Modbus Protocol Specification

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)
1.3 Modbus Message Framing
In either of the two serial transmission modes (ASCII or RTU), a Modbus message is placed by
the transmitting device into a frame that has a known beginning and ending point. This allows
receiving devices to begin at the start of the message, read the address portion and determine
which device is addressed (or all devices, if the message is broadcast), and to know when the
message is completed. Partial messages can be detected and errors can be set as a result.
On networks like MAP or Modbus Plus, the network protocol handles the framing of messages
with beginning and end delimiters that are specific to the network. Those protocols also handle
delivery to the destination device, making the Modbus address field imbedded in the message
unnecessary for the actual transmission. (The Modbus address is converted to a network node
address and routing path by the originating controller or its network adapter.)
1.3.1 ASCII Framing
In ASCII mode, messages start with a colon ( : ) character (ASCII 3A hex), and end with a
carriage return-line feed (CRLF) pair (ASCII 0D and 0A hex).
The allowable characters transmitted for all other fields are hexadecimal 0 ... 9, A ... F.
Networked devices monitor the network bus continuously for the colon character. Wh
Modbus Protocol
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