Configuring Hardware and Communication Connections STEP 7.pdf

Configuring the Distributed I/O (DP)
3.12 Setting Constant Bus Cycle Times for PROFIBUS
Subnets
Introduction
Bus Cycle Time
For PROFIBUS subnets you can set constant (of equal length) bus cycle times in
STEP 7.
Constant bus cycle time is the property of the PROFIBUS-DP that guarantees bus
cycles that are exactly the same length. "Bus cycles of the same length" means
that the DP master always begins the DP bus cycle after the same time interval.
From the viewpoint of the connected slaves, this means that they also receive their
data from the master in time intervals of exactly the same duration.
The following figure shows how the time for a bus cycle is made up.
The "variable pause" shown in the figure is always minimal if communication jobs,
for example, for other active nodes are still pending. The master (also known as
the constant-bus-cycle-time master) controls the communication parts so that the
same duration for a bus cycle is always achieved.
Configuring Hardware and Communication Connections with STEP 7
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