Rs485: Modbus Rtu Telegram Structure - Danfoss VLT HVAC Drive FC 102 Design Manual

Appendix
message, or if the slave is unable to perform the requested
action, the slave constructs an error message which it
sends in response, or a timeout occurs.
14.7.4 Drive with Modbus RTU
The drive communicates in Modbus RTU format over the
built-in RS485 interface. Modbus RTU provides access to
the control word and bus reference of the drive.
The control word allows the Modbus master to control
several important functions of the drive:
•
Start
•
Stop of the drive in various ways:
Coast stop
Quick stop
DC brake stop
Normal (ramp) stop
•
Reset after a fault trip.
•
Run at various preset speeds.
•
Run in reverse.
•
Change the active set-up.
•
Control the built-in relay of the drive.
The bus reference is commonly used for speed control. It is
also possible to access the parameters, read their values,
and, where possible, write values to them, allowing a
range of control options, including controlling the setpoint
of the drive when its internal PI controller is used.
14.7.5 Drive with Modbus RTU
To enable Modbus RTU on the drive, set the following
parameters:
Parameter Setting
Parameter 8-30 Protocol Modbus RTU
Parameter 8-31 Address 1–247
Parameter 8-32 Baud Rate 2400–115200
Parameter 8-33 Parity / Even parity, 1 stop bit (default)
Stop Bits
14.7.6 Drive with Modbus RTU
The controllers are set up to communicate on the Modbus
network using RTU mode, with each byte in a message
containing 2 4-bit hexadecimal characters. The format for
each byte is shown in Table 14.10.
Start Data byte
bit
Table 14.10 Example Format
MG16C302
Design Guide
Coding system
Bits per byte
Error check field
Table 14.11 Bit Detail
14.8 RS485: Modbus RTU Telegram
Structure
14.8.1 Modbus RTU Telegram Structure
The transmitting device places a Modbus RTU message
into a frame with a known beginning and ending point.
Receiving devices are able 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 recognize when the message is
completed. Partial messages are detected and errors set as
a result. Characters for transmission must be in
hexadecimal 00–FF format in each field. The drive contin-
uously monitors the network bus, also during silent
intervals. When the first field (the address field) is received,
each drive or device decodes it to determine which device
is being addressed. Modbus RTU messages addressed to 0
are broadcast messages. No response is allowed for
broadcast messages. A typical message frame is shown in
Table 14.12.
Start
T1-T2-T3-
T4
Table 14.12 Typical Modbus RTU Telegram Structure
14.8.2 Start/Stop Field
Messages start with a silent period of at least 3.5 character
intervals, implemented as a multiple of character intervals
at the selected network baud rate (shown as start T1-T2-
T3-T4). The first transmitted field 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
Stop/ Stop
intervals occurs before completion of the frame, the
parity
receiving device flushes the incomplete message and
assumes that the next byte is the address field of a new
message. Similarly, if a new message begins before 3.5
Danfoss A/S © 11/2017 All rights reserved.
8-bit binary, hexadecimal 0–9, A–F. 2
hexadecimal characters contained in each 8-
bit field of the message.
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.
CRC (cyclical redundancy check)
Address Function Data
8 bits 8 bits N x 8 bits
CRC End
check
16 bits T1-T2-T3-
T4
14
233
14