Fieldbus controller 750-842
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73
Data exchange
Modular I/O System
ETHERNET TCP/IP
The bit number can be defined using the following formula:
BitNo = (Word * 16) + Bitno_in_Word
Data access by the PLC functionality
When accessing the same data, the PLC functionality of the PFCs uses a dif-
ferent type of addressing.
When declaring 16 bit variables, the PLC addressing is identical to the ad-
dressing of the MODBUS master made word-by-word.
When declaring Boolean variables (1 bit) a notation different to that of the
MODBUS is used.
The bit address is composed of the elements word address and bit number in
the word, separated by a dot.
Example:
Bit access MODBUS to bit number 4097 => bit addressing in the PLC
<Wordno>.<Bitno> = 0.1
The PLC functionality of the PFC can also access the data byte-by-byte and
double word-by-double word.
With the bytewise access, the addresses are computed according to the fol-
lowing formula:
High-Byte Address = Word address*2
Low-Byte Address = (Word address*2) + 1
With the access by a double word, the address is computed according to the
following formula:
Double word address = High word address/2 (rounded off)
or = Low word address/2
3.2.5.6
Common access of MODBUS master and PLC functionality to outputs
The process illustration of outputs is described both by the MODBUS master
as well as by the PLC functionality, so that the I/O module outputs can be set
or reset from both sides. Design the user programs of the MODBUS master
and the PLC functionality such that conflicting instructions for simultaneous
setting or resetting of outputs is excluded.
