COMCommunication interface knowledge and technical information

COMCommunication interface, commonly known as serial port, also has a professional name RS-232.（RS-232 port includes a wide range, COM is just one of them）The COM port is usually 9-pin, which is applied in Lenovo consumption and commercial computers, and there is a 25-pin interface. The definition of the COM ports can be queried in other FAQs. However, the functions and uses of each definition are unknown, and the meaning is now sharing with everyone:
9 cores  Signal direction  abbreviation  Description name（Function）
1  Debug demodulator  DCD(Also known as CD)  Carrier detection
2  Debug demodulator  RXD  Receive data
3  PC  TXD  send data
4  PC  DTR  Data terminal is ready
5  GND  Signal ground
6  Debug demodulator  DSR  Communication equipment is ready
7  PC  RTS  Request to send
8  Debug demodulator  CTS  Allowed to send（Send clear）
9  Debug demodulator  RI  ring（Vibrate）Bell indicator
In addition, there is a mother head is also a slot type, although the form is different, but the pin definition is the same.（More detailed introduction is as follows）

What is a serial port?
   The serial port is a very universal computer.Device communication protocol(Do not confuse with the General Serial Bus Universal Serial Bus or USB). Most computers include two RS232-based serial ports. The serial port is also a common communication protocol for instrumentation equipment; many GPIB compatible devices also have RS232 ports. At the same time, the serial port communication protocol can also be used to obtain data of the remote acquisition device.
   The concept of serial communication is very simple, serial bit bit (BIT) sends and receives bytes. Although the parallel communication is slower than by byte, the serial port can receive data with another wire while transmitting data using a line. It is very simple and can achieve long-distance communication. For example, when IEEE488 defines a parallel traffic state, the specified equipment line must not exceed 20 meters, and the length between any two devices should not exceed 2 meters; for the serial port, the length can reach 1200 meters.
   Typically, serial port is used for transmission of ASCII code characters. Communication uses 3 wires to complete: (1) Ground, (2) transmission, (3) reception. Since the serial port communication is asynchronous, the port can send data on one line to receive data on another line. Other lines are used to shake hands, but it is not necessary. The most important parameters of serial port communications are baud rates, data bits, stop bits, and parity. These parameters must match for two ports that are passing.
   (a)baud rate:This is a parameter that measures the speed of communication. It represents the number of bits per second. For example, 300 baots indicate 300 BITs per second. When we mention the clock cycle, we refer to the vibrate rate, for example, if the protocol needs 4800 baud rate, then the clock is 4800 Hz. This means that the sampling rate of serial communication is 4800 Hz on the data line. The baud rate of usually 14400, 28800 and 36600 is 14400, 28800 and 36600. The baud rate can be much greater than these values, but the baud rate and distance are inversely proportional. High-baud rate is often used to place communication between very close instruments, typical examples are communication of GPIB devices.
   (b)Data bit:This is a parameter that measures the actual data bit in communication. When the computer sends a packet, the actual data is not 8 bits, and the standard value is 5, 7, and 8 bits. How to set up the information you want to transmit. For example, the standard ASCII code is 0 ~ 127 (7). The extended ASCII code is 0 ~ 255 (8). If the data uses a simple text (standard ASCII code), then each packet uses 7-bit data. Each package refers to a byte, including start / stop bit, data bit, and parity bit. Since the actual data bits depends on the selection of communication protocols, the term“” refers to any communication.
   (c)Stop bit:Used to represent the last bit of a single package. Typical values ??are 1, 1.5 and 2. Since the data is timed on the transmission line, and each device has its own clock, it is likely that there is a small disorder between two devices in communication. Therefore, the stop bit is not only the end of the transmission, but also the opportunity to provide a computer correction clock synchronization. The more the number of bits suitable for stopping the bit, the larger the tolerance of different clock synchronization, but the data transfer rate is also slower.
   (d)Parity bit:A simple error error in serial port communications. There are four types of error modes: even, odd, high and low. Of course, there is no check bit. For even the odd caller, the serial will set the check bit (one of the data bits behind), and use a value to ensure that the transmitted data has a plurality or aqi logical high. For example, if the data is 011, then for the doll, the check digit is 0, and the number of digits of the logic high is even. If it is aqi calibration, the calibration bit is 1, there is 3 logical high. High and low-level unmerified check data, simple set logic high or logic low check. This makes the receiving device to know a bit of a bit, and there is a chance to determine whether there is noise interference communication or whether the transmission and receive data are not synchronized.
What is RS-232?
   RS-232(ANSI/EIA-232Standard) is IBM-PC and its compatible machineSerial connection standard. Can be used in many purposes, such as connecting mice, printers, or modems, can also pick up industrial instruments. Improvements for driving and connecting, the transmission length or speed of RS-232 is often exceeded by the standard value in the actual application. RS-232 is limited to communication between PC serial ports and inter-device-point peer-to-point. RS-232 serial communication farther distance is 50 feet.
   -DB-9Needle
   -------------
   \1 2 3 4 5/
   -\6 7 8 9/
   -----------
   The cross section of the line connected from the computer.
RS-232 features:
data:
TXD (PIN 3): Serial Data Output
RXD (PIN 2): Serial Port Data Enter

RTS (PIN 7): Send a data request
CTS (PIN 8): Clear Send
DSR (PIN 6): Data Send Ready
DCD (PIN 1): Data carrier detection
DTR (PIN 4): Data terminal Ready
Ground: GND (PIN 5): Ground
Other Ri (PIN 9): Ringtone Indication
Note:COMThe interface is the physical port following the RS232 standard.
COMThe physical implementation of the inscriptions serial port, and RS232 is a serial port standard.
COMinterface
   COMThe interface refers to the Component Object Mode interface, which is a Microsoft definition.standardinterface.
   The serial port is called a serial interface, and the current PC generally has two serial port COM 1 and COM 2. The serial port is different from the parallel port in that its data and control information are transmitted by one. Although this speed will be slow, the transfer distance is longer than in parallel, so the serial port should be used when a long distance is to be used. Usually, COM 1 uses a 9-pin D-shaped connector, also known as the RS-232 interface, and some of the COM 2 uses an old-fashioned DB25 pin connector, also known as the RS-422 interface, this interface is currently Already rarely used