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Protocol & FramingModbus Term

Modbus RTU

Binary serial framing over RS-485 (or RS-232) with CRC-16 error checking.

Definition

Modbus RTU (Remote Terminal Unit) is the most widely used variant of Modbus. Messages are transmitted as compact binary bytes over an RS-485 or RS-232 serial bus. Each frame ends with a 2-byte CRC-16 checksum. There is no start or end character; instead, inter-frame silence of at least 3.5 character times signals a new frame.

Quick Facts

Error checkCRC-16
EncodingBinary
Physical layerRS-485 or RS-232
Max slaves247

Example

Frame example (FC03 request — read 2 registers at address 0):
01 03 00 00 00 02 C4 0B
│  │  ├──┤  ├──┤  ├─────┤
│  │  addr  count   CRC
│  FC (Read Holding Registers)
Slave ID (1)

Related Terms

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ModPackQT Modbus Glossary · Updated 2026