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Appendix B: Profibus
™
DP Slave Communication
58 312785C
About Profibus DP
Profibus DP communication takes the form of block
exchange. Profibus blocks are made of memory bytes
where 2 bytes make a word and 4 bytes a double word,
or Dword.
A block is a range of consecutive byte addresses and is
unidirectional. This means the PLC will exchange data
read only and write only blocks with DP slave units.
Addresses in each device are independent and only rel-
evant to the device for its own program. For example,
the address range for the first block in the master can
start at 256 with a 64 words block length and could
result in the slave with a range starting from 0 with 64
words length.
However, since blocks are consecutive, this means the
first block start address, in the above example 256, will
be the slave start address 0. Then, 257 in the master
would be 1 in the slave, and so on.
Block length can be defined in bytes, words, or double
words. Since start addresses between the master and
the slave can be different, one could give its start
address and length in bytes, and the other in words.
For example, the master start address is byte 256, with
a 64 words block length.
• This results in a range from byte 256 up to byte 383.
The slave start address is word 0, with a 64 words
block length.
• This results in a range from word 0 to word 63. F
IG.
48 illustrates this exchange.
NOTE: Do not forget that the Input block is independent
from the Output block, and although the address range
is the same, the data are different.
NOTE: F
IG. 48 has no connection with the Input Block
and Output Block in Graco Gateway. The terminology
used in the tags configuration is from the PLC point of
view (DP Master). Refer to Graco Gateway Setup for
more information.
F
IG. 48: Master/Slave Exchange
DP Master
Output Block
byte 256 to byte 383
Input Block
byte 256 to byte 383
DP Slave
Input Block
word 0 to word 63
Output Block
word 0 to word 63