message, or if the slave is unable to perform the requested
action, the slave will construct an error message, and send
it in response, or a time-out occurs.
7.5.4 Frequency Converter with Modbus
RTU
The frequency converter communicates in Modbus RTU
format over the built-in RS-485 interface. Modbus RTU
provides access to the Control Word and Bus Reference of
the frequency converter.
The Control Word allows the Modbus master to control
several important functions of the frequency converter:
•
Start
•
Stop of the frequency converter in various ways:
Coast stop
Quick stop
DC Brake stop
Normal (ramp) stop
•
Reset after a fault trip
•
Run at a variety of preset speeds
•
Run in reverse
•
Change the active set-up
•
Control the frequency converter’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 frequency converter when its internal PI controller is
used.
7.5.5 Frequency Converter with Modbus
RTU
To enable Modbus RTU on the frequency converter, set the
following parameters
Parameter Setting
8-30 Protocol
Modbus RTU
8-31 Address
1 - 247
8-32 Baud Rate
2400 - 115200
8-33 Parity / Stop Bits
Even parity, 1 stop bit (default)
Table 7.11
7.6 Modbus RTU Message Framing
Structure
7.6.1 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 2 4-bit hexadecimal
characters. The format for each byte is shown in Table 7.12.
Start
bit
Data byte Stop/
parity
Stop
Table 7.12 Format for each byte
Coding System 8-bit binary, hexadecimal 0-9, A-F. 2
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
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)
Table 7.13
7.6.2 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 in Table 7.14.
Start Address Function Data CRC
check
End
T1-T2-T3-
T4
8 bits 8 bits N x 8
bits
16 bits T1-T2-T3-
T4
Table 7.14 Typical Modbus RTU Message Structure
RS-485 Installation and Set...
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