RS-485 Tools

RS-485 enables half-duplex multi-drop bus communication where multiple devices share a single pair of wires. These tools handle direction control, request/response transactions, and bus scanning.

Note

Examples use tool-call notation: tool_name(param=value). These are MCP tool calls made by the assistant, not code you write directly. Boolean values use JSON convention (true/false).

Switch to RS-485 mode first

Ports open in RS-232 mode by default. Before using any tool on this page, switch the port to RS-485 mode:

open_serial_port(port="/dev/ttyUSB0", baudrate=9600)
set_port_mode(port="/dev/ttyUSB0", mode="rs485")

---

set_rs485_mode

Configure the hardware RS-485 settings for half-duplex communication. This sets up automatic TX/RX direction switching via the RTS line (DE/RE control) on hardware that supports it.

Parameter	Type	Default	Description
port	str	required	Device path of the port.
enabled	bool	True	Enable or disable RS-485 hardware mode.
delay_before_tx	float	0.0	Delay in seconds before enabling TX. Allows bus settling time.
delay_before_rx	float	0.0	Delay in seconds before enabling RX after TX completes.
rts_level_for_tx	bool	True	RTS level when transmitting. True = high.
rts_level_for_rx	bool	False	RTS level when receiving. True = high.
loopback	bool	False	Enable RS-485 loopback mode (see your own transmissions).

Returns: Dict with success, port, rs485_enabled, and the configured settings (rts_level_for_tx, rts_level_for_rx, delay_before_tx, delay_before_rx, loopback). Settings are None when RS-485 is disabled.

Tip

The rts_level_for_tx and rts_level_for_rx parameters control the polarity of the DE/RE enable pin. Most RS-485 transceivers use active-high for TX enable and active-low for RX enable, which is the default configuration.

# Enable hardware RS-485 with defaults
set_rs485_mode(port="/dev/ttyUSB0")

# Add bus settling delays for long cable runs
set_rs485_mode(
    port="/dev/ttyUSB0",
    delay_before_tx=0.001,
    delay_before_rx=0.001,
)

# Inverted RTS polarity
set_rs485_mode(
    port="/dev/ttyUSB0",
    rts_level_for_tx=false,
    rts_level_for_rx=true,
)

# Disable hardware RS-485 (revert to software control)
set_rs485_mode(port="/dev/ttyUSB0", enabled=false)

---

check_rs485_support

Detect whether a serial port’s hardware and driver support automatic RS-485 direction control, or whether software RTS toggling is needed. This tool queries udevadm for USB device information and tests the kernel RS-485 ioctl.

Parameter	Type	Default	Description
port	str	required	Device path to check. The port does not need to be open.

Returns: Dict with success, port, driver (e.g., ftdi_sio, cp210x, ch341), chip (product string if detected), hardware_rs485 (bool), software_fallback (bool), kernel_rs485_ioctl (bool), notes (list of informational strings), and recommendation.

Known hardware support:

Driver	Chip	Hardware RS-485
ftdi_sio	FT232, FT2232, etc.	Yes — auto-direction control
cp210x	CP2105, CP2108	Yes
cp210x	CP2102	No — use software RTS control
ch341	CH340, CH341	No — timing may be unreliable
pl2303	PL2303	No — use software RTS control
native	ttyS, ttyAMA	Yes — check transceiver wiring

check_rs485_support(port="/dev/ttyUSB0")

# Example response:
# {
#   "success": true,
#   "port": "/dev/ttyUSB0",
#   "driver": "ftdi_sio",
#   "chip": "FT232R",
#   "hardware_rs485": true,
#   "software_fallback": true,
#   "kernel_rs485_ioctl": true,
#   "notes": ["FTDI chips have hardware RS-485 auto-direction"],
#   "recommendation": "Use set_rs485_mode() for automatic DE/RE control"
# }

Note

This tool uses udevadm for USB device detection. If udevadm is not installed, detection falls back to basic checks and a note is added to the response.

