Standards for dividing serial interfaces

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

Synchronous serial refers to the abbreviation of ISP (interface Serial Peripheral), as the name implies, is the serial peripheral interface. The ISP bus system is a synchronous serial peripheral interface, which enables the MCU to communicate with various peripheral devices in a serial manner to exchange information. TRM450 is an ISP interface.

Asynchronous serial refers to UART (Universal Asynchronous Receiver/Transmitter), universal asynchronous receiving/transmitting. UART is a chip whose parallel input becomes serial output, which is 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. It defines +5~+25V as low level and -3~-25V as high level. MDS2710, MDS SD4, EL805, etc. are RS232 interfaces. 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.

RS-232 also called standard serial port, the most commonly used serial communication interface. It is a standard for serial communication formulated in 1970 by the Electronic Industries Association (EIA) of the United States in conjunction with Bell Systems, modem manufacturers and computer terminal manufacturers. 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, using a standard 25-pin D-type plug socket (DB25), and later simplified to a 9-pin D-type socket (DB9). Nowadays, 25-pin plug sockets are rarely used in applications. .

RS-232 adopts unbalanced transmission mode, the so-called single-ended communication. Since the difference between the sending level and the receiving level is only about 2V to 3V, 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 a pair of receiving and transmitting devices) communication, and its driver load is 3-7kΩ. So RS-232 is suitable for communication between local devices.

The full name of the RS422 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. There is actually 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 capability than RS232, it is allowed to connect multiple receiving nodes on the same transmission line, up to 10 nodes can be connected. That is, a master device (Master), and the rest are slave devices (Slave). The slave devices cannot communicate, so RS-422 supports point-to-many two-way communication. The receiver input impedance is 4k, so the maximum load capacity of the transmitter is 10×4k+100Ω (terminating resistance). The RS-422 four-wire interface adopts separate sending and receiving channels, so there is no need to control the data direction. Any necessary signal exchange between the devices can be done in software (XON/XOFF handshake) or hardware (a pair of separate dual Stranded wire) to achieve.

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 is only possible when the rate is below 100kb/s. Only in a short distance can the highest rate of transmission be obtained. Generally, the maximum transmission rate that can be obtained on a 100-meter-long twisted pair cable is only 1Mb/s.

RS485 is developed on the basis of RS-422, so many electrical regulations of RS-485 are similar to RS-422. If all adopt the balanced transmission method, all need to connect the terminating resistor on the transmission line, etc. RS-485 can use two-wire and four-wire methods. The two-wire system can realize true multi-point two-way communication. When four-wire connection is adopted, only point-to-many communication can be realized 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, 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 and +12V, while RS-422 is between -7V and +7V. RS-485 receiver The minimum input impedance is 12kΩ and RS-422 is 4kΩ; because RS-485 meets all RS-422 specifications, RS-485 drivers can be used in RS-422 network applications.

RS-485 is the same as RS-422, its maximum transmission distance is about 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 longest cable length may be used when the rate is below 100kb/s. Only in a short distance can the highest rate of transmission be obtained. Generally, the maximum transmission rate of a 100-meter-long twisted pair is only 1Mb/s

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