Appendix B: Profibus
®
DP Secondary Communication
36
3A7639A
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 Secondary
units. Addresses in each device are independent and
only relevant to the device for its own program. For
example, the address range for the first block in the Pri-
ority can start at 256 with a 64 words block length and
could result in the Secondary 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 Secondary start address 0. Then, 257 in the Pri-
ority would be 1 in the Secondary, and so on.
Block length can be defined in bytes, words, or double
words. Since start addresses between the Priority and
the Secondary can be different, one could give its start
address and length in bytes, and the other in words.
For example, the Priority start address is byte 256, with
a 64 words block length.
•
This results in a range from byte 256 up to byte 383.
The Secondary start address is word 0, with a 64
words block length.
•
This results in a range from word 0 to word 63. F
IG
.
23 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
. 23 has no connection with the Input Block
and Output Block in the Gateway Assembly. The termi-
nology used in the tags configuration is from the PLC
point of view (DP Priority). Refer to
Refer to your Crimson 3.1 User Manual for setup.
F
IG
. 23: Priority/Secondary Exchange
DP Priority
Output Block
byte 256 to byte 383
Input Block
byte 256 to byte 383
DP Secondary
Input Block
word 0 to word 63
Output Block
word 0 to word 63