Introduction to the definition of serial interface
Serial interface is called serial port for short, also called serial communication interface or serial communication interface (usually referred to as COM interface), which is an extended interface adopting serial communication mode. Serial interface? (serial? Interface)? It refers to the sequential transmission of data bit by bit, characterized by simple communication lines, and bidirectional communication can be realized as long as a pair of transmission lines (telephone lines can be directly used as transmission lines), thus greatly reducing the cost, especially suitable for long-distance communication, but the transmission speed is slow. The communication mode in which each bit of information is transmitted bit by bit in sequence is called serial communication. The characteristics of serial communication are: the transmission of data bits is carried out in bit order, and at least one transmission line is needed; Low cost but slow transmission speed. The distance of serial communication can be from several meters to several kilometers; According to the transmission direction of information, serial communication can be further divided into simplex, half-duplex and full-duplex.
Origin of serial interface and serial interface connector
The serial port appears around 1980, and the data transmission rate is115 kbps to 230 kbps. Serial port first appeared for the purpose of connecting computer peripherals. In the early days, serial ports were generally used to connect mice and external modems, as well as old-fashioned cameras and wordpads. Serial port can also be used for interconnection and data transmission between two computers (or devices). Because the serial port (COM) does not support hot plug and the transmission rate is low, some new motherboards and most portable computers have begun to cancel this interface. At present, serial ports are mostly used in industrial control and measurement equipment and some communication equipment.
Interface division standard of serial interface
There are two basic ways of serial communication: synchronous serial communication and asynchronous serial communication.
Synchronous serial refers to SPI (serial? Peripheral? Interface), as the name implies, is a serial peripheral interface. SPI bus system is a synchronous serial peripheral interface, which enables MCU to communicate with various peripherals in serial mode to exchange information. TRM450 is an SPI interface.
Asynchronous serial refers to UART (universal? Asynchronous? Receiver/transmitter), universal asynchronous reception/transmission. UART is a chip with parallel input and serial output, which is usually integrated on the motherboard. UART includes TTL serial port and RS232 serial port. ? TTL level is 3.3V, RS232 is negative logic level, and +5~+ 12V is defined as low level,-12~-5V is high level, MDS27 10, MDS? SD4 and EL805 are RS232 interfaces, and EL806 has TTL interface.
According to electrical standards and protocols, serial interfaces include RS-232-C, RS-422 and RS485. RS-232-C, RS-422 and RS-485 standards only specify the electrical characteristics of interfaces, and do not involve connectors, cables or protocols.
RS-232
Also known as standard serial port, the most commonly used serial communication interface. It is a serial communication standard jointly formulated by American Electronics Industry Association (EIA) in 1970 with Bell system, modem manufacturer and computer terminal manufacturer. Its full name is "technical standard for serial binary data exchange interface between data terminal equipment (DTE) and data communication equipment (DCE)". The traditional RS-232-C interface standard has 22 wires and adopts the standard 25-core D-type plug and socket (DB25). Later, it was simplified to 9-core D-socket (DB9), and now 25-core plug socket is rarely used in application.
RS-232 adopts unbalanced transmission mode, which is called single-ended communication. Because the difference between the transmitting level and the receiving level is only about 2V to 3V, its * * * mode suppression ability is poor. Coupled with the distributed capacitance on twisted pair, its maximum transmission distance is about 15m, and its maximum speed is 20kb/s ... RS-232 is designed for point-to-point communication (that is, only one pair of transceiver devices is used), and its driving load is 3 ~ 7kΩ. Therefore, RS-232 is suitable for communication between local devices.
