VLT® Serie 2800 and VLT® Serie 6000 Modbus RTU
Network
Configuration
å Network Configuration
å Remote Terminal Unit
The controllers are setup to communicate on the
Modbus network using RTU (Remote Terminal
Unit) mode, with each 8-bit byte in a message
contains two 4-bit hexadecimal characters. The
format for each byte is shown below.
Coding System: 8-bit binary, hexadecimal 0-9, A-F
Two hexadecimal characters contained in
each 8-bit field of the message
Bits Per Byte: 1startbit
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)
Start bit Data bit Stop/
parity
Stop
å Modbus RTU Message Framing Structure
A Modbus RTU message is placed by the transmitting
device 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 know when
the message is completed. Partial messages are
detected and errors set as a result.
The allowable characters transmitted for all fields
are hexadecimal 00 to FF. The frequency converter
monitor the network bus continuously, including ‘silent’
intervals. When the first field (the address field) is
received, each frequency converter or device decodes
it to determine whether it is the addressed device.
Modbus RTU messages addressed to zero are
broadcast messages. No response is permitted
on broadcast messages.
A typical message frame is shown below.
Start Address Function Data CRC Check End
T1-T2-T3-T4 8Bits 8Bits nx8bits 16 Bits T1-T2-T3-T4
Typical Modbus RTU Message Structure
å Start/Stop Field
Messages start with a silent interval of at least 3.5
character times. This is implemented as a multiple of
character times at the selected network baud rate
(shown as Start T1-T2-T3-T4). The first field then
transmitted is the device address. Following the last
transmitted character, a similar interval of at least 3.5
character times marks the end of the message. A
new message can begin after this interval.
The entire message frame must be transmitted as a
continuous stream. If a silent interval of more than
1.5 character times 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 earlier that 3.5
character times following a previous message, the
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