Serial port is called serial port. Nowadays, PC computers generally have two serial ports, COM 1 and COM 2. Serial port is different from parallel port in that its data and control information are transmitted bit by bit. Although the speed will be slower, but the transmission distance is longer than the parallel port, so long-distance communication should use serial port. COM 1 usually uses 9-pin D-connector, also known as RS-232 interface, while COM 2 uses old-fashioned DB25-pin connector, also known as RS-422 interface, which is rarely used at present.
Generally, there are two situations, or there may be only one new situation. The laptop may not have it.
Many industrial instruments use it as a standard communication port. The content and format of communication are generally attached to the user manual of the instrument.
Data transmission between computers or between computers and terminals can adopt serial communication and parallel communication. Serial communication is widely used because of its few lines and low cost, especially in long-distance transmission, which avoids the inconsistency of multiple lines. In serial communication, both sides of communication are required to adopt a standard interface, so that different devices can be easily connected for communication. RS-232-C interface (also known as EIA RS-232-C) is the most commonly used serial communication interface at present. 1970, developed by American Electronics Industry Association (EIA) in conjunction with Bell system, modem manufacturer and computer terminal manufacturer, used for serial communication. Its full name is "technical standard for serial binary data exchange interface between data terminal equipment (DTE) and data communication equipment (DCE)". The standard specifies the use of a 25-pin DB25 connector, and specifies the signal content of each pin of the connector and the level of various signals.
Signal content of (1) interface In fact, many of the 25 leads of RS-232-C are rarely used, and generally only 3-9 leads are used for communication between the computer and the terminal. See attached table 1 for the signal contents of the nine most commonly used leads of RS-232-C.
(2) Electrical characteristics of the interface The voltage of any signal line in RS-232-C has a negative logic relationship. Namely: logic series "1",-5-15v; Logic "0"+5-+15v. The noise tolerance is 2V. That is, the receiver is required to recognize signals as low as +3V as logic "0" and signals as high as -3V as logic "1".
(3) Physical structure of the interface RS-232-C interface connector generally uses a 25-core plug socket with a model of DB-25. Usually, the plug is at the DCE end and the socket is at the DTE end. Some devices are connected to the RS-232-C interface of a PC, because they only need three interface lines, namely, "sending data", "receiving data" and "signal ground", without using the other side's transmission control signal. Therefore, the 9-core plug socket of DB-9 is adopted, and the transmission line is shielded twisted pair.
(4) The length of transmission cable is specified by RS-232C standard. When the symbol distortion is less than 4%, the length of transmission cable should be 50 feet. In fact, this 4% symbol distortion is very conservative. In practical application, about 99% users work in the range of symbol distortion 10-20%, so the maximum distance in practical use will far exceed 50 feet. Among them, 1 cable is a shielded cable, and the model is DECP. 9 107723. There are three pairs of twisted pairs inside, each of which is composed of 22# AWG and covered with a shielding net. Cable No.2 is unshielded. The model number is DECP. No.9105856-04 is a 22#AWG four-core cable.
1.RS-232-C is a serial physical interface standard formulated by EIA (Electronic Industry Association). RS is the abbreviation of English "Recommended Standard", 232 is the identification number, and C stands for the revision number. The RS-232-C bus standard has 25 signal lines, including a main channel and an auxiliary channel. In most cases, the main channel is mainly used. For general duplex communication, only a few signal lines are needed, such as transmission line, reception line and ground line. The data transmission rates specified in RS-232-C standard are 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 per second. According to RS-232-C standard, the driver is allowed to have a capacitive load of 2500pF, and the communication distance will be limited by this capacitance. For example, when using the communication cable of 150pF/m, the maximum communication distance is15m; If the capacitance per meter of cable is reduced, the communication distance can be increased. Another reason for the short transmission distance is that RS-232 is a single-ended signal transmission, which has the problem of * * ground noise and cannot suppress * * mode interference, so it is generally used for communication within 20m.
2.RS-485 bus, when the communication distance is required to be tens of meters to thousands of meters, RS-485 serial bus standard is widely used. RS-485 adopts balanced transmission and differential reception, so it has the ability to suppress * * * mode interference. In addition, the bus transceiver has high sensitivity and can detect the voltage as low as 200mV, so it can recover the transmission signal thousands of meters away. RS-485 works in half-duplex mode, and only one point can be transmitted at any time. Therefore, the transmitting circuit must be controlled by the enable signal. RS-485 is very convenient for multipoint interconnection, which can save a lot of signal lines. The application of RS-485 can be networked to form a distributed system, which allows up to 32 drivers and 32 receivers to be connected in parallel.
In the past, the communication between PC and intelligent devices mostly depended on RS232, RS485 and Ethernet, mainly depending on the interface specifications of the devices. However, RS232 and RS485 can only represent the physical medium layer and link layer of communication. If you want to realize two-way data access, you must write your own communication application programs, but most of these programs can't meet the ISO/OSI specification, and can only achieve a single function, which is suitable for a single device type, and the programs are not universal. In a device network composed of RS232 or RS485 devices, if the number of devices exceeds two, RS485 must be used as the communication medium. The information exchange between devices in RS485 network can only be realized through the transmission of "master" devices. The master device is usually a PC, but only one master device is allowed in this device network, and the rest are slave devices. Fieldbus technology is based on ISO/OSI model and has a complete software support system, which can solve bus control, conflict detection, link maintenance and other problems. ...