WAGO 750-830 Manual page 74

74 • BACnet/IP Controller 750-830
Data Exchange
3.1.6.5.1 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 bus 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 by using the following formula:
Example: 4097 = (256 * 16) + 1
Data access by PLC function (CPU)
The PLC function of the PFC employs a different type of addressing for ac-
cessing 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 num-
ber in the word, separated by a decimal point.
Example:
Bit access by MODBUS to bit number 4097 => Bit addressing in the PLC
<WordNo.>.<BitNo.> = 256.1
The PLC function of the PFC can also access data by bytes and by double-
word access.
Addresses are calculated based on the following equations for byte-based ac-
cess:
Addresses are calculated according to the following equation for double-word-
based access:
BitNo = (word * 16) + BitNo. in word
High-byte address = Word address*2
Low-byte address = (Word address*2) + 1
Double-word address = High word address/2 (rounded down)
or = Low word address/2
WAGO-I/O-SYSTEM 750
BACnet/IP Controller