbclient struct {
 	keepAliveDuration  time.Duration
 	timeOut            time.Duration
 	wg                 sync.WaitGroup
+	registerOffset     uint16
 }
 
-func New(Address string, Unit uint8, KeepAlive, TimeOut time.Duration) (*Mbclient, error) {
+func New(Address string, Unit uint8, KeepAlive, TimeOut time.Duration, registerOffset uint16) (*Mbclient, error) {
 
 	c := new(Mbclient)
 	c.address = Address
@@ -30,6 +31,7 @@ func New(Address string, Unit uint8, KeepAlive, TimeOut time.Duration) (*Mbclien
 	<-c.t.C
 	c.keepAliveDuration = KeepAlive
 	c.timeOut = TimeOut
+	c.registerOffset = registerOffset
 	return c, nil
 }
 
@@ -49,7 +51,17 @@ func (m *Mbclient) closeConn() {
 	m.wg.Wait()
 }
 
-func (m *Mbclient) ReadRegisters(first uint16, numRegs uint16) ([]uint16, error) {
+// Reads one or many holding registers (Function code 03)
+func (m *Mbclient) ReadHoldingRegisters(first uint16, numRegs uint16) ([]uint16, error) {
+	return m.readRegisters(3, first, numRegs)
+}
+
+// Reads one or many input registers (Function code 04)
+func (m *Mbclient) ReadInputRegisters(first uint16, numRegs uint16) ([]uint16, error) {
+	return m.readRegisters(4, first, numRegs)
+}
+
+func (m *Mbclient) readRegisters(functionCode uint8, first uint16, numRegs uint16) ([]uint16, error) {
 	var err error
 	// If The timer is expired, conn is closed and needs to be reopened
 	if !m.t.Stop() {
@@ -71,11 +83,11 @@ func (m *Mbclient) ReadRegisters(first uint16, numRegs uint16) ([]uint16, error)
 	var mbpayload mbPDU
 
 	binary.BigEndian.PutUint16(req[0:2], m.transactionCounter)
-	binary.BigEndian.PutUint16(req[2:4], 0)
+	binary.BigEndian.PutUint16(req[2:4], 0) // Protocol identifier
 	binary.BigEndian.PutUint16(req[4:6], 6) // Length
 	req[6] = m.unit
-	req[7] = 3 //FunctionCode
-	binary.BigEndian.PutUint16(req[8:10], first-1)
+	req[7] = functionCode
+	binary.BigEndian.PutUint16(req[8:10], first-m.registerOffset)
 	binary.BigEndian.PutUint16(req[10:12], numRegs)
 	m.conn.SetDeadline(time.Now().Add(5 * time.Second))
 
@@ -114,7 +126,7 @@ func (m *Mbclient) ReadRegisters(first uint16, numRegs uint16) ([]uint16, error)
 	}
 
 	err = mbpayload.unMarshal(response)
-	if mbpayload.functionCode != 3 {
+	if mbpayload.functionCode != functionCode {
 		m.t.Reset(m.keepAliveDuration)
 		return nil, fmt.Errorf("modbus exception %v req: %x", mbpayload.functionCode&0x7F, req)
 	}