P4NR Modbus for Node-RED

Modbus was designed in 1979 for programmable logic controllers talking over a serial cable. Nearly fifty years later, it is still one of the most widely deployed protocols in industrial automation — running over TCP/IP as often as RS-485 now, sitting beneath PLCs, energy meters, building-management systems, and a staggering amount of hardware that will still be running in 2040. If you write Node.js and work anywhere near a plug and a PLC, you will eventually need to speak it.

Modbus with Node.js is a practical, code-first guide to doing that well, built around @plus4nodered/node-modbus — a governed, heavily tested fork of the long-running node-modbus-serial project. It assumes you already know modern JavaScript. It does not assume you know anything about Modbus.

The book is organized in four parts:

• Understanding Modbus builds the protocol knowledge you need from zero: RTU, ASCII, and TCP wire formats, the addressing model, and every function and exception code — including why Modbus registers are sometimes numbered 40001 and sometimes just 0, and why that distinction matters more than it looks.
• SDK Architecture tours the codebase itself: its mixin-based API design, every transport it supports (TCP, TLS, serial RTU/ASCII, Telnet, UDP, even Bluetooth Low Energy), and its client and server implementations.
• Features Deep Dive goes function code by function code — reads, writes, diagnostics, device identification, TLS security, and the reliability utilities (connection pooling, rate limiting, request logging) that separate a demo script from something you'd actually run in production.
• Learning Path is entirely hands-on: five chapters, each a small, complete, runnable Node.js 24 program — no fragments, nothing left as an exercise. You'll build a client, a server, a polling loop, a TLS-secured pair, and a custom function-code handler, then combine them into a small capstone project. No hardware required — every example talks to the SDK's own server, running right there on your machine.

What sets this book apart is that every claim in it was checked against the real SDK, not against documentation that might have drifted from what the code actually does. In the course of writing this edition, that verification process turned up two real, previously unnoticed bugs — one where a "simple" request silently hung forever, one where a legacy addressing assumption had quietly leaked into live validation code. Both are fixed, and both are told here in full: the wrong assumption, the one-line fix, and the regression test that caught it for good. Understanding how a well-tested library can still hide a bug like that — and how you find and fix one properly — is a more useful lesson than any amount of "and then it just works."

Whether you've been handed a one-off task ("read the holding registers off that PLC") or you're building a serious Modbus-based service, this book will get you from zero to production-minded, with working code the entire way.

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