node-fxplc

Node.js library for communication with Mitsubishi FX PLCs using low-level MELSEC FX series serial protocol.

Note: it is not the same as Non-Protocol Communication (or D8120) as described in FX Series Programmable Controllers manuals.

This is a Node.js port of a Python library.

Structure

• lib/ — Library source code
• test/ — Tests and example scripts

Project Status

Pre-release. Core client API + TCP/Serial transports work for basic read/write, but advanced resilience and integration testing are still in progress. See ROADMAP.md for detailed progress & planned items. Contributions / hardware test feedback are welcome.

Installation

npm install node-fxplc

Optional (for debug logging):

npm install debug

Enable logs by setting environment variable (PowerShell):

$env:DEBUG = "fxplc:*"; node yourscript.mjs

Features (current)

• Low-level MELSEC FX serial frame protocol (hex payload + checksum)
• Read / write single bit & forced coil (ON/OFF)
• Read / write signed word and generic numeric types via converters
• Batch read/write (auto-coalesces consecutive addresses)
• Raw byte read/write access for advanced use
• TCP & Serial transports with buffer accumulation & timeouts
• Simple retry + operation-level timeout
• Event emitter (error / connect / disconnect)

Protocol Notes

This is NOT the "Non-Protocol Communication" (D8120) mode. It targets the classic hex framed ASCII protocol (FX0N/FX1N style). Serial defaults: 7 data bits, even parity, 1 stop bit (7E1), commonly 9600 baud. Verify your PLC parameters.

Compatibility note: The protocol layer is electrical-transport agnostic; it should also work with many FX-compatible clone PLCs exposing an RS422 port, provided they implement the same framed command set. Use a proper RS422↔USB (or RS422↔RS232) converter and match serial parameters (7E1, baud rate). Timing and response behaviors may vary between clones—enable retries if needed and report any incompatibilities.

Supported Registers (initial)

• Bit areas (e.g. M, X, Y) via readBit / writeBit
• Data registers (e.g. D) via numeric read/write helpers Parsing is handled by RegisterDef.parse('D100'). Unsupported / unknown areas will throw until implemented.

Error Classes

Exposed error types (see errors.js):

• NoResponseError – timeout or no reply
• ResponseMalformedError – bad checksum / frame inconsistency
• NotSupportedCommandError – PLC replied NAK
• NotConnectedError – transport not open Use instanceof checks for granular handling.

Timeouts & Retries

Client option timeoutMs controls per-operation timeout. Retry policy via retry: { count, delayMs } (default 1 attempt). Example:

const plc = new FXPLCClient(transport, { retry: { count: 3, delayMs: 150 }, timeoutMs: 2500 });

Auto Reconnect (TCP)

Enable simple reconnect:

const tcp = new TransportTCP({ host: '127.0.0.1', port: 5000 });
tcp.setReconnect(true, 2000); // every 2s after disconnect

Serial transport currently has no auto-reopen loop (planned).

Usage Examples

Promise style (TCP)

import { FXPLCClient, TransportTCP } from 'node-fxplc';
const transport = new TransportTCP({ host: '127.0.0.1', port: 5000 });
await transport.connect();
const plc = new FXPLCClient(transport);
const bit = await plc.readBit('M0');
console.log('Bit M0:', bit);
const vals = await plc.batchRead(['D100','D101']);
await plc.writeBit('M10', true);
plc.close();

Callback style

plc.readBit('M0', (err, bit) => {
    if (err) return console.error('Error:', err);
    console.log('Bit M0:', bit);
});
plc.batchRead(['D100','D101'], (err, vals) => {
    if (!err) console.log('Values:', vals);
});

Events (error, connect, disconnect)

plc.on('error', err => console.error('PLC Error:', err));
plc.on('connect', () => console.log('Connected!'));
plc.on('disconnect', () => console.log('Disconnected!'));

Input error handling

try {
    await plc.readBit('Z999'); // invalid register
} catch(e) {
    console.error('Input error:', e.message);
}

Batch write

await plc.batchWrite(['D100','D101'], [123,456]);

Serial usage example

import { FXPLCClient, TransportSerial } from 'node-fxplc';
const transport = new TransportSerial({ path: 'COM3', baudRate: 9600, timeout: 1500 });
const plc = new FXPLCClient(transport, { debug: true });
const bit = await plc.readBit('M0');
console.log('M0 =', bit);
plc.close();

Enabling debug logs

Install debug and set DEBUG=fxplc:* env variable. Logs show TX/RX hex frames.

Next focus: transport robustness, CLI, integration tests, richer docs.

Note: Serial transport is functional but minimally tested; use caution in production scenarios.