diff --git a/api.go b/api.go
index fe06ab0..6c77dd1 100644
--- a/api.go
+++ b/api.go
@@ -46,4 +46,9 @@ 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 reads the device id contents of the remote device.
+	ReadDeviceId(deviceid byte, objectid byte) (results []byte, err error)
+	// UserFunction makes a user defined request of the remote device.
+	UserFunction(usersubfunc byte, data []byte) (results []byte, err error)
 }
diff --git a/client.go b/client.go
index ac3ee2e..dd46730 100644
--- a/client.go
+++ b/client.go
@@ -432,6 +432,36 @@ 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
+}
+
+func (mb *client) UserFunction(usersubfunc byte, data []byte) (results []byte, err error) {
+	request := ProtocolDataUnit{
+	FunctionCode: FuncCodeUser,
+	Data:         append([]byte{usersubfunc}, data...),
+	}
+	response, err := mb.send(&request)
+	if err != nil {
+		return
+	}
+	results = response.Data[4:]
+	return
+}
+
 // Helpers
 
 // send sends request and checks possible exception in the response.
diff --git a/modbus.go b/modbus.go
index 869473c..7e7cf29 100644
--- a/modbus.go
+++ b/modbus.go
@@ -26,6 +26,15 @@ const (
 	FuncCodeReadWriteMultipleRegisters = 23
 	FuncCodeMaskWriteRegister          = 22
 	FuncCodeReadFIFOQueue              = 24
+
+	// byte access
+	FuncCodeReadDeviceId               = 43
+	FuncCodeUser                       = 100
+)
+
+const (
+	MEITypeCANopen  = 13
+	MEITypeDeviceId = 14
 )
 
 const (
diff --git a/rtuclient.go b/rtuclient.go
index ce55bd8..998bd1d 100644
--- a/rtuclient.go
+++ b/rtuclient.go
@@ -202,6 +202,8 @@ func calculateResponseLength(adu []byte) int {
 		length += 6
 	case FuncCodeReadFIFOQueue:
 		// undetermined
+	case FuncCodeReadDeviceId:
+		length += 3
 	default:
 	}
 	return length