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Function Description
WAGO-I/O-SYSTEM 750
750-880, 750-880/025-000 ETHERNET Programmable Fieldbus Controller
Manual
Version 1.0.1
Pos: 75.43 /Serie 750 (WAGO-I/O-SYSTEM)/Funktionsbeschreibung/Datenaustausch/Datenaustausch - Datenaustausch Master und SPS-Funktionalität (CPU) @ 6\mod_1256049969000_21.doc @ 43316 @ 3 @ 1
7.3.5
Data Exchange between Master and PLC Function (CPU)
The fieldbus master and the PLC function (CPU) of the PFC have different
perspectives on data.
Variable data generated by the master are routed as input variables to the PFC,
where they are further processed.
Data created in the PFC are transmitted via fieldbus to the master as output
variables.
In the PFC, access to the MODBUS/TCP PFC variable data is possible starting
from word address 256 to 511 (double-word address 128-255, byte address 512-
1023), while access to the PFC variable data is possible starting from a word
address of 1276 to 1531 (double-word address 638-765, byte address 2552-3063).
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7.3.5.1
Example of MODBUS/TCP Master and PLC Function (CPU)
Data access by the MODBUS/TCP Master
Access to data by the MODBUS Master is always either by word or by bit.
Addressing of the first 256 data words by the I/O modules begins with word-by-
word and bit-by-bit access at 0.
Addressing of the data by the variables begins at 256 for word-based access;
bit-by-bit access then takes place starting at:
4096 for bit 0 in word 256
4097 for bit 1 in word 256
...
8191 for bit 15 in word 511.
The bit number can be determined using the following equation:
Bit No. = (word * 16) + Bit No. in word
Example: 4097 = ( 256 * 16) + 1
Data Access by PLC Function (CPU)
The PLC function of the PFC employs a different type of addressing for accessing
the same data. PLC addressing is identical with word-by-word addressing by the
MODBUS Master for the declaration of 16-bit variables. However, a different
notation is used for declaration of Boolean variables (1 bit) than that used by
MODBUS. Here, the bit address is composed of the elements word address and
bit number in the word, separated by a decimal point.
Example:
Bit access by MODBUS to bit number 4097 => Bit addressing in the PLC
<Word No.>.<Bit No.> = 256.1
The PLC function of the PFC can also access data by bytes and by doubleword
access.
