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What is RS-485? How should RS-485 be wired?

by:Rika Sensors     2021-12-16
What is RS-485? How should RS-485 be wired?
RS-485 is a physical interface, in simple terms, it is hardware. RS-485 belongs to wired transmission, so hardware transmission media is needed. In fact, two wires are enough. The two wires transmit the same signal, but the transmitter divides this signal into two. The receiving end will restore it to the original signal.

RS-485 features:

1. The electrical characteristics of RS-485: use differential signal negative logic, logic '1' is expressed by the voltage difference between the two wires as +(2~6)V; logic '0' is represented by the voltage difference between the two lines as -(2~6)V. The interface signal level is lower than RS-232-C, it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, and it is convenient to connect with the TTL circuit.

2. The highest data transmission rate of RS-485 is 10Mbps.

3. The RS-485 interface is a combination of balanced drivers and differential receivers, with enhanced anti-common-mode interference capability, that is, good anti-noise interference.

4. The maximum communication distance of RS-485 is about 1219m, and the maximum transmission rate is 10Mbps. The transmission rate is inversely proportional to the transmission distance. The maximum communication distance can be reached at a transmission rate of 100KbpS. If you need to transmit more For long distances, 485 repeaters are required. The RS-485 bus generally supports a maximum of 32 nodes. If a special 485 chip is used, it can reach 128 or 256 nodes, and the maximum can support up to 400 nodes.

RS-485 bus has been widely used due to its low cost and simple design. It is widely used in various fields such as security monitoring, intelligent transportation, intelligent buildings, computer room monitoring, and industrial automation. The RS-485 bus line is relatively simple to lay, but there are some corresponding matters that must be paid attention to, otherwise it will easily lead to communication failure, and subsequent maintenance work is very extensive. Points to note in the 485 wiring specification compiled by the following editor:

RS-485 wiring attention:

Wire problem: The wire used in the 485 bus wiring must use shielded twisted pair, the wire diameter is the best For the 0.75 or 1.0 wire diameter, many people use the network cable as the 485 wire directly during the construction for the convenience of the figure. The network cable has eight wires, and the 485 wire only needs to use two wires or four wires. The other wires are wasted. , And the wire diameter of the current network cable is relatively thin, and can not fully meet the communication requirements of the 485 bus, it is recommended not to use the network cable. In addition, parallel wires, coaxial cables or twisted pairs without shielding layers cannot be used. Because 485 is a differential balanced transmission, the use of twisted pairs can effectively offset the influence of external interference. 485 lines must not use parallel wires. For the same reason, since the shielding layer has the effect of shielding external interference, it is best to use a twisted pair with a shielding layer.

Wiring problem: The wiring of the 485 bus should be as far away as possible from the interference source. During the construction process, many people route the 485 line and the power line together in order to save trouble. This is unreasonable, and the power line will cause interference. , Causing 485 communication to be unstable, and the 485 wiring must be far away from strong voltage interference sources such as transformers and inverters.

Bus topology problem: Due to the complex on-site environment, the distribution of 485 equipment is generally scattered. For the convenience of the diagram, the construction personnel did not arrange the line in the hand-in-hand daisy chain bus topology according to the 485 specification, but arbitrarily arranged it as a star Type, tree type or even multiple topological structures are mixed, leaving too many branches that are too long. In practical applications, simple star, tree topologies or even mixed topologies can also be used sometimes, but communication It will definitely be unstable. If a star or tree topology must be deployed, it is recommended to use a 485 hub and a 485 repeater. For related applications, please refer to the 485 bus topology problem page.

Grounding problem: In the 485 bus wiring specification, it is emphasized that a single point is required for reliable grounding, but in actual construction, the 485 bus grounding sometimes has a counterproductive effect. Since the 485 wiring needs to be connected by hand in a daisy chain, the shielded twisted pair used will definitely be cut and connected to the 485 device, and most of the external shielding layer will be used as the ground wire, if the shielding layer is not well connected , The grounding may cause the 485 signal to be unstable, so the 485 line grounding must be a single-point reliable grounding.

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