RS-422 serial interface standard

rain

RS-422 serial interface standard [Copy link]

RS-422 serial interface standard
1. Balanced transmission
RS-422 data signals adopt differential transmission, also known as balanced transmission. It uses a pair of twisted pairs, one of which is defined as A and the other as B. Under normal circumstances, the positive level between the transmission driver A and B is between +2 and +6V, which is a logical state, and the negative level is between -2 and 6V, which is another logical state. There is also a signal ground C. In RS-485, there is also an "enable" terminal, which is optional in RS-422. The "enable" terminal is used to control the disconnection and connection of the transmission driver and the transmission line. When the "enable" terminal is in effect, the transmission driver is in a high-impedance state, called the "third state", that is, it is a third state different from the logical "1" and "0".
The receiver is also defined opposite to the transmitter. The receiving and transmitting ends connect AA and BB correspondingly through a balanced twisted pair. When there is a level greater than +200mV between AB at the receiving end, a positive logic level is output, and when it is less than -200mV, a negative logic level is output. The voltage level on the receiver receiving balanced line usually ranges from 200mV to 6V.

2. RS-422 electrical regulations
The full name of the RS-422 standard is "Electrical Characteristics of Balanced Voltage Digital Interface Circuits", 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, a total of 5 wires. Its DB9 connector pin definition. Since 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, up to 10 nodes. That is, one master device (Master) and the rest are slave devices (Salve). The slave devices cannot communicate with each other, so RS-422 supports point-to-multiple bidirectional communication. The receiver input impedance is 4k, so the maximum load capacity of the transmitting end is 10×4k+100Ω (termination resistance). Since the RS-422 four-wire interface uses separate transmission and reception channels, it is not necessary to control the data direction. Any necessary signal exchange between devices can be implemented in software mode (XON/XOFF handshake) or hardware mode (a pair of separate twisted pairs).
The maximum transmission distance of RS-422 is 1219 meters, and the maximum transmission rate is 10Mb/s. The length of its balanced twisted pair is inversely proportional to the transmission rate. The maximum transmission distance can only be achieved at a rate below 100kb/s. The highest transmission rate can only be obtained at a very short distance. Generally, the maximum transmission rate that can be obtained on a 100-meter long twisted pair is only 1Mb/s.
RS-422 requires a terminating resistor, and its resistance is required to be approximately equal to the characteristic impedance of the transmission cable. The terminating resistor is not required for short-distance transmission, that is, generally no terminating resistor is required below 300 meters. The terminating resistor is connected to the farthest end of the transmission cable.