Application of 485 Hub in Bus Data Acquisition System
The smart meters used in the industrial field are developed with the maturity of the single-chip microcomputer technology in the early 1980s. Now the world instrument market is basically monopolized by smart meters. The reason for this is the need for enterprise informatization. One of the necessary conditions for companies to select instruments is to have networked communication capabilities. (485 communication, Ethernet communication, optical communication, etc.)
RS485 network: RS485/MODBUS is a popular cloth network mode, which is characterized by simple and convenient implementation, and now supports special RS485 instruments and more, especially in the oil industry RS485/MODBUS is simply dominate the world, the current instrument Businesses have also turned to support RS485/MODBUS for the simple reason that, like the original HART instrument, it would be very difficult and expensive to buy a switch port. The RS485 switch interface is much cheaper and varied. At least in the low-end market, RS485/MODBUS will also be the most important networking mode and will not change in the next two to three years.
Initially it was a simple process of data analog signal output. Later, the instrument interface was an RS232 interface. This interface can implement peer-to-peer communication, but this method can not realize the networking function. Subsequent RS485 solves this problem. RS485 uses differential signal negative logic. +2V to +6V indicates "0" and -6V to -2V indicates "1." RS485 has two-wire and four-wire two wiring, four-wire system can only achieve point-to-point communication, is rarely used, and now more used is a two-wire connection, this connection is a bus topology Up to 32 nodes can be attached to the same bus.
Generally used in the RS485 communication network is the master-slave communication mode, that is, a host with multiple slaves. In many cases, the RS-485 communication link is simply connected to the "A" and "B" sides of each interface using a pair of twisted pairs. While ignoring the connection of the signal ground, this connection method can work normally in many situations, but it has buried a lot of hidden dangers.
There are two reasons for this:
(1) Common Mode Interference Problem: The RS-485 interface uses a differential transmission method. It does not need to detect the signal with respect to a certain reference point. The system only needs to detect the potential difference between the two lines. However, people often overlook the transceiver has a certain range of common-mode voltage, RS-485 transceiver common-mode voltage range of -7 ~ +12 V, only to meet the above conditions, the entire network can work properly. When the common-mode voltage in the network line exceeds this range, it will affect the stability and reliability of the communication and even damage the interface.
(2) EMI problem: The common mode part in the output signal of the transmission driver needs a return path. If there is no low-resistance return path (signal ground), it will return to the source in the form of radiation. The whole bus will be like a huge one. The antenna radiates electromagnetic waves outward
The existing network topology generally adopts a bus-type structure of terminal matching 120 ohm resistors, and does not support ring or star networks. When building a network, please note the following points:
(1) Use a twisted-pair cable as the bus and connect each node 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-out line has the lowest impact on the bus signal. Some network connections, though incorrect, may still work normally at short distances and at low speeds. However, as the communication distance increases or the communication speed increases, its adverse effects will become more and more serious. The main reason is that signals are reflected at the end of each branch. After superimposing with the original signal, the signal quality will be degraded.
(2) The continuity of the characteristic impedance of the bus should be noted. Signal reflections occur at impedance discontinuities. This type of discontinuity can easily occur in the following situations: Different sections of the bus use different cables, or there are too many transceivers on a certain bus to be installed close together, and the branches are too long to lead out to the bus. .
In the industrial field data acquisition system project, with the improvement of the degree of automation and the need for system networking, the project management department requires that the master and slave monitoring centers can view and control the entire data acquisition terminal equipment. In addition, it is required that each monitoring center can monitor the entire industrial field device through the host control acquisition terminal device, and it also requires that the data of the terminal device can be saved at the industrial site through the server. Therefore, it must be required that multiple data acquisition devices can co-exist in a control system.
During the construction process, the contractor found that if all the acquisition terminal equipments were simply connected and connected to a 485 bus, due to the potential difference between each master control device and the signal reflection from the 485 bus, the whole data acquisition system could not For use, the project owner uses the 485 bus splitter of Shenzhen Tiandi Huajie Technology Co., Ltd. to connect the 485 lines of multiple main control devices to the input port of the 485 sharer and share it with a 485 output port. There are optical isolation between the input ports of the sharer, and independent drive is used, which can effectively solve the problem of potential difference and signal reflection. As a result, multiple data acquisition terminal equipment hosts coexist in one control system
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