Appendix B: Profibus™ DP Slave Communication
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. FIG. 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: FIG. 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.
DP Master
Output Block
byte 256 to byte 383
Input Block
byte 256 to byte 383
FIG. 48: Master/Slave Exchange
DP Slave
Input Block
word 0 to word 63
Output Block
word 0 to word 63
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