# MODBUS RTU Protocols · `MODBUS RTU` · verification: source checked MODBUS RTU is the binary serial-line transmission mode of the MODBUS master-slave protocol. A master sends a request frame containing slave address, function code, data, and CRC; only the addressed slave replies. RTU is the default and mandatory mode for MODBUS over serial line, using silent intervals to delimit frames. ## Specification | Property | Value | |---|---| | Transports | serial | | Architecture | master slave | | Encoding | binary | | Standard body | Modbus.org | ### Data types | Name | Size (bits) | Description | |---|---|---| | Coils | 1 | Single-bit read-write data type | | Discrete Inputs | 1 | Single-bit read-only data type | | Input Registers | 16 | 16-bit word read-only data type | | Holding Registers | 16 | 16-bit word read-write data type | ## Connected - Supported by: [AutomationDirect BX-DM1-10AR-D](https://machineschema.com/hardware/automationdirect-bx-dm1-10ar-d.md), [AutomationDirect BX-DM1-10ED1-D](https://machineschema.com/hardware/automationdirect-bx-dm1-10ed1-d.md), [AutomationDirect BX-DM1-10ED2-D](https://machineschema.com/hardware/automationdirect-bx-dm1-10ed2-d.md), [AutomationDirect 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must be shielded, and at one end of each cable its shield must be connected to protective ground. - Line termination is required near each of the two ends of the bus; never place line termination on a derivation cable, and do not place more than two line terminations on one passive D0-D1 balanced pair. - RS232-MODBUS should only be used for short point-to-point interconnection, typically less than 20 m. - A figure of 32 devices is always authorized on any RS485-MODBUS system without repeater; line polarization reduces the maximum number of devices by 4. - For a maximum 9600 baud rate and AWG26 or wider gauge, the maximum trunk length is 1000 m; in the specific case of 4-wire cabling used as a 2-wire system, the maximum length must be divided by two. ## Study notes - RTU mode is mandatory for MODBUS serial-line devices; ASCII mode is optional, and the default setup must be RTU. - RTU frame fields are slave address (1 byte), function code (1 byte), data (0 up to 252 bytes), and CRC (2 bytes); maximum RTU frame size is 256 bytes. - RTU character format is 8 data bits LSB first, 1 start bit, 1 parity bit, and 1 stop bit; no parity requires 2 stop bits. Default parity is even. - Message frames are separated by at least 3.5 character times (t3.5); a silent interval greater than 1.5 character times between characters makes the frame incomplete and it should be discarded. - For baud rates above 19200 bps, recommended fixed timer values are 750 microseconds for t1.5 and 1.750 milliseconds for t3.5. - CRC-16 uses polynomial 0xA001 and initial register value 0xFFFF; the low-order CRC byte is transmitted first, followed by the high-order byte. - The master initiates only one transaction at a time; slave nodes never transmit without a request and never communicate with each other. - Unicast transactions use a slave address from 1 to 247; broadcast uses address 0, is write-only, and returns no response. - Typical turnaround delay after a broadcast is 100 ms to 200 ms; response timeout is application-dependent and is typically 1 s to several seconds at 9600 bps. - The MODBUS data model defines coils and discrete inputs as single-bit objects and input registers and holding registers as 16-bit words. - 9600 bps and 19.2 Kbps are required data signaling rates; 19.2 Kbps is the required default. Optional rates include 1200, 2400, 4800, 38400, 56 Kbps, and 115 Kbps. ## Sources 1. [modbusoverserial](https://www.modbus.org/file/secure/modbusoverserial.pdf) (accessed 2026-10-25) 2. [modbusprotocolspecification](https://www.modbus.org/file/secure/modbusprotocolspecification.pdf) (accessed 2026-10-25) --- Source page: https://machineschema.com/protocol/modbus-rtu JSON: https://machineschema.com/protocol/modbus-rtu.json License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite Machine Schema and the original sources.