RS485 Wiring Method

RS-485 uses balanced transmission and differential reception to achieve communication: the transmitter converts the serial port TTL level signal into differential signal a, b two outputs, and after cable transmission the differential signal is reduced to TTL level signal at the receiver.

RS-485 bus network topology generally adopts the terminal matching bus type structure. That is, a bus is used to connect each node in series, and ring or star network is not supported. If the communication distance is relatively short or the connected slave devices are relatively few, then the termination resistor can be left out, but as the distance increases and the number of slave devices increases, the signal is easily reflected at the end of the bus, resulting in unstable communication. In order to eliminate this reflection, a terminal resistor is added at the end of the bus, and the empirical value of the terminal resistor is generally 120Ω.

When the field bus is not working when the device is normal, the following investigations can be done.

1. Whether the A and B lines of one of the devices are reversed.

2. Whether the slave has pull-up and pull-down resistors on A and B

3. Whether there is a terminal resistor only at the terminal, and there should not be a terminal resistor at the intermediate device.

4. Whether the number of slave devices exceeds the maximum

5. Twisted pair cable must be used for long distance transmission.