Modbus ASCII vs RTU (Serial Data Transmission Modes)

1.2 Two Serial Transmission Modes
Controllers can be setup to communicate on standard Modbus networks using either of two transmission modes: ASCII or RTU. Users select the desired mode, along with the serial port communication parameters (baud rate, parity mode, etc), during configuration of each controller. The mode and serial parameters must be the same for all devices on a Modbus network.


1.2.1 ASCII Mode
When controllers are setup to communicate on a Modbus network using ASCII (American Standard Code for Information Interchange) mode, each eight-bit byte in a message is sent as two ASCII characters. The main advantage of this mode is that it allows time intervals of up to one second to occur between characters without causing an error.

Coding System
V Hexadecimal, ASCII characters 0 ... 9, A ... F
V One hexadecimal character contained in each ASCII character of the message

Bits per Byte
V 1 start bit
V 7 data bits, least significant bit sent first
V 1 bit for even / odd parity-no bit for no parity
V 1 stop bit if parity is used-2 bits if no parity

Error Check Field
V Longitudinal Redundancy Check (LRC)


1.2.2 RTU Mode
When controllers are setup to communicate on a Modbus network using RTU (Remote Terminal Unit) mode, each eight-bit byte in a message contains two four-bit hexadecimal characters. The main advantage of this mode is that its greater character density allows better data throughput than ASCII for the same baud rate. Each message must be transmitted in a continuous stream.

Coding System
V Eight-bit binary, hexadecimal 0 ... 9, A ... F
V Two hexadecimal characters contained in each eight-bit field of the message

Bits per Byte
V 1 start bit
V 8 data bits, least significant bit sent first
V 1 bit for even / odd parity-no bit for no parity
V 1 stop bit if parity is used-2 bits if no parity

Error Check Field
V Cyclical Redundancy Check (CRC)