Serial line transmission distance analysis

Serial interface refers to the sequential transmission of data one by one. It is characterized by simple communication lines. As long as a pair of transmission lines can realize two-way communication (you can directly use the telephone line as a transmission line), the cost is greatly reduced. Especially suitable for long-distance communication, but the transmission speed is slow. The communication method in which the data of one piece of information is transmitted in order by bit is called serial communication. The characteristics of serial communication are: the transmission of data bits, in bit order, at least one transmission line can be completed; the cost is low but the transmission speed is slow. The serial communication distance can range from a few meters to several kilometers; according to the direction of information transmission, serial communication can be further divided into simplex, half-duplex and full-duplex.

The two most basic ways of serial communication: synchronous serial communication and asynchronous serial communication.

Synchronous serial is an abbreviation of SPI (Serial Peripheral interface), which is the serial peripheral interface as its name suggests. The SPI bus system is a synchronous serial peripheral interface that allows the MCU to communicate serially with various peripherals to exchange information. The TRM450 is an SPI interface.

Asynchronous serial refers to UART (Universal Asynchronous Receiver/Transmitter), universal asynchronous receiving/sending. The UART is a parallel input that becomes a serial output chip, usually integrated on the motherboard. The UART includes a TTL level serial port and an RS232 level serial port. The TTL level is 3.3V, and RS232 is a negative logic level. It defines +5~+12V as low level, while -12~-5V is high level. MDS2710, MDS SD4, EL805, etc. are RS232 interfaces. The EL806 has a TTL interface.

The serial interface includes 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.

Also known as standard serial port, the most commonly used serial communication interface. It was developed in 1970 by the Electronic Industries Association (EIA) in conjunction with Bell Systems, modem manufacturers, and computer terminal manufacturers 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)". The traditional RS-232-C interface standard has 22 wires, using a standard 25-pin D-type connector (DB25), and later simplified to a 9-pin D-type socket (DB9). In the current application, the 25-pin connector has been rarely used. .

RS-232 adopts an unbalanced transmission method, so-called single-ended communication. Since the difference between the transmission level and the reception level is only about 2V to 3V, the common mode rejection is poor, and the distributed capacitance on the twisted pair is up to about 15 meters, and the maximum rate is 20kb. /s. RS-232 is designed for point-to-point communication (ie, using only a pair of receiving and transmitting devices) with a driver load of 3 to 7 kΩ. So RS-232 is suitable for communication between local devices.

Pt; background: rgb(255,255,255); mso-shading:rgb(255,255,255); " 》, RS485, etc. RS-232-C, RS-422 and RS-485 standards only for interfaces Electrical characteristics are specified and do not involve connectors, cables or protocols.

The full name of the standard is "the electrical characteristics of the balanced voltage digital interface circuit", which defines the characteristics of the interface circuit. A typical RS-422 is a four-wire interface. There is actually a signal ground, a total of 5 lines. Its DB9 connector pin definition. Since the receiver uses high input impedance and the transmission driver has a stronger driving capability than RS232, it is allowed to connect multiple receiving nodes on the same transmission line, and up to 10 nodes can be connected. That is, one master device (Master) and the rest slave devices (Slave), the slave devices cannot communicate with each other, so RS-422 supports point-to-multidirectional two-way communication. The receiver input impedance is 4k, so the maximum load capacity of the transmitter is 10 & TImes; 4k + 100Ω (terminating resistor). Since the RS-422 four-wire interface uses separate transmit and receive channels, there is no need to control the data direction. Any necessary signal exchange between devices can be done in software mode (XON/XOFF handshake) or hardware mode (a pair of separate pairs). Stranded wire) is achieved.

The RS-422 has a maximum transmission distance of 1219 meters and a maximum transmission rate of 10 Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate, and it is possible to reach the maximum transmission distance below the 100 kb/s rate. The highest rate transmission is only possible at very short distances. The maximum transmission rate that can be obtained on a typical 100-meter twisted pair is only 1 Mb/s.

It is based on RS-422, so many electrical regulations of RS-485 are similar to RS-422. If balanced transmission is used, it is necessary to connect the termination resistors on the transmission line. RS-485 can adopt two-wire and four-wire modes. The two-wire system can realize true multi-point two-way communication. When using four-wire connection, it can only achieve point-to-multiple communication like RS-422, that is, there can only be one master ( Master) The device, the rest is a slave device, but it is better than RS-422, and can receive up to 32 devices on the four-wire or two-wire connection bus.

RS-485 differs from RS-422 in that its common mode output voltage is different, RS-485 is between -7V and +12V, and RS-422 is between -7V and +7V, RS-485 receiver The minimum input impedance is 12kΩ and the RS-422 is 4kΩ. Since RS-485 meets all RS-422 specifications, RS-485 drivers can be used in RS-422 networks.

Like RS-422, RS-485 has a maximum transmission distance of approximately 1219 meters and a maximum transmission rate of 10 Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate, and it is possible to use the longest specified cable length below 100 kb/s. The highest rate transmission is only possible at very short distances. Generally, the maximum transmission speed of a 100-meter long twisted pair is only 1 Mb/s.

Nt face=”Arial” 》1219 meters, the maximum transmission rate is 10Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate, and it is possible to reach the maximum transmission distance below the 100 kb/s rate. The highest rate transmission is only possible at very short distances. The maximum transmission rate that can be obtained on a typical 100-meter twisted pair is only 1 Mb/s.

The USB interface is a four-pin interface on the computer motherboard. The middle two pins transmit data, and the two pins on both sides supply power to the peripherals. USB interface is fast, simple, no external power supply, transmission speed 12Mbps, latest USB2.0 up to 480Mbps; cable maximum length 5 meters, USB cable has 4 lines, 2 signal lines, 2 power lines, available 5 volt power supply, USB cable is also divided into two types: shielded and unshielded. Shielded cable transmission speed can reach 12Mbps, the price is more expensive, unshielded cable speed is 1.5Mbps, but the price is cheap; USB can connect up to 127 devices through serial connection. ; Support hot swap.

The serial port describes a lane, and the parallel port has 8 lanes that can transmit 8 bits (one byte) of data at the same time. In this way, the data transmission speed is greatly improved, but it is not said that the parallel port is fast. Due to the mutual interference (crosstalk) between the 8-bit channels, the transmission speed is limited, and the transmission is prone to errors. The serial ports do not interfere with each other. The parallel port sends a large amount of data at the same time, but it is slower than the serial port. Serial hard drives are like this.

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