fix: print original function code for exceptions

Fixes #46.

Author: guelfey

modbus.go

@@ -89,7 +89,7 @@ func (e *Error) Error() string {
 	default:
 		name = "unknown"
 	}
-	return fmt.Sprintf("modbus: exception '%v' (%s), function '%v'", e.ExceptionCode, name, e.FunctionCode)
+	return fmt.Sprintf("modbus: exception '%v' (%s), function '%v'", e.ExceptionCode, name, e.FunctionCode&0x7F)
 }
 
 // ProtocolDataUnit (PDU) is independent of underlying communication layers.

test/client.go

@@ -132,7 +132,7 @@ func ClientTestReadFIFOQueue(t *testing.T, client modbus.Client) {
 	results, err := client.ReadFIFOQueue(address)
 	// Server not implemented
 	if err != nil {
-		AssertEquals(t, "modbus: exception '1' (illegal function), function '152'", err.Error())
+		AssertEquals(t, "modbus: exception '1' (illegal function), function '24'", err.Error())
 	} else {
 		AssertEquals(t, 0, len(results))
 	}