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Protocol & FramingModbus Term

Modbus ASCII

ASCII-encoded Modbus frames with colon start and LRC error checking.

Definition

Modbus ASCII encodes each byte as two readable hexadecimal characters. Every frame begins with a colon (:) and ends with CR+LF. Error detection uses an LRC (Longitudinal Redundancy Check) rather than CRC-16. ASCII mode is less efficient than RTU — a 4-byte PDU becomes 8 ASCII characters — but is easier to debug with a serial terminal.

Quick Facts

Error checkLRC
EncodingASCII hex
Frame start':' (colon)
Frame endCR LF

Example

:0103000000024E

│ │ │    │    │  LRC
│ │ │    Count (2 regs)
│ │ Address (0x0000)
│ Function (03)
Slave ID (01)
':' start character

Related Terms

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ModPackQT Modbus Glossary · Updated 2026