RS-422
The full name of the standard is Electrical Characteristics of Balanced Voltage Digital Interface Circuit, which defines the characteristics of the interface circuit. The typical RS-422 is a four-wire interface. In fact, there is also a signal ground wire, ***5 wire. Its DB9 connector pin definition. Because the receiver adopts high input impedance and the transmitter driver has stronger driving ability than RS232, it is allowed to connect multiple receiving nodes on the same transmission line, with a maximum of 10 nodes. That is, one master, and the rest are slaves. Slaves can't communicate with each other, so RS-422 supports one-to-many bidirectional communication. The input impedance of the receiver is 4k, so the maximum load capacity of the transmitter is10× 4k+100Ω (terminal resistance). Because the RS-422 four-wire interface adopts independent sending and receiving channels, there is no need to control the data direction, and any necessary signal exchange between devices can be realized by software (XON/XOFF handshake) or hardware (a pair of independent twisted pairs).
The maximum transmission distance of RS-422 is 12 19 m, and the maximum transmission rate is100 MB/s. The length of the balanced twisted pair is inversely proportional to the transmission rate, and the maximum transmission distance can only be reached when the transmission rate is lower than 100kb/s/s ... only in a short distance. Generally, the maximum transmission rate that can be obtained on the twisted pair with the length of 100m is only1MB/s. ..
RS-485
It is developed from RS-422, so many electrical regulations of RS-485 are similar to RS-422. If the balanced transmission mode is adopted, it is necessary to connect the terminal resistor on the transmission line. RS-485 can adopt two-wire mode and four-wire mode, and the two-wire mode can realize real multi-point two-way communication. When using four-wire connection, only point-to-many communication can be realized like RS-422, that is, there can only be one master device, and the rest are slave devices, but this is an improvement of RS-422, and 32 more devices can be connected to the bus regardless of the four-wire or two-wire connection mode.
The difference between RS-485 and RS-422 is that their * * * mode output voltages are different. RS-485 is between -7V and+12V, while RS-422 is between -7V and +7V. The minimum input impedance of RS-485 receiver is12kΩ, and that of RS-422 is 4K. Because RS-485 conforms to all the specifications of RS-422, the driver of RS-485 can be applied to RS-422 network.
Like RS-422, the maximum transmission distance of RS-485 is about 12 19 m, and the maximum transmission rate is100 MB/s. The length of the balanced twisted pair is inversely proportional to the transmission rate. Only when the transmission rate is lower than 100kb/s can the specified longest cable length be used. The highest transmission rate can only be obtained in a short distance. Generally, the maximum transmission rate of 100m twisted pair is only1MB/s. ..
Application of serial interface
Serial port of switch
The English for serial port of switch is trunk, which is used for next-hop routing conversion. Each VLAN can only be connected to the external network after it points to the route through the trunk.
Pin description:
Pin？ Name? Dir？ describe
1? CD？ Operator? find
2? RXD？ Receive? data
3? TXD？ Transmission? data
4? DTR？ Data? Terminal? prepared
5? GND？ System? ground
6? DSR？ Data? Settings? prepared
7? RTS？ Request? Where to? send
8? CTS？ Is that clear? Where to? send
9? RI？ A ring? director
At present, the commonly used serial ports are 9-pin serial port (DB9) and 25-pin serial port (DB25). When the communication distance is close (
Description of common signal pins of (1)DB9 and DB25
DB9？ |? DB25
pin？ Function? Name? pin？ Function? name
1? Data carrier detection? DCD？ 8? Data carrier detection? dcd
2? Receiving data? RXD？ 3? Receiving data? receive data
3? Send data? TXD？ 2? Send data? TXD
4? Data terminal preparation? DTR？ 20? Data terminal preparation? Duty Type Rating
5? Signal ground? GND？ 7? Signal ground? GND
Is the data equipment of 6 DSR ready? 6? Is the data equipment ready? Daily Service Report (daily service report)
7? Request to send? RTS？ 4? Request to send? real-time strategy game
8? Clear send? CTS？ 5? Clear send? With ＣARPAL TUNNEL SYNDROME
9? Ringing indication? RI？ 22? Ringing indication? Indonesia
(2)RS232C serial communication connection method (three-wire system)
First of all, as long as there are data receiving pins and sending pins, serial data transmission can be realized: the receiving pins and sending pins of the same serial port are directly connected through wires, and two serial ports are connected or one serial port is connected with multiple serial ports.
