762 Modbus Protocol Simulator
Interactive visualization of Modbus RTU/TCP for Industrial IoT
762.1 Overview
Modbus is a serial communication protocol originally published by Modicon in 1979 for use with programmable logic controllers (PLCs). It has become a de facto standard in industrial environments for connecting electronic devices. This interactive simulator demonstrates Modbus RTU, TCP, and ASCII modes, showing how master devices communicate with slaves through various function codes.
Think of Modbus like a walkie-talkie system in a factory:
- Master: The “boss” who asks questions (like a PLC or SCADA system)
- Slaves: The “workers” who answer (sensors, motors, valves - each with a unique address 1-247)
- Registers: Memory locations where data is stored (like mailboxes)
- Function Codes: Different types of questions/commands (read temperature, turn on motor, etc.)
The master says: “Hey Slave #5, what’s the value in register 100?” Slave #5 responds: “The value is 2500!”
This simple request-response pattern has made Modbus the most widely used industrial protocol for over 40 years!
762.2 Chapter Topics
This comprehensive Modbus simulator is organized into four focused chapters:
762.2.1 Frame Structure
Learn how Modbus frames are constructed across RTU, TCP, and ASCII modes:
- Byte-level frame visualization with interactive controls
- CRC-16 and LRC checksum calculation
- Request and response frame formats
- Error simulation and detection
762.2.2 Registers and Communication
Understand Modbus register types and master-slave communication:
- Four register types: Coils, Discrete Inputs, Input Registers, Holding Registers
- Interactive register map visualization
- Master-slave communication patterns
- Modicon addressing conventions
762.2.3 Function Codes and Exceptions
Complete reference for Modbus operations:
- All 8 standard function codes with request/response formats
- Function code decision tree for operation selection
- Exception codes and troubleshooting
- Practical industrial application examples
762.2.4 Modes and Timing
Compare modes and analyze performance:
- RTU vs TCP vs ASCII comparison
- Timing analysis with configurable baud rate
- Protocol efficiency calculation
- Mode selection guidelines
762.3 Key Features
- Configure Devices: Set up master and slave devices with register configurations
- Select Mode: Choose between Modbus RTU (serial), TCP (Ethernet), or ASCII
- Execute Commands: Use function codes to read/write registers and coils
- Analyze Frames: See byte-level breakdown of request/response frames
- Simulate Errors: Test exception responses, timeouts, and CRC errors
762.4 Quick Reference
| Mode | Transport | Error Check | Best For |
|---|---|---|---|
| RTU | RS-232/RS-485 | CRC-16 | Most industrial applications |
| TCP | Ethernet | TCP/IP | Enterprise SCADA systems |
| ASCII | RS-232/RS-485 | LRC | Debugging, legacy systems |
| Register Type | Prefix | Access | Function Codes |
|---|---|---|---|
| Coils | 0x | R/W | 01, 05, 15 |
| Discrete Inputs | 1x | R | 02 |
| Input Registers | 3x | R | 04 |
| Holding Registers | 4x | R/W | 03, 06, 16 |
762.5 What’s Next
Start with the chapter that best matches your learning goals:
- New to Modbus? Start with Frame Structure to understand the basics
- Configuring devices? Go to Registers and Communication
- Need a reference? Check Function Codes and Exceptions
- Optimizing performance? See Modes and Timing
- WirelessHART TDMA - Industrial wireless with TDMA scheduling
- ISA100 DLMO - Another industrial wireless standard
- Protocol State Machine - Deep dive into protocol states
- OPC UA - Modern industrial protocol with security
- Network Emulator - Test under network conditions
- Packet Analyzer - Deep packet inspection