What Is RS-422?

RS-422 is a serial communication standard, published as TIA/EIA-422, that sends each signal as the voltage difference between two wires rather than as a voltage against ground. It was designed in the 1970s to extend serial links far beyond the short reach of RS-232, and it is still used wherever a controller has to talk to a device across a plant, a yard or a building.

Because the receiver compares two wires, interference that hits both equally cancels out. That lets RS-422 run at around 100 kbit/s over 1,200 m of twisted pair, or up to 10 Mbit/s over about 12 m. Like other serial standards, it defines only the electrical layer: the data format and protocol are set by the devices at each end.

Each RS-422 signal uses a twisted pair, usually labeled A and B or + and −. The driver pushes one wire up and the other down, producing a difference of at least 2 V. The receiver reads a 1 or a 0 from which wire is higher, and needs only around 200 mV of difference to decide, which leaves a wide margin for losses and noise along the cable.

A standard RS-422 link uses two pairs: one for transmit and one for receive, so data flows in both directions at once. A signal ground between the devices keeps them within the receivers’ common-mode range. Shielded twisted pair cable with an impedance of around 100 to 120 ohm suits the job.

RS-422 and RS-485 share the same differential idea and reach the same distances, which is why they are often confused. The difference is how many devices can transmit.

An RS-485 transceiver can usually drive and receive RS-422 when wired for four wires, which is why most industrial serial ports offer both modes. The reverse is not true: RS-422 drivers cannot share a bus. Our comparison of RS-232 and RS-485 sets out how the short-range RS-232 standard differs from both.

RS-422 was written as the long-distance partner to RS-232, and the two are easy to tell apart. RS-232 measures each signal against ground on a single wire, swings to around ±5 to ±12 V and is reliable to roughly 15 m between two devices. RS-422 uses a pair of wires per signal, much lower voltages and a sensitive differential receiver, so it reaches around 80 times further and can feed several receivers from one driver. RS-232 still has the advantage of being built into many instruments and console ports, which is why converters between the two remain common on older sites.

RS-422 suits any link that is too long or too noisy for RS-232 but does not need several transmitters on one bus:

The standard defines the electrical signals but not a connector, so pinouts vary between makers. Industrial computers commonly use a DB9 connector for RS-232/422/485 ports, with the transmit pair (TX+ and TX−), the receive pair (RX+ and RX−) and ground on pins that differ by model. Terminal blocks are common on I/O modules. Always check the device’s manual: swapping the + and − wires of a pair inverts the data, and is the most frequent wiring fault.

Our explainer on the serial port covers COM port settings and connectors. Where a computer needs extra or isolated ports, our serial I/O modules add RS-232/422/485 on mPCIe or PC/104, and the British Steel case study shows protocol-selectable serial on rugged panel PCs.

We fit RS-232/422/485 ports and isolated serial modules to industrial computers as standard, set the port modes and test each one before the system ships. Tell us the devices and cable runs, and we will specify the serial ports to match.