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Application Analysis of 485 Hub in Bus Data Acquisition System
The intelligent instrument used in the industrial field developed with the maturity of the single-chip technology in the early 1980s, and now the world instrument market is basically monopolized by the intelligent instrument. The reason is the need of enterprise informatization. One of the necessary conditions for enterprises to select instruments is to have the function of network communication. (485 communication, Ethernet communication, optical communication, etc.)
RS485 network: RS485/MODBUS is a popular way of network distribution now, which is characterized by simple and convenient implementation, and now there are many instruments supporting RS485, especially in the oil industry, RS485/MODBUS is simply dominating the world, and the current instruments Businesses have also turned to support RS485/MODBUS. The reason is very simple. It is very difficult and expensive to buy a conversion port for the original HART instrument. The conversion port of RS485 is much cheaper and has a wide variety. At least in the low-end market, RS485/MODBUS will still be the most important networking method, and it will not change in the past two or three years.
At first, it was a simple process quantity for data analog signal output. Later, the instrument interface was RS232 interface, which could realize point-to-point communication, but this method could not realize networking function. The subsequent RS485 solves this problem. RS485 adopts the negative logic of differential signals, +2V~+6V means “0”, -6V~-2V means “1”. RS485 has two types of wiring: two-wire system and four-wire system. The four-wire system can only realize point-to-point communication. It is rarely used now. Now, the two-wire wiring method is mostly used. This wiring method is a bus topology structure. Up to 32 nodes can be attached on the same bus.
In the RS485 communication network, the master-slave communication mode is generally used, that is, a host with multiple slaves. In many cases, when connecting the RS-485 communication link, simply use a pair of twisted pairs to connect the “A” and “B” ends of each interface. While ignoring the connection of the signal ground, this connection method can work normally in many occasions, but it has buried a lot of hidden dangers.
There are two reasons for this:
(1) Common mode interference problem: The RS-485 interface adopts the differential mode to transmit signals, and does not need to detect the signal relative to a certain reference point, the system only needs to detect the potential difference between the two lines. But people often ignore that the transceiver has a certain common-mode voltage range. The common-mode voltage range of the RS-485 transceiver is -7 to +12V. Only when the above conditions are met, the entire network can work normally. When the common mode voltage of the network line exceeds this range, it will affect the stability and reliability of communication, and even damage the interface.
(2) EMI problem: the common-mode part of the output signal of the transmitting driver needs a return path. If there is no low-impedance return path (signal ground), it will return to the source in the form of radiation, and the entire bus will be like a huge The antenna radiates electromagnetic waves outward
The existing network topology structure generally adopts a hand-in-hand bus type structure with a terminal matching 120 ohm resistance, and does not support a ring or star network. When building your network, keep the following in mind:
(1) A twisted pair cable is used as the bus, and each node is connected in series. The length of the lead wire from the bus to each node should be as short as possible, so that the reflected signal in the lead wire has the lowest impact on the bus signal. Although some network connections are incorrect, they may still work normally at short distances and low rates, but as the communication distance increases or the communication rate increases, the adverse effects will become more and more serious. The main reason is that the signal is reflected at the end of each branch. After superimposing with the original signal, the signal quality will be degraded.
(2) Attention should be paid to the continuity of the characteristic impedance of the bus, and the reflection of the signal will occur at the discontinuous point of the impedance. Such discontinuities are prone to occur in the following situations: different cables are used in different segments of the bus, or too many transceivers are installed close to each other on a segment of the bus, or too long branch lines lead to the bus .
In the industrial field data acquisition system project, with the improvement of the degree of automation and the needs of system networking, the project management department requires two monitoring centers, master and slave, to view and control the entire data acquisition terminal equipment. And it is required that each monitoring center can monitor the entire industrial field equipment through the host computer control acquisition terminal equipment, and also requires that the data of the industrial field acquisition terminal equipment can be saved through the server. Therefore, it must be required that multiple data acquisition devices can coexist in a control system.
During the construction process, the engineer found that if all the acquisition terminal devices were simply connected to a 485 bus, the entire data acquisition system would be paralyzed due to the potential difference of each main control device and the signal reflection generated by the 485 bus. Use, the engineer uses the 485 bus splitter of Shenzhen Tiandi Huajie Technology Co., Ltd. to connect the 485 lines of multiple master devices to the input port of the 485 sharer, and share it to a 485 output port. There is optical isolation between the input ports of the sharer, and the independent driving method is adopted, which can effectively solve the problems of potential difference and signal reflection. Thus, multiple data acquisition terminal equipment hosts coexist in one control system.
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