User manual

IEC61131 User and Reference Manual
April 22, 2008
502
Interval parameter is set to 60 then the master will poll the slave station every hour. Valid values
are 1 to 32767. The default value is 60.
 The Poll Offset parameter is used to distribute the load on the communication network. The Poll
Offset is entered in multiples of the base poll interval. Valid values for this parameter are 0 to the
Poll Interval value minus 1. Any non-zero value delays the start of polling for the specified
objects by that amount. The default value is 0. This control is disabled when None is selected,
and enabled otherwise. For an example of using the Poll Offset parameter see the Poll Offset
Example at the end of this section.
 Limit Maximum Events allows limiting the number of events in poll responses for Class 1/2/3
data. The checkbox is not checked by default, meaning there is no limit on the number of events.
Select the checkbox to specify a limit. The valid values for this parameter are 1 to 65535. The
default value is 65535. This control is disabled when None is selected, and enabled otherwise.
The Maximum Events parameter can be used to manage communication load on a system.
Consider the example of a master polling some data logging remotes, and the case where one of
the remotes has been offline for a long time. The remote will have built up a large number of
buffered events. If the master polled it for all events, the reply might take a long time, and cause
an unwanted delay in the master's polling cycle. However if the master limits the number of
events returned, the reply message duration will be more deterministic and the master can ensure
its poll loop timing is maintained. In this case, the event retrieval from the data logger will be
distributed over a number of poll cycles.
The Class 2 Polling section of the dialog specifies the type and rate of polling for Class 2 data.
 The None selection disables class 1 polling for the slave station. This is the default selection.
 The At Start Up Only selection will cause the master to poll the slave station at startup only.
 The Interval selection will cause the master to poll the slave station at startup and then every
Interval of the base poll interval. For example if the base poll interval is 60 seconds and the
Interval parameter is set to 60 then the master will poll the slave station every hour. Valid values
are 1 to 32767. The default value is 60.
 The Poll Offset parameter is used to distribute the load on the communication network. The Poll
Offset is entered in multiples of the base poll interval. Valid values for this parameter are 0 to the
Poll Interval value minus 1. Any non-zero value delays the start of polling for the specified
objects by that amount. The default value is 0. This control is disabled when None is selected,
and enabled otherwise. For an example of using the Poll Offset parameter see the Poll Offset
Example at the end of this section.
 Limit Maximum Events allows limiting the number of events in poll responses for Class 1/2/3
data. The checkbox is not checked by default, meaning there is no limit on the number of events.
Select the checkbox to specify a limit. The valid values for this parameter are 1 to 65535. The
default value is 65535. This control is disabled when None is selected, and enabled otherwise.
The Maximum Events parameter can be used to manage communication load on a system.
Consider the example of a master polling some data logging remotes, and the case where one of
the remotes has been offline for a long time. The remote will have built up a large number of
buffered events. If the master polled it for all events, the reply might take a long time, and cause
an unwanted delay in the master's polling cycle. However if the master limits the number of
events returned, the reply message duration will be more deterministic and the master can ensure
its poll loop timing is maintained. In this case, the event retrieval from the data logger will be
distributed over a number of poll cycles.
The Class 3 Polling section of the dialog specifies the type and rate of polling for Class 3 data.