RS-232 vs RS-422 vs RS-485

RS-232, RS-422, and RS-485 are three serial communication standards that solve different problems. RS-232 connects one device to another over short distances. RS-422 connects one transmitter to up to ten receivers over long distances with full-duplex differential signaling. RS-485 connects many devices on a shared bus over long distances with half-duplex communication. Understanding when to use each one determines which mcserial tools your MCP assistant will reach for.

Physical layer

The fundamental difference is how voltage signals are represented on the wire.

RS-232 uses single-ended signaling: each signal is measured as a voltage relative to ground. A logic HIGH is -3V to -15V, and a logic LOW is +3V to +15V (inverted from what you might expect). This means every signal needs its own wire plus a shared ground, and the voltage swings are large.

RS-422 uses differential signaling on two pairs of wires: one pair for transmit (TX+/TX-) and one pair for receive (RX+/RX-). The receiver reads the voltage difference between each pair, ignoring common-mode noise. Because both pairs are always active, RS-422 is full-duplex — you can send and receive simultaneously, just like RS-232, but with much better noise immunity and cable length.

RS-485 uses differential signaling on a single pair of wires (or two pairs for full-duplex, but half-duplex is far more common). The receiver reads the voltage difference between the two wires, ignoring any common-mode noise that affects both wires equally. Because the bus is shared, devices take turns transmitting.

Property	RS-232	RS-422	RS-485
Signaling	Single-ended (voltage to ground)	Differential (two pairs)	Differential (one or two pairs)
Voltage levels	+/-3V to +/-15V	+/-2V to +/-6V differential	+/-1.5V to +/-6V differential
Max cable length	~15 meters (50 feet)	~1200 meters (4000 feet)	~1200 meters (4000 feet)
Max devices	2 (point-to-point)	1 transmitter, 10 receivers	32 standard, 256 with high-impedance receivers
Duplex	Full-duplex (separate TX and RX lines)	Full-duplex (separate TX and RX pairs)	Half-duplex on 2 wires, full-duplex on 4 wires
Noise immunity	Low (susceptible to ground loops)	High (differential rejection)	High (differential rejection)
Typical connectors	DB-9, DB-25	Screw terminals, DB-9, RJ-45	Screw terminals, RJ-45, DB-9 (repurposed)

Communication models

RS-232: point-to-point

RS-232 is always a conversation between two devices. One device (the DTE, Data Terminal Equipment — originally a terminal or computer) talks to another device (the DCE, Data Communications Equipment — originally a modem). They have separate transmit and receive lines, so both can send data simultaneously (full-duplex).

RS-232 also defines modem control lines — dedicated signals for hardware flow control, device presence, and status:

• DTR / DSR: “I’m here and ready” handshake
• RTS / CTS: “I want to send” / “Go ahead” flow control
• RI: “There’s an incoming call” (modems)
• CD: “I have a connection” (modems)

These extra lines are what make RS-232 useful for tasks beyond raw data transfer: you can detect whether a device is connected, pause data flow when a buffer fills up, or pulse DTR to reset a microcontroller.

RS-422: differential point-to-point

RS-422 is a point-to-point or point-to-multipoint standard. One transmitter drives a differential pair, and up to 10 receivers can listen. A separate differential pair carries data in the opposite direction, providing full-duplex communication.

RS-422 has no modem control lines and no direction control. You simply read and write, just like RS-232, but with the benefits of differential signaling: noise immunity, long cable runs (up to 1200 meters), and higher data rates at distance.

Note

RS-422 and RS-485 use the same electrical interface (EIA/TIA-422 is referenced by EIA/TIA-485). The difference is topology: RS-422 is point-to-point with dedicated TX/RX pairs, while RS-485 is multi-drop with shared bus and direction control. Many “RS-485” adapters work for RS-422 when wired with separate TX and RX pairs.

RS-485: multi-drop bus

RS-485 is a shared bus. Multiple devices connect to the same pair of wires. Only one device transmits at a time while all others listen. This requires a protocol layer on top of the electrical standard to manage who talks when — Modbus RTU being the most common example.

Because the bus is shared, RS-485 devices need a way to switch between transmitting and receiving. The transceiver chip has a Driver Enable (DE) and Receiver Enable (RE) pin. When a device wants to transmit, it asserts DE to drive the bus, sends its data, then deasserts DE to go back to listening. Some adapters handle this toggling in hardware; others require software control via the RTS line.

