Modbus RTU

Setup

Open the port with parity even — that's the Modbus RTU default. Then wrap it in a SerialCommandQueue:

import { SerialPilot } from 'serialpilot'
import { SerialCommandQueue } from '@serialpilot/command-queue'
const port = new SerialPilot({
path: ‘/dev/ttyUSB0’,
baudRate: 9600,
dataBits: 8,
parity: ‘even’,
stopBits: 1,
})
const queue = new SerialCommandQueue({ port, timeout: 500 })

Send and wait

Each call to queue.command() writes the request, waits for a response, and resolves with what came back. Subsequent calls queue up:

const response = await queue.command(':010300000001F8\r\n')
console.log('Modbus response:', response)

Validate the response shape

Pass expect to require a particular pattern. The promise rejects if the response doesn't match — handy for protocols that interleave OK/ERROR replies:

const response = await queue.command('AT+CSQ', { expect: /OK/ })

Production tips

• Timeouts: 500 ms is fine for 9600 bps; bump to 2–3 s for 1200 bps or noisy lines.
• Retries: set retryCount: 3 on the queue when running over RS-485 with multiple devices on the bus.
• Parity matters: Modbus RTU is even by spec; some equipment uses none with two stop bits as a workaround.
• Inter-byte timing: if you need raw frames (no \r\n termination), pipe through InterByteTimeoutParser instead of relying on a delimiter.

Queue options

Events

• response — (command, response) when a request resolves.
• error — (error) on timeout or transport failure.
• idle — when the queue empties.