نُشر في 13 نوفمبر 2025

RS-232, RS-422 and RS-485: Why Serial Still Runs Modern Industry

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Every few years someone declares serial communication obsolete, and every year industrial PCs ship with six COM ports because customers keep asking for them. The reason is simple: a PLC commissioned in 2006 is still running a production line in 2025, and it talks RS-485. The interface did not survive out of nostalgia. It survived because it does a specific job extremely well.

Three Standards, Two Ideas

The confusion around serial comes from treating RS-232, RS-422 and RS-485 as three versions of the same thing. They are not. There are two underlying electrical ideas, and the three standards distribute themselves across them.

RS-232 is single-ended. Each signal is one wire measured against a shared ground. A voltage swing between roughly ±3 V and ±15 V carries the bit. It is simple and has been on computers since the 1960s.

The limitation is inherent: if the signal is measured against ground, then any electrical noise that appears between the two ends corrupts it. On a factory floor, with motor drives and contactors switching nearby, that is a real constraint. RS-232 is a point-to-point interface for short runs — 15 metres is the classic figure, less in a noisy plant.

RS-422 and RS-485 are differential. Each signal travels as a voltage difference between two wires in a twisted pair. The receiver reads the difference, not the absolute level. Noise induced on the cable appears almost equally on both conductors, so the difference is unchanged and the noise cancels.

That single design decision is why differential signalling reaches 1200 metres where RS-232 reaches fifteen, and why it works next to a variable-frequency drive.

What Separates RS-422 from RS-485

Both are differential. The difference is who may transmit.

RS-422 is full duplex with one driver. Two pairs, one for each direction, both ends transmitting simultaneously. One transmitter can address up to ten receivers, but only that one transmitter exists on the bus.

RS-485 allows multiple drivers on the same bus, typically up to 32 standard unit loads, with modern low-load transceivers supporting considerably more. Devices take turns: one talks, the rest listen, and a protocol arbitrates whose turn it is. Two-wire RS-485 is half duplex on a single pair; four-wire variants exist for full duplex.

This multi-drop capability is the whole reason RS-485 dominates industrial fieldbus work. One twisted pair, daisy-chained from device to device down a production line or up a building riser, connecting dozens of meters, drives or controllers to a single master — with a fraction of the cable, the conduit and the labour that individual runs would need.

A Quick Comparison

Property	RS-232	RS-422	RS-485
Signalling	Single-ended	Differential	Differential
Typical max distance	15 m	1200 m	1200 m
Devices on the bus	2	1 driver, 10 receivers	32+ drivers/receivers
Duplex	Full	Full	Half (2-wire) or full (4-wire)
Typical use	Local console, instrument	Long point-to-point link	Multi-drop fieldbus

The Protocols That Ride on Top

The standards above define electricity, not meaning. The protocol layered over them is what carries data.

Modbus RTU over RS-485 is the most widely deployed industrial protocol in existence. The master polls each slave address in turn; the addressed device replies. It is simple, well documented and implemented by nearly every industrial device ever built.

M-Bus — Meter-Bus, EN 13757 — is purpose-built for utility metering. Water, gas, heat and electricity meters chained onto a single pair, with the bus itself powering the slaves.

BACnet MS/TP runs building automation on the same physical layer, connecting controllers, thermostats and sensors across a site.

DMX512, which lights stages and architectural installations worldwide, is RS-485 underneath.

The Three Things That Break RS-485 Installations

Long RS-485 runs fail for the same three reasons nearly every time.

Termination. A transmission line needs matching resistors — typically 120 Ω — at both physical ends of the bus, and only at the ends. Without them, signals reflect off the unterminated end and interfere with the data. Terminating in the middle of a run, or at every device, is equally wrong and equally common.

Topology. RS-485 is a daisy chain, not a star. Long spur cables off a central point create reflections that no amount of termination fixes. Wire it as a single line passing through each device.

Biasing and ground. Idle bus lines need bias resistors to hold them in a defined state, or receivers see noise as data. And although the signal is differential, the transceivers still need their grounds within a common-mode range of each other — across a large site, a third conductor or a shield connection at one point is what keeps them there. Isolated ports solve this properly where ground potentials genuinely differ.

What to Look for on the Datasheet

When serial matters, read the specification closely:

Configurable modes. Many industrial PCs offer COM1 and COM2 as RS-232/422/485 selectable, with the remaining ports fixed at RS-232. Our IBOX-1226 and IBOX-1326 follow this pattern with six COM ports, of which the first two are switchable.
Native versus USB. Onboard ports driven by a real UART behave predictably under timing-sensitive protocols. USB-to-serial adapters introduce latency and can reorder or drop bytes under load — enough to break Modbus timing on a busy bus.
Isolation. Ports with galvanic isolation protect the computer from surges and ground loops. On long outdoor runs this is not optional.
Port count. Serial ports run out faster than expected. The IBOX-101PLUS and IBOX-708 offer six, the ITPC-A600 panel PC supports up to six as an option.

Serial Is Not Legacy, It Is Infrastructure

The equipment it connects has a service life measured in decades, and replacing a working meter or drive because its interface is unfashionable makes no economic sense. Modern industrial PCs act as the bridge: serial on one side, Ethernet and cloud connectivity on the other.

If your project needs a specific mix of RS-232, RS-422 and RS-485 — isolated or not — tell us the count and the protocol, and we will confirm which models provide it natively rather than through an adapter.

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