RS-422 Differential Links

RS-422 uses differential signaling on two wire pairs for full-duplex communication over long distances. It shares the noise immunity and cable length advantages of RS-485 without the complexity of half-duplex bus management. This guide covers how to use mcserial for RS-422 devices.

When you have an RS-422 device

RS-422 is common in industrial and scientific equipment where you need reliable communication over distances that RS-232 cannot handle:

• Satellite dish controllers — antenna positioners using long cable runs from the control room to the dish
• Industrial sensors — temperature, pressure, and flow sensors mounted far from the data acquisition system
• CNC machines — machine tools with control panels connected via long differential links
• Avionics data buses — aircraft instrumentation using differential serial
• Building automation — HVAC controllers, access systems, and energy monitors spread across a facility

All of these are point-to-point (or point-to-few-receivers) links that need the distance and noise immunity of differential signaling but use full-duplex communication — you send a command and read the response without any direction switching.

Opening a port in RS-422 mode

Use the mode parameter on open_serial_port to start in RS-422 mode directly:

open_serial_port(port="/dev/ttyUSB0", baudrate=9600, mode="rs422")

Or open first and switch:

open_serial_port(port="/dev/ttyUSB0", baudrate=9600)
set_port_mode(port="/dev/ttyUSB0", mode="rs422")

Tip

Opening directly in RS-422 mode saves a tool call and immediately gives the assistant the right context about which tools apply.

What changes in RS-422 mode

RS-422 mode disables tools that don’t apply to differential links:

• RS-232 modem tools blocked: get_modem_lines, set_modem_lines, pulse_line, send_break, set_break_condition — these control RS-232 signal lines that have no meaning on an RS-422 link. Your adapter may expose RTS/DTR, but they serve no RS-422 function.
• RS-485 bus tools blocked: set_rs485_mode, rs485_transact, rs485_scan_addresses — these manage half-duplex direction control. RS-422 is full-duplex, so direction control is not needed.

Everything else works normally:

• transact — send a command and read the response
• read_serial / write_serial — read and write data
• configure_serial — change baud rate, timeouts, line endings
• file_transfer_send / file_transfer_receive — XMODEM/YMODEM/ZMODEM transfers
• All other common tools

Checking adapter support

Use check_rs422_support to detect your adapter’s RS-422 capabilities:

check_rs422_support(port="/dev/ttyUSB0")

This reports the adapter chip family, whether it supports dual differential pairs natively, and any adapter-specific notes. The same USB-serial adapters that work with RS-485 generally work with RS-422 — the difference is in the wiring, not the chip.

Note

check_rs422_support requires RS-422 mode. If the port is in RS-232 or RS-485 mode, switch first with set_port_mode.

Common adapters for RS-422

Most RS-422 communication uses the same USB-serial adapters as RS-485, with different wiring:

Adapter	RS-422 Notes
FTDI FT2232H / FT4232H	Dual/quad-channel chips. Each channel can drive a differential pair via external transceiver. Native RS-422 support with appropriate breakout board.
FTDI FT232R / FT232H	Single-channel. Works with external RS-422 transceiver (MAX490, SN75179). Most common setup.
CP2105 / CP2108	Silicon Labs dual/quad-channel. Each channel supports RS-422 with external transceiver.
CH340 / CH341	Budget adapters. Work with external RS-422 transceiver but timing is less precise at high speeds.

Caution

Many adapters marketed as “USB to RS-422/RS-485” have the transceiver built in and expose screw terminals for A+, A-, B+, B-. These are ready to use. If your adapter only has a DB-9 connector, it is likely RS-232 only and needs an external transceiver for RS-422.

RS-422 vs RS-485: same wires, different topology

RS-422 and RS-485 are electrically compatible — they use the same differential signaling levels and the same transceiver chips (like the MAX485/MAX490 family). The difference is topology:

Aspect	RS-422	RS-485
Wiring	4 wires: TX+, TX-, RX+, RX-	2 wires: A, B (shared bus)
Direction	Full-duplex (always)	Half-duplex (most common)
Transmitters	1 per pair	Many, taking turns
Receivers	Up to 10 per pair	Up to 32 (or 256)
Direction control	None needed	DE/RE pin toggling
Bus arbitration	None needed	Protocol-dependent (Modbus, etc.)

If your device uses 4 wires with dedicated TX and RX pairs and doesn’t need bus arbitration, it is RS-422. Use RS-422 mode in mcserial.

If your device uses 2 wires with multiple devices taking turns transmitting, it is RS-485. Use RS-485 mode.

Example: long-distance sensor link

A temperature sensor is mounted 500 meters from the control room, connected via RS-422 differential cable:

# Discover the adapter
list_serial_ports(grep="FTDI")

# Open in RS-422 mode at the sensor's baud rate
open_serial_port(port="/dev/ttyUSB0", baudrate=9600, mode="rs422")

# Set line ending for the sensor protocol
configure_serial(port="/dev/ttyUSB0", line_ending="crlf")

# Query the sensor
transact(port="/dev/ttyUSB0", data="READ TEMP", response_terminator="\r\n")

# Check adapter capabilities
check_rs422_support(port="/dev/ttyUSB0")

The workflow is identical to RS-232 — just transact to send commands and read responses. The difference is entirely at the physical layer (differential signaling over long cable), which the adapter handles transparently.