PLC | RS485 can not connect how to do??

RS485 communication error, it is often not error messages,
Hard to check that part of the problem.

Here are a few to trace the order:
1 Confirm the PLC is set up correctly.
2 confirm there RS485 data sent.
3 Confirm whether to receive RS485 data sent to their ownformat (MODBUS / ASCII / RTU) is correct. (Optional)
4 Check the wiring + / -.
5 external devices to connect one.
6 External device station number is set correctly.

If you still can not connect:

If you have the confirmation, can be + / - exchange.
If you could only pick one, you can not connect more than one, you can view the terminal resistance.

Other .... not encountered.