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RS485 Installation and Set-...
•
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.
8.7 Network Configuration
To enable Modbus RTU on the frequency converter, set the
following parameters:
Parameter
8-30 Protocol
8-31 Address
8-32 Baud Rate
8
8
8-33 Parity / Stop Bits
8.8 Modbus RTU Message Framing
Structure
8.8.1 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 2 4-bit hexadecimal
characters. The format for each byte is shown in Table 8.1.
Start
bit
Coding System
Bits Per Byte
Error Check Field
8.8.2 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
76
VLT Micro Drive Design Guide
Setting
Modbus RTU
1 - 247
2400 - 115200
Even parity, 1 stop bit (default)
Data byte
Stop/
parity
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
Cyclical Redundancy Check (CRC)
MG02K102 - VLT
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 8.1.
Start
Address
T1-T2-T3-
8 bits
T4
Table 8.1 Typical Modbus RTU Message Structure
8.8.3 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
Stop
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.
8.8.4 Address Field
The address field of a message frame contains 8 bits. Valid
slave device addresses are in the range of 0 – 247 decimal.
The individual slave devices are assigned addresses in the
range of 1 – 247. (0 is reserved for broadcast mode, which
all slaves recognize.) A master addresses a slave by placing
the slave address in the address field of the message.
When the slave sends its response, it places its own
address in this address field to let the master know which
slave is responding.
8.8.5 Function Field
The function field of a message frame contains 8 bits. Valid
codes are in the range of 1-FF. Function fields are used to
send messages between master and slave. When a
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Function
Data
CRC
check
8 bits
N x 8
16 bits
bits
End
T1-T2-T3-
T4
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