RS232/RS422/RS485 communication interface difference - Database & Sql Blog Articles

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The RS-232, RS-422, and RS-485 standards only specify the electrical characteristics of the interface, and do not involve connectors, cables, or protocols. On this basis, users can establish their own high-level communication protocols. For example, the video server has multiple RS422 serial communication interfaces, and each interface can be recorded and played by an external computer through the RS422 communication line. In addition to providing various control hardware interfaces, the video server also provides a protocol interface. For example, the RS422 interface supports the RS422 profile protocol, and also supports protocols controlled by RS422 such as Louth, Odetics, and BVW.
RS-232, RS-422 and RS-485 are serial data interface standards, which were developed and released by the Electronic Industries Association (EIA). RS-232 was released in 1962. RS-422 was developed from RS-232. To improve the shortcomings of short RS-232 communication and low speed, RS-422 defines a balanced communication interface that increases the transmission rate to 10 Mbps and the transmission distance to 4000 ft ( The rate is below 100Kbps) and allows up to 10 receivers to be connected on a balanced bus. RS-422 is a one-way, balanced transmission specification for single-machine transmission and multi-machine reception. It is named TIA/EIA-422-A standard. In order to expand the scope of application, EIA developed the RS-485 standard based on RS-422 in 1983, adding multi-point, two-way communication capability, that is, allowing multiple transmitters to connect to the same bus, and adding a transmitter. The drive capability and conflict protection features extend the bus common-mode range and are later named TIA/EIA-485-A.
1. S-232 Serial Interface Standard RS-232 is currently the most widely used serial interface in the PC and communications industries. RS-232 is defined as a single-ended standard that increases communication distance in low-rate serial communications. RS-232 adopts an unbalanced transmission method, so-called single-ended communication. The data signals at the receiving and transmitting ends are relative to the signal ground. A typical RS-232 signal swings between positive and negative levels. When transmitting data, the output of the transmitter driver outputs a positive level of +5 to +15V and a negative level of -5 to -15V. When there is no data transmission, the line is TTL, from the beginning of the data transmission to the end, the line level returns from the TTL level to the RS-232 level and then back to the TTL level. Typical operating levels of the receiver are +3~+12V and -3~-12V. 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 about 15 meters at the maximum, and the maximum rate is 20Kbps. RS-232 is designed for point-to-point communication (that is, using only a pair of receiving and transmitting devices), and its driver load is 3kΩ~7kΩ. So RS-232 is suitable for communication between local devices.
2. RS-422 and RS-485 serial interface standard (1) balanced transmission
RS-422 and RS-485 are different from RS-232. The data signal uses differential transmission, also called balanced transmission. It uses a pair of twisted pairs, one of which is defined as A and the other is defined as B. Normally, the positive level between the transmit drivers A and B is +2~+6V, which is a logic state, and the negative level is -2V~6V, which is another logic state. There is also a signal ground C, which has an "enable" end in RS-485, which is available and not available in RS-422. The "Enable" terminal is used to control the disconnection and connection of the transmit driver and the transmission line. When the "enable" terminal is active, the transmit driver is in a high-impedance state, referred to as the "third state", ie it is the third state distinct from the logic "1" and "0".
(2) RS-422 electrical regulation Because the receiver uses 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, and up to 10 nodes can be connected. That is, one master device (Master) and the rest slave devices (Salve), the slave devices cannot communicate with each other, so RS-422 supports point-to-multidirectional two-way communication. 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 4,000 feet (about 1219 meters) and a maximum transmission rate of 10 Mbps. 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 100 Kbps. 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 Mbps. RS-422 requires a terminating resistor that requires a resistance equal to approximately the characteristic impedance of the transmission cable. There is no need to terminate the resistor when transmitting at the moment distance, that is, the resistor is not required to be terminated below 300 meters. The terminating resistor is connected to the farthest end of the transmission cable. (3) RS-485 electrical regulations Since RS-485 is developed from RS-422, many electrical regulations of RS-485 are similar to those of RS-422. If balanced transmission is used, it is necessary to connect the termination resistors on the transmission line. RS-485 can be used in two-wire and four-wire modes, and two-wire system can realize true multi-point two-way communication. The RS-485 bus is widely used in the RS-485 serial bus standard when the communication distance is required to be several tens of meters to several kilometers. RS-485 uses balanced transmit and differential receive, so it has the ability to reject common-mode interference. In addition to the high sensitivity of the bus transceiver, it can detect voltages as low as 200mV, so the transmitted signal can be recovered beyond the kilometer. RS-485 uses a half-duplex mode of operation, and only one point can be sent at any time. Therefore, the transmitting circuit must be controlled by an enable signal. RS-485 is very convenient for multi-point interconnection and can save many signal lines. Applications RS-485 can be networked to form a distributed system that allows up to 32 drives and 32 receivers to be connected in parallel. RS-485 is different 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 meets all RS-422 specification, so 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 Mbps. The length of the balanced twisted pair is inversely proportional to the transmission rate. Below 100Kbps, the longest specified cable length is possible. 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 Mbps.