HEIDENHAIN PROFIBUS-DP (DPV2) - Manual (Page 50)

PROFIBUS IO data description
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TMSG (Message Time)
The times TMSG may elapse to handle all acyclic services
between the master and slave. These acyclic services shall be
executed after the cyclic services. To ensure an Isochronous DP
cycle this part shall be limited.
TDP (DP-Cycle Time)
TDP is the time a DP cycle lasts.
Content of a DP cycle:
SYNCH: Global_Control telegram for synchronization. The end of
the Global_Control (GC) telegram marks the beginning of a new
DP cycle.
DX: Data_Exchange
With the service Data_Exchange, user data exchange between
master and slave 1-n is executed sequentially.
MSG: acyclic services. After cyclic transmission the master may
transmit an acyclic service. e.g. parameter request via MS1/MS2
AR.
RES: Reserve
The reserve consists of the "active spar time" which is used as an
active rest (master transmits to itself) and the "passive spar time".