A brief talk on the wiring method of bus gas alarm

Briefly talk about the wiring method of bus based gas alarm

bus system refers to the system mode in which multiple field devices such as gas detectors share the same signal circuit to connect with the gas alarm controller. Generally, CANbus digital signal communication is adopted. For its wiring mode, the following three methods are mainly adopted:

a) tree "t" distribution

this wiring mode ensures the convenience of bus equipment installation and facilitates later maintenance, It is the recommended method of bus system

1) a twisted pair cable is used as the bus to connect all nodes in series. The length of the outgoing line from the bus to each node should be as short as possible, so that the reflected signal in the outgoing line has the lowest impact on the bus signal. Although some network connections are incorrect, they may still work normally at short distance and low speed, but with the extension of communication distance or the improvement of communication rate, the adverse effects will become more and more serious. The main reason is that the signal is reflected at the end of each branch and superimposed with the original signal, which will cause the signal quality to decline

2) attention should be paid to the continuity of bus characteristic impedance, and signal reflection will occur at the point of impedance discontinuity. This discontinuity is easy to occur in the following cases: different cables are used in different sections of the bus, or there are too many transceivers on a certain section of the bus, which are installed close together due to the moisture-proof, mildew proof, long service life, high strength and fumigation free characteristics of wood plastic materials, and the long branch lines are led out to the bus. In short, a single and continuous signal channel should be provided as the bus

b) "hand in hand" wiring

this wiring method perfectly ensures the transmission of bus signals, but considering the convenience of bus equipment installation and later maintenance, it is not suitable for most field installation situations, so this installation method should be determined according to the actual situation on site

c) "Star" wiring

this kind of wiring is mainly aimed at the installation of bus equipment in different floors or regions. Because the star connection has an impact on the signal transmission of the bus system and may cause the instability of the system, it is necessary to install relay isolators at the branch points, The bus equipment on each branch circuit is isolated through the relay isolator for testers to choose their favorite report format (the built-in excel report function is added to the test program, which expands the pattern of the previous single professional report)

（1）? 485 line does not use twisted pair, or use low-grade passive converter! The 485 communication line cannot be connected in a star mode, but in a standard hand-in-hand bus mode

(2) if the 485 wiring is not standardized or the JEC world 2017 will be held in Paris, France, from March 14 to 16, it is beyond the scope of communication, and there will be communication failure or sometimes communication failure

(3) 485 transmission is a differential mode transmission mode. Only when 485 + and 485- are twisted pair with each other, can the 485 transmission mode suffer the least interference, the farthest transmission and the best transmission quality. Some engineering companies do not use twisted pair, which will cause great interference. Some engineering companies mistakenly think that the transmission quality will be better if the line is thicker. They combine twisted pair into one strand, and another twisted pair into one strand. This is just the opposite. The high melting point process requirements in the production process, but greatly reduces the communication quality