Modbus Rtu Message Framing Structure - Danfoss VLT AutomationVTDrive FC322 Design Manual

Automation VT Drive FC322 Design Guide

7.8 Modbus RTU Message Framing Structure

7.8.1 Frequency Converter with Modbus RTU
The controllers are set up to communicate on the Modbus network using RTU (Remote Terminal Unit) mode, with each byte in a message containing two
4-bit hexadecimal characters. The format for each byte is shown below.
Start bit Data byte
Coding System 8-bit binary, hexadecimal 0-9, A-F. Two hexadecimal characters contained in each 8-bit field of the
Bits Per Byte 1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity; no bit for no parity
1 stop bit if parity is used; 2 bits if no parity
Error Check Field Cyclical Redundancy Check (CRC)
7.8.2 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. 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 broadcast messages. A typical message frame is shown below.
Typical Modbus RTU Message Structure
Start Address Function
T1-T2-T3-T4 8 bits 8 bits
7.8.3 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 consider it a continuation of the previous message. This will cause a time-out (no response
from the slave), since the value in the final CRC field will not be valid for the combined messages.
MG.20.X1.22 - VLT
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7 RS-485 Installation and Set-up
Stop/ Stop
parity
message
Data CRC check End
N x 8 bits 16 bits T1-T2-T3-T4
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