Add Read DeviceId function

Function code 43 in modbus spec, can also be extended for CAN

api.go

@@ -46,4 +46,6 @@ type Client interface {
 	//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)
+
+	ReadDeviceId(deviceid byte, objectid byte) (results []byte, err error)
 }

client.go

@@ -432,6 +432,23 @@ func (mb *client) ReadFIFOQueue(address uint16) (results []byte, err error) {
 	return
 }
 
+func (mb *client) ReadDeviceId(deviceid byte, objectid byte) (results []byte, err error) {
+	request := ProtocolDataUnit{
+	FunctionCode: FuncCodeReadDeviceId,
+	Data:         []byte{MEITypeDeviceId, deviceid, objectid},
+	}
+	response, err := mb.send(&request)
+	if err != nil {
+		return
+	}
+	if len(response.Data) < 7 {
+		err = fmt.Errorf("modbus: response data size '%v' is less than minimum '%v'", len(response.Data), 7)
+		return
+	}
+	results = response.Data[2:]
+	return
+}
+
 // Helpers
 
 // send sends request and checks possible exception in the response.

modbus.go

@@ -26,6 +26,12 @@ const (
 	FuncCodeReadWriteMultipleRegisters = 23
 	FuncCodeMaskWriteRegister          = 22
 	FuncCodeReadFIFOQueue              = 24
+	FuncCodeReadDeviceId               = 43
+)
+
+const (
+	MEITypeCANopen  = 13
+	MEITypeDeviceId = 14
 )
 
 const (