How to improve the application of RS-485 bus system reliability – fire alarm, 485, fire alarm – Fire Industry
Introduction Recent years, as people continue to increase awareness of fire risks and relevant laws and regulations of the constant improvement of automatic fire alarm system has been the rapid development and wide application, has become the prevention of fires, protecting life and property safety of the most important means. This requires that automatic fire alarm system must have high reliability and stability.
Present, many domestic fire alarm system bus using RS485 half-duplex asynchronous communication interconnection between the realization of the fire alarm control, and fire alarm control and communication between the fire indicating panels. However, in actual use, often due to the large number of devices, communications lines and the scene of a variety of well interference, resulting in communication reliability, stability is not high, resulting in the quality of networked systems can not be guaranteed.
Author networking automatic fire alarm system design, by a large number of tests, found that when using the RS485 bus, if the simple design of the circuit by conventional means, then there may be in the actual project the following two questions: First, the data communication send and receive unreliable; Second, in multi-machine communication, a node failure (such as crashes) often makes the whole system of communication framework collapse, and to the failure of the investigation difficult. In response to these problems, on the 485 bus interface hardware and software design improvements have taken effective measures to significantly improve network reliability and stability.
1 RS-485 bus interface hardware design
Shown in Figure 1, 89C51 microcontroller comes with asynchronous communication interface, an external RS485 transceiver 75LBC184, 89C51 asynchronous communication between the mouth and 75LBC184 the electrical coupler using three isolation.
1.1 75LBC184DE the client, design
As fire alarm control system of the host and the extension far apart, the total length of the lines of communication are often more than 1000m, and the extension system power or reset it is often not at the same time to complete. If at this time of the DE-side potential of a 75LBC184 1, its output will be in 485 to send the state, which is occupied by a communication bus, so other extensions can not communicate with the host. This is especially manifested in an abnormal extension (such as crashes), the cause of the communication breakdown. Therefore, circuit design should ensure that the system reset 75LBC184 the DE-side potential of 0. As 89C51 in the reset period, I / O port output is high, so in Figure 1 connected in electrical reset can be solved effectively during the extension "bite" the bus issue.
Figure 1, the improved theory Road 485 communication interface
1.2 isolation optocoupler circuit parameter selection
The fire alarm system, to be conducted on-site real-time monitoring and response, so that the communication baud rate of data is often done relatively high (the system controller and display the communication rate between the disk in 6250bps). Improve the communication baud rate limit the "bottleneck" is not the site of wire (non-site construction generally shielded twisted pair), but the single chip system for signal isolation optocoupler circuit. Here by TIL117, circuit design can be considered high-speed optocoupler, such as the 6N137, 6N136 and other chips; also can optimize the design of common optocoupler circuit parameters, so that work in the best condition. For example: resistor R2, R3, if the election made bigger, the optocoupler's LED will enter the saturation by the end become slower; If you select too small, then exit the saturation will be very slow. So this two resistor values should be carefully selected, different types of coupler and drive circuit makes little difference in the two resistance values in the circuit design should be particularly cautious, usually determined through experiments.
1.3 485 bus, part of the design of output circuit
Output circuit lines to give full consideration to all types of interference and line impedance matching.
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