5.2 Modbus RTU Message Framing Structure
5.2.1 Remote Terminal Unit
The controllers are set up to communicate on the Modbus network using RTU (Remote Terminal Unit) mode, with each 8-bit byte in a message containing
two 4-bit hexadecimal characters.
The format for each byte is shown below.
Start bit
Data bit
Stop/parity Stop
Coding system:
8-bit binary, hexadecimal 0-9, A-F. Two hexadecimal characters contained in each 8-bit field of
the message.
Bits per byte:
1 start bit
8 data bits, least significant bit sent first
Parity: 1 bit for even/odd parity is used; 1 or 2 stop bits if no parity is selected (see par. 8-33).
Error Check Field:
Cyclical Redundancy Check (CRC).
5.2.2 Modbus RTU Message Structure
The transmitting device places a Modbus RTU message into a frame with a known beginning and ending point. This allows receiving devices to begin at
the start of the message, read the address portion, determine which device is addressed (or all devices, if the message is broadcast), and to recognise
when the message is completed. Partial messages are detected and errors set as a result - or timeouts occur. Characters for transmission must be in
hexadecimal 00 to FF format in each field.
The frequency converter continuously monitors the network bus, also during “silent” intervals. When the first field (the address field) is received, each
frequency converter or device decodes it to determine which device is being addressed. Modbus RTU messages addressed to zero are broadcast messages.
No response is permitted for boradcast messages. A typical message frame is shown below.
Start
Address
Function
Data
CRC check
End
T1-T2-T3-T4
1 byte
1 byte
N x 1 byte
2 bytes
T1-T2-T3-T4
Table 5.1: Typical Modbus RTU Message Structure
5.2.3 Start/Stop Field
Messages start with a silent period of at least 3.5 character intervals. This is implemented as a multiple of character intervals at the selected network
baud rate (shown as Start T1-T2-T3-T4). The first field to be transmitted is the device address. Following the last transmitted character, a similar period
of at least 3.5 character intervals marks the end of the message. A new message can begin after this period.
The entire message frame must be transmitted as a continuous stream. If a silent period of more than 1.5 character intervals occurs before completion
of the frame, the receiving device flushes the incomplete message and assumes that the next byte will be the address field of a new message. Similarly,
if a new message begins prior to 3.5 character intervals after a previous message, the receiving device will ignore both messages. This will cause a time-
out (no response from the slave).
5 Modbus RTU
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