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Communication protocols
Modbus > Modbus Slave - Function codes
Conversion
15
14
13
12
start_register-offset_DB_no. = 00h
Further values
start_register
hex
decimal
0000h
0
01FAh
500
0384h
900
03FFh
1023
6.4.8.4
FC 04 - Read Input Registers
84
For e.g. start_register = 80 (0050h) the conversion takes place with the following
approach:
start_register = 0050h
11
10
9
n
Data block DB = Base-DB-Number + start_register-offset_DB_no.
–
Data block DB = 800 + 0 = 800
n
Data word DBW = start_register-word-no. x 2
–
Data word DBW = 80 x 2 = 160
offset_DB_no.
word_no.
decimal
hex
0
000h
0
1F4h
1
184h
1
1FFh
n
This function is identical to FC 03.
n
The corresponding address allocation of the CPU memory area are established by
the properties of "FC 04" in the parameterization of the CP. Here the fixed "Modbus
address in transmission message" 0 may be assigned to a Base-DB-Number in the
"SIMATIC memory area".
n
For more information see FC 03.
page 83
8
7
6
5
start_register-word-no. = 50h
Base DB
Number
decimal
decimal
0
800
500
800
388
800
511
800
Ä Chap. 6.4.8.3 'FC 03 - Read Output Registers'
VIPA System 300S
4
3
2
1
DB
DBW
decimal
decimal
800
0
800
1000
801
776
801
1022
HB130 | CP | 341-1CH01 | en | 18-40
0
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