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What is DB9 RS232 Serial interface ?

What is DB9 RS232 Serial interface ?


Serial interface is referred to as serial port, also known as serial communication interface or serial communication interface, usually refers to COM interface, which is an expansion interface using serial communication.

Serial interface refers to the sequential transfer of data bit by bit.

Its characteristic is that the communication line is simple, as long as a pair of transmission lines can realize two-way communication, and the telephone line can be directly used as the transmission line, thereby greatly reducing the cost, especially suitable for long-distance communication, but the transmission speed is slow.

Chinese name Serial Interface Foreign name Serial Interface Alias ​​usually refers to COM interface, referred to as serial port

Interface division standard

▪ RS-232
▪ RS-422
▪ RS-485

Serial port of the switch

Computer motherboard serial port

The serial port appeared around 1980, and the data transmission rate was 115kbps to 230kbps.

The initial stage of the serial port was to realize the purpose of connecting computer peripherals. In the early stage, the serial port was generally used to connect the mouse and external Modem, as well as the old-fashioned camera and tablet and other devices.

Serial ports can also be used for interconnection and data transmission between two computers (or devices).

Because the serial port (COM) does not support hot-plugging and the transfer rate is low, some new motherboards and most portable computers have begun to cancel this interface. Serial ports are mostly used in industrial control and measurement equipment and some communication equipment.

Synchronous Serial Interface (English: Synchronous Serial Interface, SSI) is a commonly used industrial communication interface.

Asynchronous serial refers to UART (Universal Asynchronous Receiver/Transmitter), Universal Asynchronous Receive/Transmit.

UART is a chip whose parallel input becomes serial output, usually integrated on the motherboard.

UART includes TTL level serial port and RS232 level serial port.

TTL level is 3.3V, while RS232 is negative logic level, which defines +5~+12V as low level, and -12~-5V as high level, MDS2710, MDS SD4, EL805, etc. are RS232 interfaces, EL806 has TTL interface.

Serial interfaces include RS-232-C, RS-422, RS485, etc. according to electrical standards and protocols. The RS-232-C, RS-422 and RS-485 standards only specify the electrical characteristics of the interface and do not involve connectors, cables or protocols.

RS-232, also known as standard serial port, is the most commonly used serial communication interface. It is a standard for serial communication jointly formulated by the Electronic Industries Association (EIA) of the United States in 1970 in conjunction with Bell Systems, modem manufacturers and computer terminal manufacturers.

Its full name is "[1] Serial Binary Data Exchange Interface Technical Standard between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE)".

The traditional RS-232-C interface standard has 22 wires, using a standard 25-pin D-type socket (DB25), which was later simplified to a 9-pin D-type socket (DB9), and now 25-pin sockets are rarely used in applications. .

RS-232 adopts the unbalanced transmission method, which is the so-called single-ended communication. Because the difference between the transmit level and the receive level is only about 2V to 3V, so its common mode rejection capability is poor, coupled with the distributed capacitance on the twisted pair, the maximum transmission distance is about 15 meters, and the maximum rate is 20kb /s.

RS-232 is designed for point-to-point (that is, only one pair of receiving and sending devices) communication, and its driver load is 3 to 7kΩ. So RS-232 is suitable for communication between local devices.
The full name of the RS-422 standard is "Electrical Characteristics of Balanced Voltage Digital Interface Circuits", which defines the characteristics of interface circuits. A typical RS-422 is a four-wire interface. In fact, there is also a signal ground wire, a total of 5 wires. Its DB9 connector pin definition.

Because the receiver adopts high input impedance and the transmission driver has stronger driving ability than RS232, it allows to connect multiple receiving nodes on the same transmission line, up to 10 nodes can be connected. That is, one master device (Master), the rest are slave devices (Slave), and the slave devices cannot communicate with each other, so RS-422 supports point-to-multiple two-way communication.

The input impedance of the receiver is 4k, so the maximum load capacity of the transmitter is 10×4k+100Ω (terminating resistance). Since the RS-422 four-wire interface adopts separate sending and receiving channels, it is not necessary to control the data direction. Any necessary signal exchange between devices can be performed in software (XON/XOFF handshake) or hardware (a pair of separate dual stranded wire) is implemented.

The maximum transmission distance of RS-422 is 1219 meters, and the maximum transmission rate is 10Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate, and the maximum transmission distance can only be achieved when the rate is below 100kb/s. The highest transmission rates are only obtained over very short distances. Generally, the maximum transmission rate that can be obtained on a twisted pair line with a length of 100 meters is only 1Mb/s.

RS-485 is developed on the basis of RS-422, so many electrical regulations of RS-485 are similar to RS-422. If the balanced transmission method is adopted, a terminating resistor needs to be connected to the transmission line. RS-485 can use two-wire and four-wire mode, and the two-wire system can realize true multi-point two-way communication. When using four-wire connection, it can only realize point-to-multiple communication like RS-422, that is, there can only be one master ( Master) device, the rest are slave devices, but it is an improvement over RS-422, no matter the four-wire or two-wire connection mode, more than 32 devices can be connected to the bus.

The difference between RS-485 and RS-422 is that its common mode output voltage is different, RS-485 is between -7V to +12V, while RS-422 is between -7V to +7V, RS-485 receiver The minimum input impedance is 12kΩ and RS-422 is 4kΩ; since RS-485 meets all RS-422 specifications, the RS-485 driver can be used in RS-422 networks.

Like RS-422, RS-485 has a maximum transmission distance of about 1219 meters and a maximum transmission rate of 10Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate. Only when the rate is below 100kb/s can the specified longest cable length be used. The highest transmission rates are only obtained over very short distances. Generally, the maximum transmission rate of a 100-meter-long twisted pair is only 1Mb/s.


RS232 level serial port Cable

RS-232 Serial interface Cable

RS-422 Serial interface Cable

RS485 Serial interface Cable

UART TTL level serial port Cable



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