# MODBUS ASCII Protocols · `MODBUS ASCII` · verification: source checked 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. ## Specification | Property | Value | |---|---| | Transports | serial | | Architecture | master slave | | Encoding | text | | Standard body | Modbus.org | ## Connected - Supported by: [AutomationDirect DURApulse GS11N-10P2](https://machineschema.com/hardware/automationdirect-gs11n-10p2.md), [AutomationDirect DURApulse GS11N-20P5](https://machineschema.com/hardware/automationdirect-gs11n-20p5.md), [AutomationDirect DURApulse GS11N-21P0](https://machineschema.com/hardware/automationdirect-gs11n-21p0.md), [AutomationDirect DURApulse GS21-21P0](https://machineschema.com/hardware/automationdirect-gs21-21p0.md), [AutomationDirect DURApulse GS21-23P0](https://machineschema.com/hardware/automationdirect-gs21-23p0.md), [AutomationDirect DURApulse GS23-21P0](https://machineschema.com/hardware/automationdirect-gs23-21p0.md), [AutomationDirect DURApulse GS23-4020](https://machineschema.com/hardware/automationdirect-gs23-4020.md), [AutomationDirect DURApulse GS23-40P5](https://machineschema.com/hardware/automationdirect-gs23-40p5.md), [AutomationDirect DURApulse GS31-21P0](https://machineschema.com/hardware/automationdirect-gs31-21p0.md), [AutomationDirect DURApulse GS33-2040](https://machineschema.com/hardware/automationdirect-gs33-2040.md), [AutomationDirect DURApulse GS33-21P0](https://machineschema.com/hardware/automationdirect-gs33-21p0.md), [AutomationDirect DURApulse GS33-22P0](https://machineschema.com/hardware/automationdirect-gs33-22p0.md), [AutomationDirect DURApulse GS33-4015](https://machineschema.com/hardware/automationdirect-gs33-4015.md), [AutomationDirect DURApulse GS33-41P0](https://machineschema.com/hardware/automationdirect-gs33-41p0.md), [AutomationDirect DURApulse GS4-2010](https://machineschema.com/hardware/automationdirect-gs4-2010.md), [AutomationDirect DURApulse GS4-2030](https://machineschema.com/hardware/automationdirect-gs4-2030.md), [AutomationDirect DURApulse GS4-2050](https://machineschema.com/hardware/automationdirect-gs4-2050.md), [AutomationDirect DURApulse GS4-25P0](https://machineschema.com/hardware/automationdirect-gs4-25p0.md), [AutomationDirect DURApulse GS4-4020](https://machineschema.com/hardware/automationdirect-gs4-4020.md), [AutomationDirect DURApulse GS4-4040](https://machineschema.com/hardware/automationdirect-gs4-4040.md), [AutomationDirect DURApulse GS4-4060](https://machineschema.com/hardware/automationdirect-gs4-4060.md), [AutomationDirect DURApulse GS4-4125](https://machineschema.com/hardware/automationdirect-gs4-4125.md), [AutomationDirect DURApulse GS4-4175](https://machineschema.com/hardware/automationdirect-gs4-4175.md), [AutomationDirect DURApulse GS4-42P0](https://machineschema.com/hardware/automationdirect-gs4-42p0.md), [AutomationDirect GS11N-10P5](https://machineschema.com/hardware/automationdirect-gs11n-10p5.md), [AutomationDirect GS11N-11P0](https://machineschema.com/hardware/automationdirect-gs11n-11p0.md), [AutomationDirect GS11N-20P2](https://machineschema.com/hardware/automationdirect-gs11n-20p2.md), [AutomationDirect GS21-10P2](https://machineschema.com/hardware/automationdirect-gs21-10p2.md), [AutomationDirect GS21-10P5](https://machineschema.com/hardware/automationdirect-gs21-10p5.md), [AutomationDirect GS21-11P0](https://machineschema.com/hardware/automationdirect-gs21-11p0.md), [AutomationDirect GS21-20P2](https://machineschema.com/hardware/automationdirect-gs21-20p2.md), [AutomationDirect GS21-20P5](https://machineschema.com/hardware/automationdirect-gs21-20p5.md), [AutomationDirect GS21-22P0](https://machineschema.com/hardware/automationdirect-gs21-22p0.md), [AutomationDirect GS23-2010](https://machineschema.com/hardware/automationdirect-gs23-2010.md), [AutomationDirect GS23-2020](https://machineschema.com/hardware/automationdirect-gs23-2020.md), [AutomationDirect GS23-20P2](https://machineschema.com/hardware/automationdirect-gs23-20p2.md), [AutomationDirect GS23-20P5](https://machineschema.com/hardware/automationdirect-gs23-20p5.md), [AutomationDirect GS23-22P0](https://machineschema.com/hardware/automationdirect-gs23-22p0.md), [AutomationDirect GS23-23P0](https://machineschema.com/hardware/automationdirect-gs23-23p0.md), [AutomationDirect