Modbus serial "Point value may not be reliable"

I have created a data source of modbus serial for reading from energy meter. I could read the data during checking the pointer location section. I created the point and enabled. but i am unable to read the data from the watch window. I am getting only “pointer value may not be reliable”

The program works well with USB to RS485 convertor , but it is not working with Serial RS232 to RS485 convertor?