LearnSCADA

Companion course · 19 chapters · 4 parts

Understanding Modbus

A practical guide to the protocol, one byte at a time. This course follows the book Understanding Modbus by Charles Vance chapter by chapter: every concept gets an animated figure showing the frames on the wire, a tool to try it yourself, and a short check.

Start Chapter 1 →
A SCADA client polls three field devices in turn on a shared bus. Each device answers only when it is addressed. SCADA client Flow meter ID 1 VFD ID 2 Power meter ID 3 REQ 1 RSP 1 REQ 2 RSP 2 REQ 3 RSP 3
Modbus is a polling protocol. The client asks each device in turn, and a device speaks only when it is spoken to.

Part I · Foundations

The mental model: what Modbus is, the four data tables, and the conversation between client and server.

Chapter 1

What Modbus Is and Why It Still Matters

A protocol, not a wire. Clients, servers, and the three transports.

 Chapter 2

The Modbus Data Model

Four tables, two questions, and the 40001 addressing trap.

 Chapter 3

The Request–Response Cycle

Data, exception, or silence, and the polling loop.

Part II · The Protocol in Detail

The exact bytes: the frame, the three transports, the function codes, and the error vocabulary.

Chapter 4

The Modbus Frame

PDU and ADU, big-endian order, and size limits.

 Chapter 5

Modbus RTU

RS-485, serial settings, framing by silence, and the CRC.

 Chapter 6

Modbus ASCII

Bytes as text, colon and CR/LF delimiters, and the LRC.

 Chapter 7

Modbus TCP/IP

The MBAP header, transaction IDs, and gateways.

 Chapter 8

Function Codes in Depth

The eight that matter, byte by byte.

 Chapter 9

Exceptions and Diagnostics

Every exception code, loopback tests, and diagnostic counters.

Part III · Hands-On with a Microcomputer

Real traffic from Python and pymodbus on a Raspberry Pi or any Linux machine, ending with a monitoring-and-control project.

Chapter 10

Setting Up Your Microcomputer

Linux, Python, a virtual environment, and a practice server.

 Chapter 11

Your First Modbus TCP Client

Connect, read, check, scale, close.

 Chapter 12

Building a Modbus Server

Data blocks, contexts, and a device that comes alive.

 Chapter 13

Modbus RTU over RS-485

A USB adapter, the serial client, and a two-adapter test rig.

 Chapter 14

Reading and Writing Real-World Data

Signed values, scaling, 32-bit numbers, floats, and word order.

 Chapter 15

A Monitoring and Control Project

A pump-station monitor that closes the loop. Run it in your browser.

Part IV · Doing It Well

The craft around the protocol: troubleshooting, performance, security, and where Modbus fits.

Chapter 16

Troubleshooting Modbus

Start from the reply, fault signatures, and isolate-simplify-expand.

 Chapter 17

Performance, Polling, and Scaling

Where the time goes, batching, tiering, and slow devices.

 Chapter 18

Security Considerations

What Modbus can't protect, and defense in depth.

 Chapter 19

Modbus in the Real World

PLCs, SCADA, gateways, and the protocol landscape.

 Appendices A–E

Quick Reference

Function codes, exception codes, pymodbus, wiring, and a glossary.

Who this is for

Technicians, apprentices, and engineers who want to understand what is actually on the wire, and then prove it by running real Modbus traffic. Parts I and II need no programming. Part III uses short Python programs; a little comfort with the command line helps.

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