User manual

IEC61131 User and Reference Manual
April 22, 2008
503
 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 Time Synchronization Rate section of the dialog specifies the rate of issuing a time
synchronization to this device, as a multiple of the base poll interval. Valid selections for this
parameter are:
 The None selection will disable issuing a time sync to this device. This is the default selection.
 The At Start Up Only selection will cause issuing a time synchronization at startup only.
 The Interval selection will cause the RTU to issue a time synchronization at startup and then
every Interval of the base poll interval seconds. Valid entries for Interval are between 1 and
32767 the base poll interval seconds. The default value is 60.
The Unsolicited Responses section is used in conjunction with the Enable Unsolicited Responses on
Start Up parameter on the Application Layer page. Certain non-SCADAPack slave devices are
designed to start with their Enable Unsolicited Responses on Start Up parameter set to No. Selecting
Enabled for any class causes the master to (after it detects the slave come online) send a command
allowing the slave to begin sending Unsolicited Responses of that class.
With SCADAPack slaves the Enable Unsolicited Responses on Start Up parameter may be set to
Yes, and the Accept Class parameters may be left at Disabled.
 The Accept Class 1 selection displays the enable/disable status of unsolicited responses from the
slave device for Class 1 events. The default selection is disabled.
 The Accept Class 2 selection displays the enable/disable status of unsolicited responses from the
slave device for Class 1 events. The default selection is disabled.