The Electronic Industries Association (EIA) formulated and released the RS 485 standard in 1983, and it was revised by the Telecommunications Industry Association (TIA) and named TIA/EIA-485-A.
Since the proposed standards proposed by EIA are all prefixed with “RS”, in the communications industry, it is still customary to refer to the above standards with the prefix RS.
RS-485_Working principle
Using differential transmission, also called balanced transmission:
Using a pair of twisted pairs, the – line is defined as A and the other – line is defined as B;
The level between lines A and B is 2~6V, which is logic “0”;
The level between lines A and B is between -2~- 6V, which is logic “1”:
Differential transmission method can effectively improve anti-interference ability
No interference: (DT) =(D)-(D-)
There is interference: (DT) =[ (D ) Noise]-[ (D-) Noise]
=(D )-(D-)
RS-485 connection method
The network topology generally adopts a terminal-matched bus structure and does not support ring or star networks.
A, B, C are wrong connection methods
D, E and F are the correct connection methods
Notice:
Different cables are used for different sections of the bus;
The lead wires of each branch are too long:
No terminal resistor (120 ohms)
RS-485 terminating resistor
During the communication process, there are two situations that will cause signal reflection: impedance discontinuity and impedance mismatch
Solution:
At the end of the cable, connect a terminal resistor with the same size as the cable’s characteristic impedance to make the cable’s impedance continuous; the characteristic impedance of ordinary twisted pairs is 120 ohms, so the terminal resistor is generally selected to be 120 ohms.
When the equipment is small and the distance is short, no terminal resistor is needed (less than 300 meters);
RS-485 network communication distance
When using 0.5m2 twisted pair as communication cable, according to the baud rate
The theoretical value of the maximum transmission distance is as follows:
| velocità di trasmissione | Maximum transmission distance |
| 2400BPS | 1800 meters |
| 4800BPS | 1200 meters |
| 9600BPS | 800 meters |
RS-485 network communication distance
The relationship between communication distance and speed:
Communication cables not only have resistance, but also distributed capacitance and distributed inductance; the pulse signal in RS-485 communication can be considered as an AC signal; distributed capacitance and distributed inductance act as impedance on the AC signal;
The influence increases as the frequency increases.
The longer the communication distance, the greater the signal attenuation and waveform deformation, delay, and reflection interference. To ensure the reliability of the transmission signal, the communication baud rate must be reduced.
RS-485 network ground
To eliminate common mode interference:
The common mode voltage range of the 485 transceiver is 7~12V. Only when the above conditions are met, the entire network can work normally. When the common-mode voltage in the network line exceeds this range, it will affect the stability and reliability of communication, and even damage the interface.
Elimination of EMI:
The common mode part of the transmit driver output signal requires a return path. If there is no low-resistance return path (signal ground), it will return to the source in the form of radiation, and the entire bus will be like a huge antenna radiating electromagnetic waves outward. .
RS-485 field application considerations
485 and 485-data lines must be twisted pairs for each other, preferably double-shielded; the 485 bus must have a hand-in-hand bus structure, and star connections and bifurcated connections must be avoided; the AC power supply for the equipment and the chassis must be real Grounded, and well grounded. Good grounding can prevent equipment from being struck by lightning and surges. In order to avoid interference from strong current, the 485 bus should avoid running together with strong current.
Common RS485 products
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