WAGO 750 Series Manual page 51

The MODBUS PFC variables are mapped after the process image of the I/O
modules. This memory area contains 256 words (word 256 to 511).
If the quantity of I/O data is greater than 256 words, the additional data is
appended after the MODBUS PFC variables in word 512 to 1275. Like the
first physical I/O process image area, there is a separate memory area for input
and output data, but both are referenced with an index of 512 to 1275 for word
operations.
After the remaining physical I/O data is the Ethernet IP PFC variables. This
memory area is word 1276 to 1531.
For future protocol additions, the area above word 1532 is reserved for
additional PFC variables.
With all WAGO fieldbus controllers, the method used by PLC functions to
access process data is independent of the fieldbus system. This access always
takes place via an application-related IEC 61131-3 program.
In contrast to the above, access from the fieldbus side is fieldbus specific. For
the ETHERNET TCP/IP fieldbus controller, either a MODBUS/TCP master
or an Ethernet/IP master is used. MODBUS/TCP accesses the data via
implemented MODBUS functions. Here decimal and/or hexadecimal
MODBUS addresses are used. With Ethernet/IP, data access occurs with the
use of an object model.
Further information
A detailed description of these fieldbus-specific data access operations is
given in the sections "MODBUS functions" and "Ethernet/IP
(Ethernet/Industrial Protocol)".
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
Fieldbus Controller 750-841
Process Image
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