According to long-term engaged in the items technology work of Comrade told: ① RS485 communications short distance (800m within) of General can using no shield of General double twisted line II interference more heavy Shi can using with shield layer of double twisted line (shield layer empty hanging/or only end grounding–to and “strong electric to” especially “frequency device to” separate) ③ signal to General empty hanging (not wiring), if grounding bad instead cannot normal communications.
Most users are also more than the idea. But there are also some one think: connected devices should be common ground; or, if the receiving end has high common–mode input, it is possible to damage the receiver device.
In most cases, when connecting to the RS-485 link you can simply use twisted pair interface “a“ and “b“ respectively together, and talking about signals vacant—this connection method is working on many occasions, but it should be noted that the following two questions:
(1) common mode interference problems: RS-485 interface using differential methods receive input signals, do not need to detect signals relative to a reference point, the system only need to measure the potential difference between the two input lines can. But people tend to ignore the receiver has some common–mode voltage range, common–mode voltage range of -7~+12V RS-485 receivers, only to meet the above conditions, the entire network to work properly. When communication lines common mode voltage is beyond the scope of this communication will be affected is stable and reliable, even corrupted interface.
(2) radiation: drive output common–mode part of the signal requires a return channel, if no return is a low impedance channel (signal), it returned as a radiation source, the entire bus will be like a giant antenna radiates electromagnetic waves.