Appendix 4 MODBUS Communication
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226
5:38400 17ms
50ms
*notes:
In the way of choosing even parity checking, user can obtain the fastest communication
response.
The minimum transmission period is the interval between sending data from master
station and receiving the correct data from the slave.
If the transmission period is smaller than the minimum one, the master station is likely to
receive disorderedly coded data.
Format of MODBUS Messages
When the ac drive communicates with master controller (Such as PC, PLC, etc.), the
master send message to the ac drive and the ac drive sends an answer message to the
master. The process or of MODBUS communication is like the right diagram
description.
As the MODBUS instructions are various, the content may be different. The distance
between two MODBUS messages must keep the under mentioned time.
PLC
Inverter
PLC
Inverter
Inverter
t(ms)
3.5char
Pb.04
response
delay time
3.5char
more than 5ms
send
response
send
PLC
Ac drive Address
: (0
～
31)
When the ac drive address value is set to 0, in broadcast mode, the master sends out
message simultaneously, the ac drive will not give a response to the master.
The Supported MODBUS Instructions:
Instruction
l
h
Right response
l
h
Abnormal
l
h
Instruction
number
(16 bits )
Functions
Minimu
m
Number
of Data
Items
Handled
by One
Messag
e
Maximu
m
Number
of Data
Items
Handled
by One
Messag
e
Minimu
m
Number
of Data
Items
Handled
by One
Messag
e
Maximu
m
Number
of Data
Items
Handled
by One
Messag
e
Minimu
m
Number
of Data
Items
Handled
by One
Messag
e
Maximu
m
Number
of Data
Items
Handled
by One
Message
03H
Read-out of holding
register Content
8 8 7 7 5
5
06H
Write-in to single
8 8 8 8 5 5
08H Loop
back
Test 8 8 8 8 5 5
Slave Address
Function Code
Content
CRC (Error check)
