summarize
RS-422 (EIA RS-422-A Standard) is a standard for Apple's Macintosh computers.Serial Connection Standard The full name is "Balanced Voltage Digital Interface Circuit" and its electrical characteristics: RS-422 uses differential signals (balanced transmission).RS-232Signals using an unbalanced reference ground. Differential transmission uses two wires to send and receive signals, which provides better noise immunity and longer transmission distances than RS-232. Better noise immunity and longer transmission distances are a big advantage in industrial environments.
RS-422 has been improved to produce aRS-485(EIA-485 standard), RS-485 increases the number of devices from 10 to 32 and defines the electrical characteristics at the maximum number of devices to ensure sufficient signal voltage.
Features of RS-422
- The maximum transmission rate of RS-422 is 10Mb/s, and the maximum transmission rate of 100 meters of twisted-pair cable is 1Mb/s (the length of its balanced twisted-pair cable is inversely proportional to the transmission rate).
- Maximum transmission distance is 1200 meters (400 feet), but requires a relatively low transmission rate
- Point-to-multipoint full-duplex bidirectional communication with one master and multiple slaves
- Includes master device, maximum number of nodes is 10
- Long-distance transmission requires a matching resistor of 100 ohms at the farthest end of the cable (related to the characteristic impedance of the transmission cable); if the distance is less than 300 meters, the matching resistor can be left out.
RS-422 Interface Definition
From the figure below, RS-422 as well as RS-485 data are transmitted using differential transmission, also known as balanced transmission, using a pair of twisted-pair wires (4 wires).RS-422 is used less nowadays due to the fact that it is easy to make a mistake by using 4 wires, so most of them use RS-485 instead.
Comparison of three serial interfaces
| sports event | RS-232 | RS-422 | RS-485 |
| transmission method | single-ended | increment | increment |
| Number of nodes | 1 receive, 1 transmit | 1 send, 10 receive | 1 send, 32 receive |
| Maximum transmission distance | 15 meters | 1200 meters | 1200 meters |
| Maximum transmission rate | 200kb/s | 10Mb/s | 10Mb/s |
| Maximum Drive Output Voltage | +/- 25V | -0.25V-+6V | -7V-+12V |
| Output Signal Level (min. with load) | +/- 5V-+/- 15V | +/- 2.0V | +/- 1.5V |
| Output signal level (no load max.) | +/- 25V | +/- 6V | +/- 6V |
| Driver Load Impedance | 3KΩ-7KΩ | 100Ω | 54Ω/120Ω |
| Oscillation rate (maximum) | 30V/us | inapplicable | inapplicable |
| Receiver Input Voltage Range | +/- 15V | -10V-+10V | -7V-+12V |
| Receiver Input Threshold | +/- 3V | +/- 200mV | +/- 200mV |
| Receiver Input Impedance | 2KΩ-7KΩ | 4KΩ | >=12KΩ |
| Driver Common Mode Voltage | inapplicable | -3V-+3V | -1V-+3V |
| Receiver Common Mode Voltage | inapplicable | -7V-+7V | -7V-+12V |
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