{"id":"modbus-ascii","model_id":"MODBUS ASCII","name":"MODBUS ASCII","summary":"MODBUS ASCII is a serial transmission mode of the MODBUS over Serial Line protocol. It sends each 8-bit message byte as two hexadecimal ASCII characters, delimits a message with a colon start character and a CRLF end pair, and uses an LRC field for frame checking. It operates as a master-slave protocol in which the master initiates requests and slave nodes respond.","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."],"safety_notes":["Some wide-area-network applications may require a character timeout in the 4 to 5 second range."],"sources":[{"url":"https://www.modbus.org/file/secure/modbusoverserial.pdf","title":"modbusoverserial","accessed":"2026-10-25"}],"verification":{"status":"source_checked","checked_at":"2026-10-25","values":3,"method":"llm_source_check"},"updated_at":"2026-10-25","type":"protocol","spec":{"transports":["serial"],"architecture":"master_slave","encoding":"text","default_ports":[],"data_types":[],"standard_body":"Modbus.org","related":[]},"url":"https://machineschema.com/protocol/modbus-ascii","license":"https://creativecommons.org/licenses/by/4.0/"}