---

rs485_transact

Send data and receive a response on the RS-485 bus in a single half-duplex transaction. Handles TX-to-RX turnaround timing automatically, including manual RTS control when hardware RS-485 is not available.

Note

For mode-agnostic write+read without RTS direction control, see transact. Use rs485_transact when you need the RS-485 turnaround timing and automatic RTS toggling.

Parameter	Type	Default	Description
port	str	required	Device path (must be open in RS-485 mode).
data	str	required	Data string to send.
response_timeout	float	1.0	Maximum time in seconds to wait for a response.
response_terminator	str | None	None	Stop reading when this sequence is received.
response_length	int | None	None	Expected response length in bytes. Alternative to response_terminator.
encoding	str	"utf-8"	Character encoding for send and receive data.
turnaround_delay	float	0.005	Delay in seconds after TX completes before switching to RX.

Returns: Dict with success, port, bytes_sent, data_sent, response (decoded string), response_bytes, response_hex, and hardware_rs485 (whether hardware RS-485 was used).

The transaction flow:

1. Clear the input buffer
2. Assert RTS for TX (if no hardware RS-485)
3. Send encoded data and flush
4. Wait for TX completion + turnaround delay
5. De-assert RTS for RX (if no hardware RS-485)
6. Read response using terminator, length, or timeout

# Simple query with timeout-based response
rs485_transact(port="/dev/ttyUSB0", data="?ID\r\n")

# Modbus-style with known response length
rs485_transact(
    port="/dev/ttyUSB0",
    data="\x01\x03\x00\x00\x00\x01",
    response_length=7,
    encoding="latin-1",
)

# Wait for a specific terminator
rs485_transact(
    port="/dev/ttyUSB0",
    data="READ\r\n",
    response_terminator="\r\n",
)

# Adjust turnaround for slow devices
rs485_transact(
    port="/dev/ttyUSB0",
    data="PING",
    turnaround_delay=0.01,
    response_timeout=2.0,
)

Tip

When neither response_terminator nor response_length is set, the tool waits for half the response_timeout, then reads all available bytes. For protocols with predictable framing, always specify a terminator or length for faster, more reliable reads.

---

rs485_scan_addresses

Scan the RS-485 bus for responding devices by sending a probe message to each address in a range. Useful for discovering Modbus or similar addressed devices.

Parameter	Type	Default	Description
port	str	required	Device path (must be open in RS-485 mode).
start_address	int	1	First address to scan.
end_address	int	247	Last address to scan. Default 247 is the Modbus maximum.
probe_template	str	"{addr:02x}03000001"	Message template with {addr} placeholder for the address. Default is a Modbus “read holding register” frame. Customize for your protocol.
response_timeout	float	0.1	Time in seconds to wait for a response per address.
encoding	str	"latin-1"	Encoding for the probe string. Use latin-1 for raw byte protocols.

Returns: Dict with success, port, addresses_scanned, devices_found, responding_addresses (list of objects with address, response_length, response_hex), and hardware_rs485.

Caution

Scanning 247 addresses at 0.1s per address takes approximately 25 seconds. Narrow the address range when possible, or reduce response_timeout for faster scans on responsive buses.

# Scan the full Modbus range
rs485_scan_addresses(port="/dev/ttyUSB0")

# Scan a known range of addresses
rs485_scan_addresses(
    port="/dev/ttyUSB0",
    start_address=1,
    end_address=10,
)

# Custom probe template for a different protocol
rs485_scan_addresses(
    port="/dev/ttyUSB0",
    probe_template="{addr:c}ID?\r\n",
    response_timeout=0.2,
    encoding="ascii",
)

# Faster scan with short timeout
rs485_scan_addresses(
    port="/dev/ttyUSB0",
    start_address=1,
    end_address=32,
    response_timeout=0.05,
)

Note

If the probe_template contains the {addr} placeholder, it is formatted with the current address as an integer. If no placeholder is found, the address is prepended as a single character (chr(addr)) to the template string.