What it is RS232 interface

- May 31, 2019-

The RS232 interface is the serial port, the 9-pin socket on the back of the computer case, usually has a "| O | O |" like Mark.

There are two general chassis, and there may be only one new chassis. The laptop may not be available.

There are many industrial instruments that use it as a standard communication port. The content and format of the communication is usually attached to the user manual of the instrument.


Data transfer between the computer and the computer or between the computer and the terminal can be performed by serial communication and parallel communication. Since the serial communication method has less use lines and low cost, especially in remote transmission, it is widely used because it avoids inconsistencies in multi-line characteristics. In serial communication, both parties need to use a standard interface so that they can easily connect to different devices for communication. The RS-232-C interface (also known as EIA RS-232-C) is the most commonly used serial communication interface. It was developed by the Electronics Industry Association (EIA) and Bell Systems, a modem manufacturer and computer terminal manufacturer for serial communication standards. Its full name is "the serial binary data exchange interface technology standard between data terminal equipment (DTE) and data communication equipment (DCE)". This standard specifies a 25-pin DB25 connector for each connector of the connector. Specifies the signal content of the foot and also specifies the level of each signal.

(1) Signal content of the interface In practice, many of the 25 leads of the RS-232-C are rarely used, and usually only 3-9 leads are used in communication between the computer and the terminal. Table 1 lists the signal content of the nine most commonly used RS-232-C leads.

(2) Electrical characteristics of the interface The voltage of any one of the RS-232-C signals is in a negative logical relationship. Namely: logic "1", -5 - -15V; logic "0" + 5 - + 15V. The noise margin is 2V. That is, the receiver needs to recognize signals as low as +3V as logic "0" and signals up to -3V as logic "1".

(3) Physical structure of the interface The RS-232-C interface connector generally uses a DB-25 type 25-pin connector. Usually, the plug is located at the DCE end, and the socket is located at the DTE end. Some devices are connected to the PC via the RS-232-C interface because it does not use the other party's transmission control signals. Only three interface lines are required, namely "send data", "receive data" and "signal ground". Therefore, with the DB-9 9-pin connector, the transmission line is a shielded twisted pair.

(4) The length of the transmission cable is specified by the RS-232C standard. When the symbol distortion is less than 4%, the transmission cable length should be 50 feet. In fact, this 4% symbol distortion is very conservative. In practice, 99% of users work within 10-20% of symbol distortion, so the maximum distance in actual use will far exceed 50 feet. DEC specifies that the allowed symbol distortion is 10% and gets Schedule 2. Experimental results. Line 1 is a shielded wire, model number DECP.NO.9107723. There are three pairs of twisted pairs, each pair consisting of 22#AWG, which is covered with a screen mesh. Cable #2 is an unshielded cable. The model number is DECP.NO.9105856-04 is a 22#AWG four-core cable.

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