RS-232, RS-422 or RS-485 — choosing the right COM port

These three standards all use the same nine-pin DB9 connector and look identical. But plug in the wrong one and nothing runs — and worse, some cases run intermittently, which is far harder to track down than a link that is completely dead.

Where they differ

	RS-232	RS-422	RS-485
Devices	1 ↔ 1	1 driver, 10 receivers	up to 32 devices on one line
Distance	~15 m	~1,200 m	~1,200 m
Direction	full duplex	full duplex	half duplex
Noise immunity	poor	good	good
Signal wires	2 + ground	4	2

Choosing by situation

Use RS-232 when

Exactly two devices placed close together, joined by a single cable. There is no third device anywhere in the picture. — One cable, two ends, done. The cable is short because RS-232 cannot go far.

Connecting one device over a short distance, inside a cabinet or on the same bench. Examples: barcode readers, electronic scales, modems, older HMI screens. The simplest option — but do not run it beyond 10–15 metres and do not route it parallel to power cables.

Use RS-485 when

A long trunk line running horizontally, five devices tapped onto it by short stubs. At each end of the trunk sits a termination resistor block. — One trunk, many devices tapped along it. Both ends of the trunk need a resistor — both, not one.

Connecting many devices on the same line, or going far. This is the most common standard in automation: VFDs, power meters, sensors, PLCs — most of them run Modbus RTU over RS-485. Devices are daisy-chained along one twisted pair, each with its own address.

Use RS-422 when

One large device on the left transmitting; the line branches into four legs reaching four smaller devices on the right. The arrows on every branch point one way, left to right. — One transmitter, many receivers at once. The direction is fixed — no role-swapping as in RS-485.

Less common. For one transmitter feeding several receivers simultaneously, with transmission in both directions at the same time. Some measurement instruments and older machine tools use this standard.

Three common RS-485 wiring mistakes

Swapped A/B. Nothing burns, but no data arrives. Swap the two wires back and it works — always try this before suspecting a faulty device.
Missing termination resistors. A long line needs 120Ω resistors at both ends of the trunk. Without them the signal reflects; the symptom is scattered errors at high baud rates or on long runs — the link works, but packets occasionally drop.
No common reference ground. Two devices in two different cabinets can sit at different earth potentials. Without a shared ground wire, the receiver's levels shift and it reads wrong.

Two devices joined by two wires, but the wires cross each other exactly once in the middle. The crossing point is circled. Both devices are intact. — Swapped A/B — nothing burns, there is simply no data.

A trunk line with three devices. The left end has a termination resistor block, the right end is bare, and from that bare end the signal bounces back. — One termination resistor missing — the signal reflects, packets drop intermittently.

Two electrical cabinets standing apart, one device in each, a signal pair running between them. The earth symbols under the two cabinets sit at different heights and no wire joins them. — Two cabinets, two different ground levels, no common ground wire.

Optical isolation — when you need it

A computer on the left, a field device on the right, a vertical barrier between them. The signal line stops at the barrier and a separate line continues on the far side — the two never touch. A surge racing in from the device side stops dead at the barrier. — The isolation barrier severs the electrical path between the two sides. The signal still crosses; the surge does not.

A COM port with optical isolation (isolated) completely separates the computer electrically from the external device. Worth having when:

The cable runs between two cabinets, two buildings, or outdoors
Near VFDs, welders, large motors — strong noise sources
The system has a history of burnt COM ports

Isolation is usually specified with a withstand rating, e.g. 2,500 VDC. If the machine lacks it, a separate external isolator can be added — but having it built into the machine is neater and more durable.

Industrial machines let you select the mode

Most industrial computers list their COM ports as RS-232/422/485 — meaning one port runs all three, with the mode selected in BIOS or software. Convenient, but remember two things: each port runs only one mode at a time, and the mode must be set correctly before wiring.