Bus system refers to the system mode in which multiple field devices such as gas detectors share the same signal circuit to connect with gas alarm controller. Generally, CANbus digital signal communication is adopted. For its wiring mode, the following three methods are mainly adopted:
A) Tree type “T-type” distribution
This wiring method not only ensures the convenience of bus equipment installation, but also facilitates later maintenance. It is the recommended method of bus system.
1) A twisted pair cable is used as the bus to connect each node in series. The length of the outgoing line from the bus to each node shall be as short as possible, so as to minimize the impact of the reflected signal in the outgoing line 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 impact 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 reduce the signal quality.
2) Attention shall be paid to the continuity of bus characteristic impedance. Signal reflection will occur at the point of impedance discontinuity. This discontinuity is easy to occur in the following situations: different cables are used in different sections of the bus, or too many transceivers are installed close together on a certain section of the bus, and too 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 signal, 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 on different floors or regions. Because the star connection method has an impact on the signal transmission of the bus system and may cause the instability of the system, it is necessary to install a relay isolator at the branch point to isolate the bus equipment on each branch circuit through the relay isolator.
(1) 485 line does not use twisted pair, or use low-grade passive converter! 485 communication line can not go through star connection, but must go through standard hand-in-hand bus mode.
(2) If the 485 wiring is not standardized or out of the communication range, the communication will fail or sometimes the communication will fail.
(3) 485 transmission is a differential mode transmission mode. Only when 485 + and 485 – are twisted pair, can 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 thicker lines will have better transmission quality. They combine twisted pair into one strand and another twisted pair into one strand. This is counterproductive, but greatly reduces the communication quality.