The receiving pin and sending pin of the same serial port are directly connected through wires, and the 9-pin serial port and the 25-pin serial port are directly connected through 2 and 3.
Two different serial ports (whether two serial ports of the same computer or serial ports of different computers)
Serial connection line
DB9-DB9
2-3,3-2,5-5
DB25-DB25
2-3,3-2,7-7
DB9-DB25
2-3,3-2,5-7
DB9-DB25 serial port jumper
2-3,3-2,5-7
The above is a standard serial port for microcomputer, and there are many non-standard devices, such as receiving GPS data or electronic compass data. Just remember one principle: the receiving data pins (or wires) are connected with the sending data pins (or wires), which cross each other and the signals are correspondingly connected.
Several points for attention in serial port debugging;
Different coding mechanisms cannot be mixed. For example, RS232C can't be directly connected with the RS422 interface, and all kinds of special converters are sold in the market, so they can only be connected through converters.
When debugging, prepare debugging tools, such as serial debugging assistant and serial wizard, which will get twice the result with half the effort;
It is strongly recommended not to plug and unplug the serial port with electricity, otherwise it is easy to damage the serial port.
For large-scale industrial control and measurement equipment, it is suggested to use photoelectric coupler to isolate the equipment, and the specific withstand voltage value is determined according to the actual demand.
Computer motherboard serial port
Serial transmission interface, only 1 bit can be transmitted at a time. The serial port can be used to connect an external modem, plotter or serial printer. It can also connect network devices such as routers and switches through console connections, which are mainly used to configure them.
What's the difference between serial interface and parallel interface?
serial interface
Serial interface, COM interface for short, is an extended interface using serial communication protocol. The serial port appears around 1980, and the data transmission rate is115 kbps to 230 kbps. Serial port is generally used to connect mouse and external modem, old camera and tablet. At present, some new motherboards have begun to cancel this interface.
parallel interface
Parallel interface, LPT interface for short, is an extended interface using parallel communication protocol. The data transmission rate of parallel port is 8 times faster than that of serial port, and the data transmission rate of standard parallel port is 1Mbps, which is generally used to connect printers and scanners. So the parallel port is also called the printing port. ? In addition, both serial port and parallel port can be interconnected by direct cable connection, in which data can only be transmitted at low speed. There are basically two types of interfaces between microcomputer host and external devices: serial interface and parallel interface. ? Parallel interface refers to the simultaneous transmission of data bits, which is characterized by high transmission speed. However, when the transmission distance is long and the number of bits is large, the communication line is complicated and the cost increases. Serial communication refers to the sequential transmission of data bit by bit, which is characterized by simple communication lines, two-way communication as long as a pair of transmission lines can be realized, and telephone lines can be used, which greatly reduces the cost, and is especially suitable for long-distance communication, but the transmission speed is slow. ? Serial communication itself is divided into asynchronous communication and synchronous communication. ? Serial communication lines transmit digital signals, and the index of the ability to transmit digital signals is Data rate (data? Rate), its unit is bps(bit? Persecond), which is the binary number transmitted every second. ? Serial interface standard:? At present, the widely used serial interface standard is RS-232-C standard. RS-232-C interface standard adopts 25-pin connector (D socket). RS-232-C stipulates that there are 25 wires.
The above is the introduction of serial port. From the historical development of serial interface, there are two basic ways to divide standards, synchronous serial communication and asynchronous serial communication. There are also applications of serial interface, such as serial interface applied to switches. Then there is the pin description of the serial port, the basic connection of the serial port and the matters needing attention in debugging the serial port, and the setting of the interface of the computer motherboard, all of which are serial ports. It can be seen that serial interface is widely used, and finally there is the difference between serial interface and parallel interface, which can be identified. I hope these introductions are helpful to you.