When to use RS-232

RS-232 is the right choice when:

• You are connecting directly to a single device — a sensor, a microcontroller, a modem, or a piece of lab equipment.
• You need modem control lines — DTR for resetting an Arduino, CTS for hardware flow control, DSR for detecting device presence.
• Cable runs are short — within the same desk, rack, or room (under 15 meters).
• You are using a USB-serial adapter — the vast majority default to RS-232 behavior.

Note

Most USB-serial adapters (FTDI, CP210x, CH340, PL2303) present as RS-232 ports by default. Even adapters marketed as “RS-485” typically include an RS-485 transceiver chip on the board but still appear as a standard serial port to the operating system. The difference is in the electrical output, not the software interface.

When to use RS-422

RS-422 is the right choice when:

• You need long cable runs with full-duplex communication — industrial sensors, weather stations, satellite dish controllers, or any device more than 15 meters away that needs simultaneous bidirectional data.
• Noise immunity matters but you don’t need a multi-drop bus — RS-422’s differential signaling rejects common-mode noise just like RS-485, but without the complexity of bus arbitration.
• You have a single device (or small number of receivers) — RS-422 supports up to 10 receivers on a single transmit pair.
• You don’t need modem control lines — RS-422 has no DTR, CTS, or other control signals. If you need those, use RS-232.
• Typical applications: industrial sensor links, antenna rotator controllers, CNC machine interfaces, long-distance point-to-point data links, avionics data buses.

When to use RS-485

RS-485 is the right choice when:

• Multiple devices share one bus — sensor networks, building automation, industrial control. RS-485 supports up to 32 devices (standard) or 256 (with high-impedance receivers) on a single pair of wires.
• Long cable runs — RS-485’s differential signaling works reliably over 1200 meters. RS-232 becomes unreliable beyond 15 meters.
• Industrial environments — factories, power plants, and outdoor installations where electrical noise from motors, inverters, and power supplies would corrupt RS-232 signals. Differential signaling rejects common-mode noise.
• Industrial protocols — Modbus RTU, PROFIBUS, and DMX-512 all run over RS-485.
• Noisy environments — even in non-industrial settings, long cable runs near power wiring benefit from differential signaling.

How mcserial handles the difference

mcserial tracks which mode each port is in and restricts tools accordingly. This prevents accidentally sending RS-232 modem control commands on an RS-422 link (where they have no meaning) or an RS-485 bus (where they would disrupt direction control). Your MCP assistant uses the set_port_mode tool to switch between modes on an open port, or passes the mode parameter when opening.

Default: RS-232 mode

Every port opens in RS-232 mode (unless you specify mode="rs422" or mode="rs485" on open_serial_port). The following mode-specific tools are available:

• get_modem_lines — read CTS, DSR, RI, CD, RTS, DTR
• set_modem_lines — set RTS and DTR
• pulse_line — pulse RTS or DTR for reset sequences
• send_break — send a timed break signal
• set_break_condition — hold or release sustained break

RS-422 mode

Use set_port_mode(mode="rs422") or open_serial_port(mode="rs422") to switch. RS-232 modem control tools and RS-485 direction control tools are both unavailable. One mode-specific tool is available:

• check_rs422_support — detect adapter type and RS-422 capabilities

All common tools work normally in RS-422 mode: transact, read_serial, write_serial, configure_serial, file transfers, and everything else. RS-422 is full-duplex, so you read and write freely without any direction management.

RS-485 mode

Use set_port_mode(mode="rs485") or open_serial_port(mode="rs485") to switch. The RS-232 tools become unavailable and these tools become available:

• set_rs485_mode — configure hardware DE/RE direction control
• rs485_transact — send data and receive response with automatic turnaround
• rs485_scan_addresses — discover devices on the bus
• check_rs485_support — determine hardware capabilities

Shared tools

These tools work in all three modes:

• open_serial_port / close_serial_port
• read_serial / write_serial / write_serial_bytes
• read_serial_line / read_serial_lines / read_until
• transact
• configure_serial / flush_serial
• detect_baud_rate
• set_flow_control / set_low_latency_mode
• file_transfer_send / file_transfer_receive / file_transfer_send_batch

You can switch modes on an open port at any time with set_port_mode. No reconnection is needed.