Download Communication for Windows

Communication Management for Allen-Bradley
14.4
Optimization
Acquisition cycle
and update times
The acquisition cycles defined in the configuration software for the area
pointer for the tags are major factors in respect of the real update times
which are achieved. The update time is the acquisition cycle plus transmission time plus processing time.
In order to achieve optimum update times, the following points should be
observed during configuration:
When setting up the individual data areas, make them as large as necessary but as small as possible.
Define data areas that belong together as contiguous areas. The effective
update time is improved by setting one single large area rather than several smaller areas.
Setting acquisition cycles which are too short unnecessarily impairs overall performance. Set the acquisition cycle to correspond to the modification time of the process values. The rate of change of temperature of a
furnace, for example, is considerably slower than the acceleration curve
of an electric motor.
Guideline value for the acquisition cycle: Approx. 1 second.
If necessary, dispense with cyclic transmission of user data areas (acquisition cycle = 0) in order to improve the update time. Instead, use PLC jobs
to transfer the user data areas at random times.
Store the tags for a message or a screen in a contiguous data area.
In order that changes on the PLC are reliably detected by the operating
unit, they must occur during the actual acquisition cycle at least.
Screens
In the case of screens, the real update time which can be achieved is dependent on:
the number of data areas used,
the type and volume of data to be displayed,
the distribution of data within a particular data area.
In the interests of achieving rapid update times, the following points should
be observed during configuration:
Use only one data block for the tags of a particular screen.
Store the items of data to be used as closely as possible to one another in
the DB.
Only configure short acquisition cycles for those objects which actually
need to be updated quickly.
14-6
Communication for Windows-based Systems User’s Manual
Release 01/99