RS485 vs RS422: which differential bus for Modbus?

The rs485 vs rs422 question comes up the moment you wire a differential serial link and find two standards that look almost identical on paper. Both carry their signal as the difference between two wires, both reach well over a kilometre, and both use the same serial settings. Yet Modbus RTU consistently chooses RS485 and not RS422, and that is no accident. The difference is how many devices may drive the line and which way traffic flows, not distance. This guide compares RS485 and RS422 the way you meet them in the field: at an energy meter, an inverter, a legacy drive, or an encoder.

You will read how many drivers and receivers each standard allows, why only RS485 forms a true multidrop bus, whether distance matters, which one Modbus expects, and when RS422 stays the clean choice.

Key points

• RS485 (TIA/EIA-485-A) is a multipoint bus: up to 32 unit loads on one pair, any node can drive, 2-wire half-duplex or 4-wire full-duplex.
• RS422 (TIA/EIA-422-B) has exactly one driver and up to 10 receivers, always full-duplex over 4 wires. The driver stays on.
• Modbus RTU expects RS485, because several slaves must answer in turn on the same wires. Distance does not decide: both reach about 1200 m.

Quick verdict: which one do you need?

RS485 and RS422 are both differential physical-layer standards. They define voltage levels and wiring, not the meaning of the data. Choose on the number of drivers and the direction, not on distance or speed.

• Choose RS485 when several devices must talk on one bus, such as a Modbus RTU field network of meters, inverters, or heat pumps. Each node drives the same pair in turn.
• Choose RS422 when one source streams continuously and simultaneously to one or more fixed receivers, such as an encoder, a servo interface, or a display link, and you want to keep the simultaneous send and receive of full-duplex.

At a glance: RS485 vs RS422 compared

The table below lays out the specifications installers ask about most. The values come from the TIA-485-A and TIA-422-B standards and from the Texas Instruments overview note (SLLA070D).

What is the real difference between RS485 and RS422?

The real difference between RS485 and RS422 is the number of drivers a line can carry. RS422 defines exactly one driver per line plus up to 10 receivers (unit loads; one unit load is 4 kOhm). RS485 lifts that to 32 unit loads on a bus, and any node may drive. According to the Renesas application note (AN1989), that works because RS485 drivers are tri-state: they go high-impedance the moment they stop transmitting, so several drivers can share the same pair without shorting each other.

An RS422 driver cannot do this. It stays on, so only one device can talk on a line. That is why the standard describes one driver broadcasting to receivers, not a bus where everyone transmits in turn. A second difference is driver strength: Analog Devices (AN-960) quotes a minimum of +/-55 mA for RS485 against +/-20 mA for RS422. An RS485 driver can therefore drive an RS422 application, but not the other way around.

Does RS422 reach the same distance as RS485?

Yes, distance is not a distinguishing criterion. Both standards are differential and reach up to 10 Mbit/s over short runs and up to about 1200 m at lower bit rates. Texas Instruments describes the same curve for RS485 in SLLA272: below about 100 kbps the ceiling stays around 1200 m, and above it the maximum length falls roughly inversely with the bit rate, down to about 120 m at 1 Mbps. RS422 follows the same curve in practice, because it is the same differential electrical family.

Anyone hesitating between RS485 and RS422 on distance is asking the wrong question. Both bridge the same metres. Cable choice does matter for that distance, which is why we wrote a separate RS485 cable selection guide that applies to RS422 as well.

Which one does Modbus use, RS485 or RS422?

Modbus chooses RS485. The modbus.org Modbus over Serial Line Specification and Implementation Guide V1.02 names RS485 two-wire as the required interface for a Modbus serial line, names RS485 four-wire as an option, and does not name RS422 as a Modbus bus. The reason follows directly from the difference above. A Modbus RTU master polls its slaves one by one, and each slave answers in turn on the same wires. That needs several tri-state drivers on one line, so RS485.

On RS422 only one device can drive the return line, so only one slave could ever answer. A four-wire Modbus bus therefore also uses RS485 transceivers on the slave return pair, not RS422. An RS485 segment carries 32 unit loads, in practice one master and up to 31 slaves; with fractional-unit-load transceivers or a repeater you extend that to 256 nodes. Each Modbus address runs from 1 to 247, as described in Modbus RTU explained. This is separate from the choice between Modbus RTU and Modbus TCP, which is about how the message is packaged, not about the wire.

When is RS422 the better fit?

RS422 is the better fit when one source has to stream continuously and simultaneously to fixed receivers, without taking turns. Because the driver stays on, there is no bus arbitration and no turnaround time between transmit and receive. That makes RS422 strong for:

• encoder and servo feedback, where position must arrive continuously and without delay;
• long full-duplex point-to-point links where transmit and receive happen at the same time;
• equipment that specifies RS422 explicitly, such as some industrial drives, measurement instruments, and display buses.

In short, RS422 excels at a fixed, simultaneous stream from one source to listeners. As soon as you want to read several devices on a shared line, you switch to RS485. For the comparison with the older single-ended serial world, also read RS485 vs RS232.

When a gateway is the answer

In existing installations you almost always find an RS485 Modbus bus, and sometimes a lone RS422 device beside it. To see that data in the cloud, a Modbus gateway is the clean bridge. On the serial side it reads the RS485 bus; on the other side it publishes the registers as Modbus TCP over Ethernet or sends them outbound to a cloud platform. The RS485 wiring guide covers the wiring of that bus in detail.

Frequently asked questions

Conclusion

RS485 and RS422 are not rivals but two answers to different questions. RS422 is the full-duplex link for a source that streams continuously and simultaneously to fixed receivers, and keeps its place on encoders, drives, and display lines. RS485 is the multipoint bus where up to 32 nodes transmit in turn over a terminated pair, which is why the Modbus serial specification is built around it. Distance never chooses between the two, because both reach the same roughly 1200 m. To lift an existing RS485 Modbus bus to the cloud, a gateway with outbound TLS is the clean bridge that leaves your wiring exactly as it is.