1. RS-232 communication interface

RS-232 communication interface is the most widely used serial interface in data communication. It is the interface for data exchange between data terminal equipment and data communication equipment. At present, the most popular is RS-232C, which is the C version of RS-232.

The physical connector (plug) of RS-232 interface is defined as 25 core plug, usually the plug is at the end of data terminal equipment (DTE) and the socket is at the end of data communication equipment (DCE). “RS” is the abbreviation of “recommended standard”, “232” is the identification number, “C” is the number of times this standard has been modified. It is not only a protocol standard, but also an electrical standard, which specifies the mode and function of information exchange between terminal and communication equipment. Serial communication is adopted to transmit data, and baud rates are 19200b / s, 9600b / s, 4800b / s, etc.

The RS-232C interface is a standard 25 pin D-type connector. RS-232C adopts negative logic, which stipulates that the logic “1” level is in the range of (- 5 ~ – 15) V, and the logic “0” level is in the range of (5 ~ 15) V, so it has high anti-interference ability.

2. RS-422 communication interface

RS-422 communication interface defines 10 circuit functions that RS-232C communication interface does not have, and 37 pin connector is specified. The transmitter and receiver use + 5V power supply, so the communication rate, communication distance and anti-interference ability are greatly improved compared with RS-232C communication interface. The data transmission rate can reach 10MB / s, and the communication distance is 12 ~ 1200m.

3. RS-485 communication interface

RS-485 communication interface is an improvement of RS-422. RS-485 adopts half duplex communication mode, and its electrical interface circuit adopts differential transmission mode, with enhanced anti common mode interference ability, low output impedance and no grounding circuit, which is suitable for long-distance data transmission.

PLC is mainly through the general communication interface such as RS-232, RS-422 and RS-485 for network communication. If the two devices of network communication have the same type of interface, they can communicate directly through the adaptive cable connection. If the communication interfaces of the two devices are different, certain hardware devices should be used to convert the interface types.

This kind of equipment produced by Mitsubishi company adopts two basic structure forms: function expansion board and independent case type. The communication interface of function expansion board (such as fx2n-485-bd type) has no shell and is installed in the PLC chassis for use. The maximum communication distance is 50m. The independent chassis interface (such as fx2n-485adp) belongs to the class of expansion module, and its communication distance composed of adapter is up to 500m.

In many industrial environments, it is required to use the least signal line to realize the communication task in a long distance. At present, RS-485 serial interface bus is widely used in PLC LAN. RS-485 is used to form a distributed system. There are 32 stations at most in the system. The new interface device allows 128 stations to be connected.

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