Rockwell Automation 2711 PanelView 550/900 Profibus Communications User Guide - Manual (Page 14)

PROFIBUS DP Communications 14
Publication 2711-6.3
Real-Time Module Data Exchange
Real Time data is automatically exchanged and updated in the DP
Master’s real-time module without any action required by the DP
Master. There is no confirmation (handshaking) to indicate the
availability of new data. Use the real-time modules to transfer data
that must be updated most frequently.
Real Time I/O is limited to a maximum of 16 words of both input
and output data.
Note: Real Time module data is not consistent.
Multiplex Module Data Exchange
New multiplex data blocks are exchanged when a request is made by
the DP Master’s program. The “command word” (word 0) in the
multiplex output module and the “status word” (word 0) in the
multiplex input module are used to manage the exchange of
multiplex data blocks.
Since a module is 16 words in length, the amount of data in a
multiplex input or output block is up to 15 words. The DP Master
program is responsible for managing the command and status words
and for moving multiplex blocks in or out of the multiplex modules.
Note: Multiplex module data is consistent.