RS-485 vs RS-232: Serial Terminal Settings Explained

Same DB9 connector, very different electrical rules: the terminal settings that work for one interface can break the other.

A serial terminal hides the electrical layer behind a few port settings, but RS-232 and RS-485 are different physical interfaces with different rules. Plugging an RS-485 device into settings tuned for RS-232, or ignoring bus termination, produces intermittent failures that look like software bugs.

This article compares the two interfaces at the level that matters for terminal configuration: signaling type, topology, distance, and the settings you must get right before any protocol runs.

Single-ended vs differential

RS-232 is single-ended: each signal is referenced to a common ground, and a mark or space is decided by the voltage on one wire relative to that ground. It is inherently point-to-point, one driver talking to one receiver, with separate TX and RX lines for full-duplex.

RS-485 is differential: a logic state is decided by the voltage difference between two wires, conventionally called A and B. Rejecting common-mode noise is what lets it run over long cables in electrically noisy environments, and it supports multi-drop topologies on a single pair.

Distances are typical field figures; the reachable length falls as baud rate rises, and repeaters extend both interfaces beyond the single-segment limits.

Topology and addressing

Because RS-232 is point-to-point, the terminal just needs the two ends matched. RS-485 on a multidrop bus means many devices share one pair, so the protocol layer (Modbus addresses, for example) is what selects the target; the electrical layer only carries the bits.

With many nodes on one bus, the total receiver count is bounded by unit-load limits. Up to about 32 unit loads are supported on a segment without a repeater, and fractional-unit-load transceivers raise that count by design.

Termination and the shield

A long RS-485 line is a transmission line, and unterminated ends reflect the signal back into the bus. The standard fix is a 120 ohm resistor across A and B at each far end of the cable, matching the cable's characteristic impedance.

The shield, if present, should be grounded at a single point to avoid forming a ground loop that injects noise. Grounding both ends of the shield is a common mistake that makes the noise worse rather than better.

Terminal settings that actually matter

Baud rate, data bits, parity, and stop bits are shared concerns for both interfaces and must match the device. RS-485 adds a direction concern: a 2-wire half-duplex link needs the transmitter enabled only while sending, so the terminal or adapter must toggle driver direction at the right moment.

If you see data on transmit but nothing coherent on receive with RS-485, suspect termination and A/B polarity before suspecting the protocol. Swapping A and B rarely damages anything but it produces a dead bus until corrected.

• Match baud / data / parity / stop bits to the device manual.
• Terminate both ends with 120 ohm across A and B on long runs.
• Single-point shield ground to avoid ground loops.
• Check A/B polarity on a half-duplex 2-wire link.

Choosing between them

Use RS-232 for a single nearby device on the bench: it is simple, full-duplex, and needs no bus planning. Reach for RS-485 when you have many devices on one cable run, longer distances, or a noisy environment where the differential receiver earns its keep.

The terminal configuration work is similar in both cases; the difference is that RS-485 asks you to think about the bus as a whole, while RS-232 lets you think about one link at a time.

Once the bus is correct, the next step is reading frames in a Modbus RTU capture; if bytes look wrong, revisit baud and framing, and set up the port in the Windows serial terminal guide.

Common questions

Can I use the same serial cable for RS-232 and RS-485?

Not interchangeably. RS-232 is point-to-point with ground-referenced signals, while RS-485 uses a differential A/B pair and often a transceiver for direction control. The connector may look the same but the wiring is different.

Why do I need 120 ohm termination on RS-485?

A long RS-485 cable behaves like a transmission line; an unterminated end reflects the signal and corrupts bits. A 120 ohm resistor at each far end matches the cable impedance and absorbs the reflection.

How many devices can share one RS-485 bus?

Up to about 32 unit loads on a segment without repeaters. Fractional-unit-load transceivers increase that count by design, and repeaters extend the bus further.

Should I ground the shield at both ends?

No. Ground the shield at a single point. Grounding both ends can form a ground loop that adds noise instead of removing it.

What happens if I swap A and B?

On a half-duplex 2-wire RS-485 link the bus stays silent or garbled until the pair is corrected. Swapping them is generally harmless to the hardware but stops communication.

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