L-VIS User Manual
178
LOYTEC
Version 6.2
LOYTEC electronics GmbH
10.1.4 Persistency
Data point values are by default not persistent. This means that their value is lost after a
power-on reset. There exist different strategies for initializing data points with an
appropriate value after the device has started.
For input data points, the value can be actively polled from the network when starting up.
Use the Poll-on-Startup feature for this behavior. Polling the network values has the
advantage that intermediate changes on the network are reflected. An input data point can
be made persistent, if the last received value shall be available after a power-on reset before
a poll-on-startup completes. This can be beneficial, if the remote device is temporarily
offline and the last value is considered usable.
For output data points, the value can be restored after starting up by the application. For
example, if the output data point’s value is determined by an input data point and a math
object, or the output data point is in a connection with an input, the input can poll its value
on startup. If the output data point has no specific other value source, e.g., it is a
configuration parameter set by the user, it can be made
persistent
.
To make a data point persistent, enable the Persistent property of the respective data point.
The persistency option is only available for the base data point classes analog, binary,
multi-state, string and user. More complex objects such as calendars, schedules, etc., have
their own data persistency rules.
For structured data points, only all or none of the structure members can be made persistent.
The configuration of the top-level data point, which represents the entire structure, serves as
a master switch. Setting the top-level data point to be persistent enables persistency for all
sub-data points. Clearing it disables persistency for all sub-data points.
NOTE:
Persistent input data points may receive value updates after system start up; regardless of
the data point update flag System Startup (see Section 9.6.2). This happens when a saved
data point value is restored after system boot up. Data points which use updates as events
to trigger certain actions should therefore not be set persistent.
10.1.5 Parameters
A data point can be qualified as a
parameter
data point. This is accomplished in the
configuration software by setting a
Parameter
check box on the data point. Those
parameter data points are automatically persistent and will typically have a default value.
Their purpose is to store parameterization values, which can be changed from the default
value at run time and influence the behavior of the device or the logic running on the
device. This way, a number of devices can have the same basic configuration and be
adapted by parameter values. Examples are sunblind run times for control logic or
descriptive strings for the L-WEB visualization.
The qualified parameter data points are also exported via a parameter file, which contains
the entire set of current parameter values including meta-information for external tools to
display parameter data in a human-readable way. The LWEB-821/900 master parameter
editor can process such parameter data points and manage them for a large number of
devices.
10.1.6 Behavior on Value Changes
The value of a data point can change, if it is written by the application or over the network.
For all data points (input, output and value) the application (connection, user control, etc.)
can be notified, when the value is written to. The property
Only notify on COV
defines,
whether the notification is done with each write or only if the value changes (change-of-
value, COV). If only notify on COV is disabled, writing the same value multiple times will
result in multiple notifications.