GS23-25P0](https://machineschema.com/hardware/automationdirect-gs23-25p0.md), [AutomationDirect GS23-27P5](https://machineschema.com/hardware/automationdirect-gs23-27p5.md), [AutomationDirect GS23-4010](https://machineschema.com/hardware/automationdirect-gs23-4010.md), [AutomationDirect GS23-4015](https://machineschema.com/hardware/automationdirect-gs23-4015.md), [AutomationDirect GS23-4025](https://machineschema.com/hardware/automationdirect-gs23-4025.md), [AutomationDirect GS23-4030](https://machineschema.com/hardware/automationdirect-gs23-4030.md), [AutomationDirect GS23-41P0](https://machineschema.com/hardware/automationdirect-gs23-41p0.md), [AutomationDirect GS23-42P0](https://machineschema.com/hardware/automationdirect-gs23-42p0.md), [AutomationDirect GS23-43P0](https://machineschema.com/hardware/automationdirect-gs23-43p0.md), [AutomationDirect GS23-45P0](https://machineschema.com/hardware/automationdirect-gs23-45p0.md), [AutomationDirect GS23-47P5](https://machineschema.com/hardware/automationdirect-gs23-47p5.md), [AutomationDirect GS23-5010](https://machineschema.com/hardware/automationdirect-gs23-5010.md), [AutomationDirect GS23-51P0](https://machineschema.com/hardware/automationdirect-gs23-51p0.md), [AutomationDirect GS23-52P0](https://machineschema.com/hardware/automationdirect-gs23-52p0.md), [AutomationDirect GS23-53P0](https://machineschema.com/hardware/automationdirect-gs23-53p0.md), [AutomationDirect GS23-55P0](https://machineschema.com/hardware/automationdirect-gs23-55p0.md), [AutomationDirect GS23-57P5](https://machineschema.com/hardware/automationdirect-gs23-57p5.md), [AutomationDirect GS31-20P5](https://machineschema.com/hardware/automationdirect-gs31-20p5.md), [AutomationDirect GS31-22P0](https://machineschema.com/hardware/automationdirect-gs31-22p0.md), [AutomationDirect GS31-23P0](https://machineschema.com/hardware/automationdirect-gs31-23p0.md), [AutomationDirect GS33-2010](https://machineschema.com/hardware/automationdirect-gs33-2010.md), [AutomationDirect GS33-2015](https://machineschema.com/hardware/automationdirect-gs33-2015.md), [AutomationDirect GS33-2020](https://machineschema.com/hardware/automationdirect-gs33-2020.md), [AutomationDirect GS33-2025](https://machineschema.com/hardware/automationdirect-gs33-2025.md), [AutomationDirect GS33-2030](https://machineschema.com/hardware/automationdirect-gs33-2030.md), [AutomationDirect GS33-2050](https://machineschema.com/hardware/automationdirect-gs33-2050.md), [AutomationDirect GS33-20P5](https://machineschema.com/hardware/automationdirect-gs33-20p5.md), [AutomationDirect GS33-23P0](https://machineschema.com/hardware/automationdirect-gs33-23p0.md), [AutomationDirect GS33-25P0](https://machineschema.com/hardware/automationdirect-gs33-25p0.md), [AutomationDirect GS33-27P5](https://machineschema.com/hardware/automationdirect-gs33-27p5.md), [AutomationDirect GS33-4010](https://machineschema.com/hardware/automationdirect-gs33-4010.md), [AutomationDirect GS33-4020](https://machineschema.com/hardware/automationdirect-gs33-4020.md), [AutomationDirect GS33-4025](https://machineschema.com/hardware/automationdirect-gs33-4025.md), [AutomationDirect GS33-4030](https://machineschema.com/hardware/automationdirect-gs33-4030.md), [AutomationDirect GS33-4040](https://machineschema.com/hardware/automationdirect-gs33-4040.md), [AutomationDirect GS33-4050](https://machineschema.com/hardware/automationdirect-gs33-4050.md), [AutomationDirect GS33-4060](https://machineschema.com/hardware/automationdirect-gs33-4060.md), [AutomationDirect GS33-4075](https://machineschema.com/hardware/automationdirect-gs33-4075.md), [AutomationDirect GS33-40P5](https://machineschema.com/hardware/automationdirect-gs33-40p5.md), [AutomationDirect GS33-4100](https://machineschema.com/hardware/automationdirect-gs33-4100.md), [AutomationDirect GS33-42P0](https://machineschema.com/hardware/automationdirect-gs33-42p0.md), [AutomationDirect GS33-43P0](https://machineschema.com/hardware/automationdirect-gs33-43p0.md), [AutomationDirect GS33-45P0](https://machineschema.com/hardware/automationdirect-gs33-45p0.md), [AutomationDirect GS33-47P5](https://machineschema.com/hardware/automationdirect-gs33-47p5.md), [AutomationDirect