72
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Fieldbus controller 750-842
Data exchange
Modular I/O System
ETHERNET TCP/IP
3.2.5.5
Data exchange between master and PLC functionality (CPU)
The MODBUS master and the PLC functionality (CPU) of the PFC regard the
data in a different manner.
Variables data created by the master reach the PFC as input variables and are
further treated there.
Data created in the PFC is sent to the master through the fieldbus as output
variables.
In the PFC the system can access the variable’s data as from I/O word address
256 (double word address 128, byte address 512).
0x000
0x0FF
0x100
0x1FF
%IW0
%IW255
%QW256
%QW511
0x000
(0x200)
0x0FF
(0x2FF)
0x100
(0x300)
%QW0
%IW256
%QW255
%IW511
0x1FF
(0x3FF)
PII
= Process Input
Image
PIO = Process Output
Image
MODBUS master
PII
PIO
PLC functionality (CPU)
Inputs
Outputs
Programmable Fieldbus Controller
Data from point of view
of PLC functionality
Data from point of view
of MODBUS master
I/O modules
variables
I/O modules
variables
I/O modules
variables
I/O modules
variables
PII
SPS-PII
SPS-PIO
Fig. 3-29: Data exchange between MODBUS master and PLC functionality
g012944e
Data access by the MODBUS master
The data can only be accessed by the MODBUS master either word by word or
bit by bit.
Addressing the data from the bus modules starts with word 0 for a word-
by-word access, and also with 0 in word 0 for bit 0 for a bit-by-bit access.
Addressing the data from the variables starts with word 256 for a word-by-
word access, and then, with a bit-by-bit access, addressing starts from:
4096 for bit 0 in word 256
4097 for bit 1 in word 256
...
8191 for bit 15 in word 511.