Modbus Rtu Message Framing Structure - Danfoss VLT AQUA Drive FC 202 Design Manual

System Integration
3.8.9.3 Adjustable Frequency Drive with
Modbus RTU
The adjustable frequency drive communicates in Modbus
3 3
RTU format over the built-in RS485 interface. Modbus RTU
provides access to the control word and bus reference of
the adjustable frequency drive.
The control word allows the Modbus master to control
several important functions of the adjustable frequency
drive:
•
Start
•
Stop of the adjustable frequency drive in various
ways:
•
Reset after a fault trip
•
Run at a variety of preset speeds
•
Run in reverse
•
Change the active set-up
•
Control the adjustable frequency drive's built-in
relay
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. This permits a
range of control options, including controlling the setpoint
of the adjustable frequency drive when its internal PI
controller is used.
3.8.9.4 Network Configuration
To enable Modbus RTU on the adjustable frequency drive,
set the following parameters:
Parameter
8-30 Protocol
8-31 Address
8-32 Baud Rate
8-33 Parity / Stop Bits
Table 3.33 Modbus RTU Parameters
88
- Coast stop
- Quick stop
- DC brake stop
- Normal (ramp) stop
Setting
Modbus RTU
1-247
2400-115200
Even parity, 1 stop bit (default)
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VLT AQUA Drive FC 202
3.8.10 Modbus RTU Message Framing
3.8.10.1 Adjustable Frequency Drive with
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 3.34.
Start
bit
Table 3.34 Format for Each Byte
Coding system
Bits per byte
Error check field
3.8.10.2 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 recognize when the message is
completed. Partial messages are detected, and errors are
set as a result. Characters for transmission must be in
hexadecimal 00 to FF format in each field. The adjustable
frequency drive continuously monitors the network bus,
also during silent intervals. When the first field (the address
field) is received, each adjustable frequency drive 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 in Table 3.35.
Start
T1-T2-T3-
T4
Table 3.35 Typical Modbus RTU Message Structure
Structure
Modbus RTU
Data byte
8–bit binary, hexadecimal 0–9, A–F.
Two 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.
Cyclical redundancy check (CRC).
Address Function Data
8 bits 8 bits N x 8 bits
Stop/ Stop
parity
CRC End
check
16 bits T1-T2-T3-
T4
MG20N622