MODBUS ASCII

Study notes

• ASCII mode encodes each 8-bit byte as two ASCII characters; for example, byte 0x5B is sent as 0x35 and 0x42 ("5" and "B").
• An ASCII frame starts with a colon (ASCII 3A hex) and ends with CRLF (ASCII 0D and 0A hex); the LF character can be changed by a MODBUS application command.
• ASCII frame fields are Start (1 char), Address (2 chars), Function (2 chars), Data (0 up to 2x252 chars), LRC (2 chars), and End (CR LF); maximum frame size is 513 characters.
• The ASCII data field can carry up to 2x252 characters because each data byte is encoded as two characters.
• In ASCII mode the character frame is 10 bits: 1 start bit, 7 data bits sent least significant bit first, 1 parity bit, and 1 stop bit; with no parity, 2 stop bits are used.
• Even parity is required; odd parity and no parity may also be used. The default parity mode is even parity.
• ASCII mode uses Longitudinal Redundancy Checking (LRC): the LRC is one byte, calculated by adding successive 8-bit message bytes, discarding carries, and two's-complementing the result, excluding the colon and CRLF.
• The LRC is ASCII-encoded into two bytes and placed before the CRLF at the end of the ASCII frame.
• Devices monitor the bus continuously for the colon character; receipt of a colon starts a new message and discards any message then in reception.
• Intervals of up to one second may elapse between characters within an ASCII message; unless a longer timeout is configured, an interval greater than one second is an error.