Configuring Hardware and Communication Connections STEP 7.pdf

Configuring the Distributed I/O (DP)
Constant Bus Cycle Time Behavior
!
When recalculating times, STEP 7 recommends a value for the constant bus cycle
time. This value is based on the configuration in question. However, you can
change this value.
When STEP 7 calculates this value for the constant bus cycle time, it includes in its
calculation the user data traffic of the DP master as well as an allowance for a few
errors that may possibly occur.
STEP 7 also calculates a minimum value for the constant DP bus cycle time. The
cycle time cannot fall below this value. When STEP 7 calculates the minimum
value, it considers only the normal message frames for each bus cycle. If errors
occur, there may be violations of the constant bus cycle time.
Times that are longer than the recommended times are possible without any
problems.
Caution
If you select times that are shorter than the time that the system recommends,
under certain circumstances the communication of the additional active nodes that
are connected to the PROFIBUS subnet is delayed, or, in the worst case, comes to
a standstill. If you set values close to the minimum possible constant bus cycle
time that is displayed, bus faults can cause the entire PROFIBUS subnet to be shut
down in certain cases.
Relationship: Constant Bus Cycle Time and SYNC/FREEZE
For PROFIBUS-DP, if you configure both "constant bus cycle time" and
SYNC/FREEZE groups, note the following:
• Group 8 must not be used (reserved for constant bus cycle time clock). If you
configure the group assignment first and have assigned group 8, you can no
longer set constant bus cycle time.
• If you configure group 7 when the constant bus cycle time is set, you cannot
use the SYNC or FREEZE functions for the slaves in this group.
Configuring Hardware and Communication Connections with STEP 7
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