1.What kind of communication line should be used for the 485 bus?
Internationally accepted shielded twisted pair must be used. The recommended type of shielded twisted pair is RVSP2*0.5 (two-core shielded twisted pair, each consisting of 16 strands of 0.2mm wire). The use of shielded twisted pairs helps to reduce and eliminate the distributed capacitance generated between the two 485 communication lines and the common mode interference generated around the communication lines.
Most of the engineering contractors are accustomed to using 5 kinds of network lines or over 5 kinds of network lines as 485 communication lines, which is wrong. This is because:
(1) The common network cable has no shielding layer and cannot prevent common mode interference.
(2) The network cable is only 0.2mm square, and the wire diameter is too thin, which will result in a reduction in transmission distance and a reduction in the number of devices that can be attached.
(3)The network cable is a single-strand copper wire that is easily broken compared to a multi-core wire.
2.Why is it grounded?
The 485 transceiver will operate normally when the specified common mode voltage is between -7V and +12V. Exceeding this range will affect communication and severely damage the communication interface. Common mode interference increases the above common mode voltage. One of the effective means to eliminate common mode interference is to use the shielding layer of the 485 communication line as the ground line, connect the devices in the network such as implements and computers, and reliably connect to the earth from one point.
3.How should the 485 communication line be connected?
Keep the communication lines away from high-voltage wires as much as possible, not in parallel with the power cables, or bundle them together.
4. Why should the 485 bus adopt a hand-in-hand structure instead of a star structure?
The star structure produces a reflected signal that affects 485 communications. The length of the branch line from the bus to each terminal device should be as short as possible, generally not exceeding 5 meters. If the branch line is not connected to the terminal, there will be a reflected signal, which will cause strong interference to the communication and should be removed.
5.Can there be contacts between devices on the 485 bus?
Use the same cable in the same network system to minimize contact in the line.Make sure the joint is well welded and tightly packed to avoid loosening and oxidation.A single and continuous signal channel is guaranteed as a bus.
6. What is common mode interference and differential mode interference? How to eliminate interference on the communication line?
The 485 communication line consists of two twisted pairs of wires that transmit signals through the voltage difference between the two communication lines, so it is called differential voltage transmission.
Differential mode interference is transmitted between two signal lines ,which belongs to symmetric interference. The method of eliminating differential mode interference is to add a bias resistor in the circuit and use a twisted pair.
Common mode interference is transmitted between the signal line and ground,which belongs to the asymmetric interference. Methods to eliminate common mode interference include:
(1) Use shielded twisted pair and effectively ground.
(2) The galvanized tube shield should also be considered in the strong electric field.
(3) Keep away from the high voltage line during wiring, and do not bundle the high voltage power line and signal line together.
(4) Do not share the same power supply with the electronic lock.
(5) Adopt linear stabilized voltage supply or high-quality switching power supply (ripple interference is less than 50mV).
7. Under what circumstances should the terminal resistance be added to bus 485?
Under normal circumstances, it is not necessary to add the terminal resistance. Only when the 485 communication distance exceeds 100 meters,the terminal resistance should be added at the beginning and end of 485 communication.
8. How to extend the communication distance of 485?
One of the specifications of the 485 network is 1.2 km in length and the number of 32 nodes.If this limit is exceeded, a 485 repeater or 485 hub must be used to extend the network distance or number of nodes.
With a 485 repeater or 485 hub, a large 485 network can be divided into several network segments. A 485 repeater or 485 hub is like a "bridge" between the 485 network segments.Of course, each network segment still follows the above 485 specification, which is 1.2 kilometers in length and the number of 32 nodes.
The 485 hub is used to construct the star 485 network.The 485 hub is a broad extension of the 485 repeater concept.It not only solves the problem of multi-forking, but also solves the problem of isolation between network segments, that is, there is a problem in one network segment (such as short circuit, etc.), which does not affect other network segments, thereby greatly improving the security and stability of large networks.
We can understand the benefits of the star-shaped cabling network from the development of the LAN from the bus type to the star type. Similarly, the star 485 network composed of 485 concentrator will also be a direction for the development of the 485 network.