User Guide

FS-8700-20 Profibus DP Driver Manual Page 17 of 22
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Appendix B. Driver Notes
Appendix B.1. Understanding buffer types and Map Descriptor functions
The following DP_Buffer types and Map Descriptor functions are normally used together
when the FieldServer is used as a DP Master:
DP Master:
DP_Buffer Function
Output WRBC
Input RDBC
The Buffer functions are reversed when using the FieldServer as a DP Slave:
DP Slave:
DP_Buffer Function
Output RDBC
Input WRBC
The diagram below explains the data flows for both buffers when using the FieldServer as a
DP Master:
Note that for the Input buffer, the data is coming from the external DP Slave and is
transferred into the DP Master card’s internal input buffer. The Map Descriptor being of the
RDBC function ( Read Data Block Continuous ) transfers the data from the card buffer into
the FieldServer’s Data Array packing it correctly according to the specified DP_Type.
For the Output buffer, the data is written from the FieldServer’s Data Array by the WRBC
function Map Descriptor ( Write Data Block Continuous ) into the DP Master card’s internal
output buffer from where it is transferred across the Profibus network to the external DP
Slave’s input buffer.