L-Gate User Manual 50 LOYTEC
Version 3.2 LOYTEC electronics GmbH
5.2.4 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.
5.2.5 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 and output) 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.
When the value of an output data point is updated, an update is usually sent out onto the
network. The property Send-On-Delta decides how the update is reflected on the network.
If send-on-delta is inactive, each update of the value is sent. If send-on-delta is active,
value changes only are sent. The send-on-delta property is only valid for output data points.
For analog data points, the COV or send-on-delta takes an extra argument, which specifies
by what amount the value must change to regard it as a change for action. Both, COV and
send-on-delta for analog data points check the Analog Point COV Increment property. A
change is detected, if the value increment is bigger or equal to the specified increment. If
the property is zero, all updates are considered.
5.2.6 Custom Scaling
Custom scaling is applied to all analog data points when they communicate values to or
from the network. This feature can be used, if a network data point has engineering units
not suitable for the application (e.g., grams instead of kilograms). The scaling is linear and
applied in the direction from the network to the application as:
A = k N + d,
where N is the network value, k the custom scaling factor, d the custom scaling offset, and
A the application value. When sending a value to the network, the reverse scaling is
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