In the actual use of video monitoring system, in order to complete the information point (video, audio, alarm signal) the collection of information, as well as to the front-end equipment (yuntai, lens, shield, lighting, etc.) of the control, we need a large number of cable laid. By 485 bus to connect and control the front-end equipment, the cable wiring method. Mainly has the star connection and T connect the two connection mode:

Star join is the most commonly used in television monitoring system wiring method (as shown in figure 1), video lines, audio lines and most alarm sensor signal lines are using this method. Star join convenient construction and maintenance of the method is simple; The disadvantage is that comparative cost line. Front-end equipment easy to lose control at the same time, or your mobile.

T connection is a bit of multipoint bus wiring method is commonly used in control system (see figure 2). That is to say, choose the distance between the two nodes farthest lines as the main bus, other nodes are received main through a tee joint. The tee joint of the line to the shorter, the better, had better not exceed 1/10 of the trunk line, otherwise may lead to communication failures. To prevent the occurrence of this problem, actually the most simple wiring method is adopt concatenated, from node to node 1 connects two core 2, then connect the two core from node 2 to node 3, and so on, until the furthest node, so that you can guarantee the tee joint approximate to 0 lead. Although concatenated would increase the line length, but the line communication performance, best can achieve maximum communication rate. T connection has the advantage of province line, can save cost; The disadvantage is that more complex wiring and maintenance.

In video monitoring system, most of the host through the RS485 bus control front decoders, to complete the yuntai, lenses and other auxiliary equipment control. RS485 bus is a kind of half-duplex differential serial bus, transmission medium for twisted pair, maximum transmission distance is 1200 meters.

T should be used in standard RS485 bus connection, and standard decoder wiring method.

However, due to the video monitoring system in other cables (video, audio and alarm) often USES the star connection, if decoder wiring must use T connection, sometimes make people feel uncomfortable and inconvenient. In practical engineering, the vast majority of the host control of the decoder is one-way, i.e. using one-way RS485 transmission of control signal, then the wiring can be less strict, a lot of practical engineering decoder star join is proved practicable (as shown in figure 4). Of course, the wiring methods in decoder number and distance limit, experience value is, the number of decoder, within 10 distance of 400 meters. Engineering environment is different, may cause larger difference of experience and actual numerical value. One-way RS232 and RS485 converter module (TD - 108) and one-way RS232 and RS485 converter (TD - 108), is designed for this function. Is important to emphasize that for RS485 system must be two-way communication, can only be used when wiring is shown in figure 3 T connection, at this time can choose the bidirectional RS232 and RS485 converter (TD - 108 - a).

In medium and large television monitoring system, the decoder is more, the number of physical distance from the host. RS485 wiring, often need to adopt the star connection and T connection method of combining the, to solve the problem. At this point, you can choose RS485 hub (TD - 1204), and RS485 amplifier (TD - 109), by increasing the number of RS485 bus and the RS485 signal relay drive, to solve the problems such as limited RS485 drivers and driving distance is not enough.

RS485 bus wiring, had better use shielded twisted-pair cable. This is because the twisted-pair can offset most of the distribution parameters, the greater the degree of twisted-pair, the stronger the ability to offset. The shield can more effectively prevent outside interference.

In order to achieve the purpose of the lightning protection, the shielding net at one end of shielded twisted-pair cable shall be the decoder of ground wire (RS232 - side, beside the RS485B) phase. At the other end should be and system into the ground. Especially for outdoor decoder, pay particular attention to this.

In T connection systems, when the line distance is longer (500 meters), will be caused by line impedance mismatch reflection interference signal, may cause the communication failure. At this point need to be considered in the two nodes farthest A and B on the side of each and A 120 euro balance matching resistance, to ensure the smooth process of communication. But notice cannot meet the balance match on other nodes and resistance.

As we know, RS485 signal lines for two, one is A, another one for B, in general, A should connect with A, B and B. It is important to note that different manufacturers ex-factory products, definition of A and B is different, so in the application, you can try the exchange of A and B connection (as long as A and B do not short circuit, there is no damage to the device), to solve the problem of commissioning impassability.

About the number of RS485 drivers load, general standard for the 32. Driving load quantity still depends on the product chooses RS485 interface chip, some can drive 64 load, some can drive 128. TD series of related products are used to drive 64 load of RS485 interface chip. But in actual engineering, in order to the reliability of communication, suggest the number of up to 80% of the maximum load driver access to load.