GS4-2015](https://machineschema.com/hardware/automationdirect-gs4-2015.md), [AutomationDirect GS4-2020](https://machineschema.com/hardware/automationdirect-gs4-2020.md), [AutomationDirect GS4-2025](https://machineschema.com/hardware/automationdirect-gs4-2025.md), [AutomationDirect GS4-2040](https://machineschema.com/hardware/automationdirect-gs4-2040.md), [AutomationDirect GS4-2060](https://machineschema.com/hardware/automationdirect-gs4-2060.md), [AutomationDirect GS4-2075](https://machineschema.com/hardware/automationdirect-gs4-2075.md), [AutomationDirect GS4-2100](https://machineschema.com/hardware/automationdirect-gs4-2100.md), [AutomationDirect GS4-21P0](https://machineschema.com/hardware/automationdirect-gs4-21p0.md), [AutomationDirect GS4-22P0](https://machineschema.com/hardware/automationdirect-gs4-22p0.md), [AutomationDirect GS4-23P0](https://machineschema.com/hardware/automationdirect-gs4-23p0.md), [AutomationDirect GS4-27P5](https://machineschema.com/hardware/automationdirect-gs4-27p5.md), [AutomationDirect GS4-4010](https://machineschema.com/hardware/automationdirect-gs4-4010.md), [AutomationDirect GS4-4015](https://machineschema.com/hardware/automationdirect-gs4-4015.md), [AutomationDirect GS4-4030](https://machineschema.com/hardware/automationdirect-gs4-4030.md), [AutomationDirect GS4-4050](https://machineschema.com/hardware/automationdirect-gs4-4050.md), [AutomationDirect GS4-4075](https://machineschema.com/hardware/automationdirect-gs4-4075.md), [AutomationDirect GS4-4100](https://machineschema.com/hardware/automationdirect-gs4-4100.md), [AutomationDirect GS4-4150](https://machineschema.com/hardware/automationdirect-gs4-4150.md), [AutomationDirect GS4-41P0](https://machineschema.com/hardware/automationdirect-gs4-41p0.md), [AutomationDirect GS4-4200](https://machineschema.com/hardware/automationdirect-gs4-4200.md), [AutomationDirect GS4-4250](https://machineschema.com/hardware/automationdirect-gs4-4250.md), [AutomationDirect GS4-4300](https://machineschema.com/hardware/automationdirect-gs4-4300.md), [AutomationDirect GS4-45P0](https://machineschema.com/hardware/automationdirect-gs4-45p0.md), [AutomationDirect GS4-47P5](https://machineschema.com/hardware/automationdirect-gs4-47p5.md), [AutomationDirect SV2A-2040](https://machineschema.com/hardware/automationdirect-sv2a-2040.md), [AutomationDirect SV2A-2075](https://machineschema.com/hardware/automationdirect-sv2a-2075.md), [AutomationDirect SV2A-2150](https://machineschema.com/hardware/automationdirect-sv2a-2150.md), [AutomationDirect SV2A-2200](https://machineschema.com/hardware/automationdirect-sv2a-2200.md), [AutomationDirect SV2A-2300](https://machineschema.com/hardware/automationdirect-sv2a-2300.md), [AutomationDirect SV2A-2550](https://machineschema.com/hardware/automationdirect-sv2a-2550.md), [AutomationDirect SV2A-2750](https://machineschema.com/hardware/automationdirect-sv2a-2750.md), [AutomationDirect SV2A-2F00](https://machineschema.com/hardware/automationdirect-sv2a-2f00.md), [AutomationDirect SV2A-4040](https://machineschema.com/hardware/automationdirect-sv2a-4040.md), [AutomationDirect SV2A-4075](https://machineschema.com/hardware/automationdirect-sv2a-4075.md), [AutomationDirect SV2A-4150](https://machineschema.com/hardware/automationdirect-sv2a-4150.md), [AutomationDirect SV2A-4200](https://machineschema.com/hardware/automationdirect-sv2a-4200.md), [AutomationDirect SV2A-4300](https://machineschema.com/hardware/automationdirect-sv2a-4300.md), [AutomationDirect SV2A-4550](https://machineschema.com/hardware/automationdirect-sv2a-4550.md), [AutomationDirect SV2A-4750](https://machineschema.com/hardware/automationdirect-sv2a-4750.md), [AutomationDirect SV2A-4F00](https://machineschema.com/hardware/automationdirect-sv2a-4f00.md) ## Safety - Some wide-area-network applications may require a character timeout in the 4 to 5 second range. ## 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. ## Sources 1. [modbusoverserial](https://www.modbus.org/file/secure/modbusoverserial.pdf) (accessed 2026-10-25) --- Source page: https://machineschema.com/protocol/modbus-ascii JSON: https://machineschema.com/protocol/modbus-ascii.json License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite Machine Schema and the original sources.