Q & A about 485 network "wiring" (1)

What kinds of communication lines should the 1.485 bus adopt?

The internationally accepted shielded twisted pair wires must be used. The recommended shielded twisted pair model is RVSP2 * 0.5 dual-core shielded twisted pair wires, each of which consists of 16 2mm wires ). The use of shielded twisted pair wires helps reduce and eliminate the distribution capacitance generated between two 485 communication lines and common mode interference from around the communication lines.
Most engineers are used to using 5 or more 5 network cables as 485 communication lines. This is wrong. This is because:

(1) The common network cable does not have a shielded layer and cannot prevent common mode interference.
2) The network cable is only 2mm square meters, and the wire diameter is too small, which will reduce the transmission distance and the number of mounted devices.
3) The network line is a single-strand copper wire, which is easier to break than the multi-core wire.
2. Why grounding?
485 the transceiver works properly only when the specified common mode voltage is-7 V to + 12 V. If the communication exceeds this range, the communication interface is damaged. Common Mode Interference increases the above common mode voltage. One of the effective methods to eliminate common-mode interference is to use the shielded layer of the 485 communication line as the ground wire, connect equipment in the machine, computer and other network devices together, and access the earth reliably from a point of view.
4.485 how should the communication line be laid out?
The communication line should be kept away from high-voltage wires as far as possible. Do not bind it together with the power cord, or even bundle it together.
5. Why does the 485 bus adopt a hand-in-hand structure instead of a star structure?
The star structure generates a reflection signal, which affects 485 of communication. The length of the branch line from the bus to each terminal device should be as short as possible, generally not more than 5 meters. If the branch line is not connected to a terminal, there will be a reflection signal, which will cause strong interference to communications, it should be removed.

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Can there be contacts between devices on the. 485 bus?
In the same network system, use the same cable to minimize the contact points in the line. Make sure that the joints are well welded and tightly wrapped to avoid looseness and oxidation. Ensure a single, continuous signal channel as the bus.
7. What are common mode interference and differential mode interference? How can we eliminate interference on communication lines?
The 485 communication line is composed of two twisted wires. It transmits signals through the voltage difference between the two communication lines. Therefore, it is called differential voltage transmission.
Differential Mode Interference transmits between two signal lines, which is symmetric interference. The method for eliminating the differential mode interference is to add a bias resistance in the circuit and use twisted pair wires;
Common Mode Interference is transmitted between the signal line and the ground and is a non-symmetrical interference. Common Mode Interference Elimination methods include:
1) Use shielded twisted pair wires for effective grounding
2) galvanized pipe shielding should be considered for areas with strong electric fields.
3) Stay away from high-voltage cables during cabling, and do not bundle the high-voltage power cords and signal cables together for cabling.
4) do not share the same power supply with the electric control lock
5) linear regulated power supply or high-quality switching power supply (ripple interference less than 50mV)
8. Under what circumstances should the terminal Resistance be increased on the 485 bus?
Generally, you do not need to increase the terminal resistance. Only when the distance between 485 and 100 is over 485, you need to increase the terminal resistance at the start and end ends.