What are the RS-422 and RS-485 serial interface standards?
1. Balanced transmission
RS-422, RS-485 and RS-232 are different, the data signal uses differential transmission, also known as balanced transmission, it uses a pair of twisted pairs, one of which is defined as A, the other is defined as B
Normally, the positive level between the sending drivers A and B is +2~+6V, which is a logic state, and the negative level is -2~6V, which is another logic state. There is another signal ground C, there is an "enable" terminal in RS-485, and this is available or not 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 active, the sending driver is in a high-impedance state, called the "third state", that is, it is a third state different from logic "1" and "0".
The receiver also makes a regulation relative to the sending end. The receiving and sending ends connect AA and BB through balanced twisted pairs. When there is a level greater than +200mV between the receiving end AB, the output is a positive logic level, less than -200mV , Output negative logic level. The level range of the receiver receiving balance line is usually between 200mV and 6V.
2. RS-422 electrical regulations
The full name of the RS-422 standard is "Electrical Characteristics of Balanced Voltage Digital Interface Circuit", which defines the characteristics of the interface circuit. Figure 2 is a typical RS-422 four-wire interface. There is actually a signal ground wire, a total of 5 wires. Figure 1 is the pin definition of its DB9 connector. Because the receiver uses high input impedance and the transmission driver has a stronger driving capability than RS232, it allows multiple receiving nodes to be connected on the same transmission line, and up to 10 nodes can be connected. That is, a master device and the rest are slave devices. The slave devices cannot communicate, so RS-422 supports point-to-many bidirectional communication. The input impedance of the receiver is 4k, so the maximum load capacity of the sending end is 10×4k+100Ω (termination resistance). Because the RS-422 four-wire interface uses separate transmission and reception channels, there is no need to control the data direction. Any necessary signal exchange between the devices can be software (XON/XOFF handshake) or hardware (a pair of separate dual Twisted wire). The maximum transmission distance of RS-422 is 4000 feet (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 maximum transmission distance can only be reached below the 100kb/s rate. Only at short distances can the highest rate transmission be achieved. 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, which requires its resistance to be approximately equal to the characteristic impedance of the transmission cable. Termination resistors are not required for moment distance transmission, that is, termination resistors are generally not required below 300 meters. The termination resistor is connected to the farthest end of the transmission cable.
3. RS-485 electrical regulations
Because RS-485 is developed on the basis of RS-422, many electrical regulations of RS-485 are similar to RS-422. If balanced transmission methods are used, terminating resistors need to be connected to the transmission line. RS-485 can adopt two-wire and four-wire methods, and the two-wire system can realize true multi-point two-way communication.
When using four-wire connection, it can only achieve point-to-many communication like RS-422, that is, there can be only one master device, and the rest are slave devices, but it is improved over RS-422, whether it is four-wire or 32 devices can be connected to the bus in the two-wire connection mode.
The difference between RS-485 and RS-422 is that the 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 12k. The S-422 Sword is 4k. The S-485 in the old gorge meets all RS-422 specifications, so the RS-485 driver can be used in the RS-422 network.
RS-485 is the same as RS-422, its maximum transmission distance is about 1219 meters, and its maximum transmission rate is 10Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate. Only under the 100kb/s rate can the specified maximum cable length be used. Only at short distances can the highest rate transmission be achieved. Generally, the maximum transmission rate of a 100-meter long twisted pair is only 1Mb/s.
RS-485 requires 2 termination resistors, whose resistance value is equal to the characteristic impedance of the transmission cable. Termination resistors are not required for moment distance transmission, that is, termination resistors are generally not required below 300 meters. The termination resistor is connected at both ends of the transmission bus.
If you want to know more, our website has product specifications for RS-422 and RS-485 serial interface, you can go to ALLICDATA ELECTRONICS LIMITED to get more information