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Protocol & FramingModbus Term

LRC (Longitudinal Redundancy Check)

A 1-byte checksum used in Modbus ASCII frames.

Definition

The LRC (Longitudinal Redundancy Check) is the error-detection method for Modbus ASCII. It is calculated by summing all the message bytes (excluding the start colon and end CR/LF), taking the two's complement, and appending the result as two ASCII hex characters before the CR/LF. LRC is weaker than CRC-16 but sufficient given ASCII's per-character framing.

Quick Facts

Size1 byte (2 ASCII chars in frame)
AlgorithmTwo's complement of byte sum
Used inModbus ASCII only

Related Terms

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ModPackQT Modbus Glossary · Updated 2026