Modbus ASCII transmits each byte as 2 ASCII hexadecimal characters (e.g. 0x4F becomes "4F"), with the frame delimiter ":" at the start and CR+LF at the end, and a Longitudinal Redundancy Check (LRC) checksum instead of the CRC used in RTU.

Modbus ASCII (legacy, barely deployed in 2026) encodes each byte as two ASCII hex characters, so 0x4F is transmitted as "4F". A frame is structured as ":" (start) + Address + Function + Data + LRC + CR + LF. The LRC is a 1-byte Longitudinal Redundancy Check (compared to the 16-bit CRC of RTU), and the effective bandwidth is halved compared to RTU because each byte takes twice as many characters on the wire. The theoretical advantage is that frames are human-readable on a serial terminal; the practical drawback is that it is about twice as slow as RTU and is now nearly extinct.

If you ever find Modbus ASCII still in use, plan its replacement at the next maintenance window: there is no operational reason to keep it on a modern installation.