Example Using The Modbus Function - ABB AC 31 Technical Manual

ABB Control - AC 31
1SBC 260400 R1001-b 06.99
Communication

1.3.4. Example using the MODBUS function

An example of MODBUS communication between two 40 series central units.
- A program isn't required in the slave central unit.
- A program example in the master central unit:
Read the 6 bits I62.00 to I62.05 in the slave 1 central unit.
The information is placed in O62.00 to O62.05 of the master central unit.
The MODBUS address of variable I62.00 in the slave 1 is 992 in decimal.
A delay should be defined to enable re-sending the MODBUS message in case of error. This
delay is called TIME_OUT and is generally 2 seconds.
READY
SLAVE_1
READ_BITS
TIME_2S
992
6
%O62.00
In case of MODBUS communication between one master 50 serie central unit and different
slaves 50 series central units, different MODBUS functions can be used. It is possible to link
them together to optimize the refresh time.
Check example : Read 6 bits continuously I62.00 to I62.05 in the slave 1 central unit. The
information is placed in O62.00 to O62.05 of the master central unit. Write 20 words continuously
IW02.00 to IW02.19 in the slave 2 central unit. The information is received from master central
unit in MW00.00 to MW00.19.
SLAVE_1
READ_BITS
TIME_2S
992
6
%O62.00
40 series
MODBUS
master
MODBUS protocol
RS232 line
MODBUS
FREI
SLAV
FCT
TIME
ADDR RDY
NB ERR
DATA ERN
MODBUS
FREI
SLAV
FCT
TIME
ADDR RDY
ERROR1
NB ERR
ERROR_CODE1
DATA ERN
SLAVE_2
WRITE_WORDS
TIME_2S
0000
20
%MW000.00
40 series
MODBUS
slave 1
READY
ERROR
ERROR_CODE
MODBUS
FREI
SLAV
FCT
TIME
ADDR RDY
ERROR2
NB ERR
ERROR_CODE2
DATA ERN
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