Types of Protocols Modbus RTU Basics CRC Details Serial Processing Constructing Commands Modbus Usage in our Products

Modbus RTU Message Structure

Message Structure

Slave ID

Device Address

Function

Read Holding Registers

8C A0

Start Address

00 0A

Length/Value

10 Registers

XX XX

CRC

Error Check

Read Request (FC=03)

01 03 30 00 00 0A XX XX
│  │  │     │     └─ CRC
│  │  │     └─────── Length (10 registers)
│  │  └───────────── Register 36000(0x8CA0)
│  └──────────────── Function Code (Read)
└─────────────────── Slave ID

Write Request (FC=06)

01 06 30 00 00 64 XX XX
│  │  │     │     └─ CRC
│  │  │     └─────── Value (100)
│  │  └───────────── Register 36000(0x8CA0)
│  └──────────────── Function Code (Write)
└─────────────────── Slave ID

Common Function Codes

Read Operations

01 - Read Coils
02 - Read Discrete Inputs
03 - Read Holding Registers
04 - Read Input Registers

Write Operations

05 - Write Single Coil
06 - Write Single Register
15 - Write Multiple Coils
16 - Write Multiple Registers

Important Notes

All values are in hexadecimal
Register addresses are 16-bit (0x0000 to 0xFFFF)
CRC is calculated over all preceding bytes
Maximum message length is 256 bytes
Minimum message gap is 3.5 character times

Modbus RTU Communication Pattern

Key Points

Single master communicates with multiple slaves
Each slave has a unique ID
Communication is sequential - one request/response at a time
Master must wait for response before sending next request
Collisions occur if multiple devices transmit simultaneously
Failed communications require retry mechanisms

Multiple Masters: Communication Failure

Why Multiple Masters Fail

Multiple masters can transmit simultaneously
No built-in arbitration mechanism in Modbus RTU
Collisions corrupt data on the bus
Slaves cannot properly decode corrupted requests
Both masters receive no valid response
System reliability becomes unpredictable

This is why Modbus RTU requires a single master architecture!