1. Forget to get the plc power on, PLC indicator is power off(POWER). Then just get it power on again.
2. The baud rate setting is wrong,and the default communication format of Haiwell PLC is :19200,8 N 2.
3. PC port selection is error (especially laptops),then re-select the correct port.
4. There is a communication program which takes up the port in the PLC program,then dial the RUN (RUN out) lights to stop-state, finally make it online again..
5. Communication line is not solid, plug it tightly again.
6. PLC download port is broken (usually the 485 port goes wrong)
7. There's a faulty connection of the communication line, especially the 485 wire is reversed.
8. The download line is broken,change a new one then try it again.
9. There is something wrong with the computer serial port,change a new one.