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Correct connection method and application of RS485 communication cable

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Before talking about RS485 communication, it is necessary to mention RS232 communication. If you don't know about RS232 communication, don't worry. You must have seen and used RS232 interface (if you don't believe it, you can look at the back of the case of desktop computer at home)

In fact, many people have used Siemens "pc-ppi" programming cable when programming S7-200 series PLC. This cable realizes the conversion of RS232 and RS485 communication. Here, "PC" can be understood as the RS232 interface of the computer, "PPI" can be understood as the RS485 port of PLC, "PPI" is a protocol, with RS485 as the medium (physical layer).

Why RS485 communication

For example, RS232 communication can only be used for point-to-point communication, such as computer to PLC. If the number of nodes that need to communicate with each other exceeds two in the system, it can not directly meet the requirements. Therefore, EIA (Electronic Industry Association) has formulated a new interface standard RS485, which can support point to multipoint communication.

The electrical standard of RS485 communication is RS422 (Mitsubishi PLC programming port) communication standard. RS485 is a variant of RS422A. RS422A is full duplex, and two pairs of balanced differential signal lines are used for transmitting and receiving respectively. RS485 is half duplex, only a pair of balanced differential signal lines, can not send and receive at the same time.

Emphasis: RS485 is half duplex, master-slave communication.

The serial communication network can be formed by using RS485 communication interface and twisted pair. The following will focus on the composition of the distributed system. There can be up to 32 stations in the system. If the number of devices connected in a network exceeds 32, RS485 repeaters can be used, and 128 stations can be connected.

Key points: RS485 supports multi station and multi device communication.

RS485 communication connection
In order to effectively suppress interference, shielded twisted pair is generally used as communication medium, such as PROFIBUS DP cable of Siemens

The RS485 communication wiring of each device is very easy, and the premise is to ensure that each device is connected "hand in hand" (it costs a lot of lines or cables. If there are many devices, it will be easier to use a hub or repeater, as described below). In order to avoid communication failure and reduce interference, please do not have star connection or bifurcation.

When connecting with RSDP / 485 cable, it is convenient to connect with RSDP connector, which can be used for communication terminal.


RS485 communication bus wiring
In the simple application of RS485 communication, the bus topology is generally used. However, in the complex system, the wiring construction of bus topology is not only very cumbersome, but also wastes a lot of wiring. Flexible use of 485 hub or 485 repeater can connect bus topology into star or tree topology. It greatly facilitates the early construction and later maintenance work.


485 bus star topology



Tree topology of 485 bus



Connection method of terminal resistance
In the practical application of 485 bus, when the transmission distance exceeds a certain length, the anti-interference ability of the bus will decline. In this case, 120 ohm terminal matching resistance should be connected at both ends of 485 bus to ensure the stability of 485 bus.

The correct connection method of terminal matching resistance is to connect a 120 ohm terminal resistance at the exit of the equipment at the first end and the entrance of the equipment at the end of the 485 bus, which is connected in parallel between the positive and negative wires of the 485 bus.

Some misunderstandings and supplements about RS485 communication

I remember that a student called me at the scene and said that his PLC could not read the data of flow meter. After checking, there was no problem with the program, no problem with the hardware connection, no problem after the computer was tested with the serial port debugging software, but the instrument data could not be read.

My first sentence is: what communication protocol does your PLC and instrument adopt?

He was silent for a while and said to me: PLC and instrument communication are not all using RS485 communication protocol, I this is RS485 communication protocol!

His answer probably answered the voice of many friends. The words "RS485 communication protocol" seem to be harmonious, but in fact they are not. Because the protocol is a protocol and RS485 communication is a medium (physical layer), it can not be confused.

For example, when making a phone call, the phone is the physical layer, and the language spoken between the people who are talking is the protocol. The same physical layer can transmit different protocols, just as people on both sides of the phone can communicate in various languages (protocols). RS485 communication is just like a telephone. It can transmit PPI Protocol, MODBUS protocol, PROFIBUS protocol and so on.

After more than an hour of communication, it was known that the instrument was surrounded by frequency converter, power line and communication line. So the power line and the signal line are separated from each other, and the terminal resistance is added to the outlet and end of the communication line.

The reason why the terminal resistance is not added is because he thinks that the distance between the instrument and PLC is relatively close (several meters to more than ten meters), which is actually a misunderstanding, because the distance is not a problem, the main problem is signal interference. Of course, some communication ports have been isolated, so it is not necessary to add them at this time!

For the selection of terminal resistance value:

I think the size of terminal resistance has something to do with the equipment interface and cable, and there is also a special calculation formula. Frankly speaking, it is a range of values. My personal practice is from no match to 120! The farther the distance, the more obvious the effect. Therefore, in order to ensure the communication quality, it is better to add!
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