Technical data

List of all BACnet Properties in Native Operation • 301
Present_Value
WAGO-I/O-SYSTEM 750
BACnet/IP Controller
The normal calculation of the value of the Present_Value
property is illustrated as follows (the actual algorithm used
is a local matter but must yield the same results as this one):
1. Determine the Exception_Schedule field element us-
ing the highest relative priority (as defined in section
6.26, "Exception_Schedule") that is valid for the cur-
rent day and whose current value (see method be-
low) is not ZERO. Assign this value to the property
Present_Value.
2. If the Present_Value was not assigned in the previ-
ous step, then evaluate the current value of the
Weekly_Schedule array element for the current day
and if that value is not ZERO, assign it to the Pre-
sent_Value property. If its value is not ZERO, assign
the value to the Present_Value property.
3. If the Present_Value was not assigned in the previ-
ous steps, then assign the value of the Sched-
ule_Default property to the Present_Value property.
The method for evaluating the current value of a schedule
(either Exception or Weekly) is to find the latest element in
the list of BACnetTimeValues that occurs on or before the
current time. That element's value is used as the current
value for the schedule. If no such element is found, then the
current value for the schedule is ZERO.
These calculations are such that they can be performed at
any time and the correct value of Present_Value property
will result. These calculations must be performed at 00:00
each day, whenever the device resets, whenever properties
that can affect the results are changed, whenever the time in
the device changes by an amount that may have an effect on
the calculation result, and at other times, as required, to
maintain the correct value of the Present_Value property
through the normal passage of time.
Note that the Present_Value property is assigned the value
of the Schedule_Default property at 00:00 of any given day,
unless there is an entry for 00:00 in effect for that day. If a
scheduled event logically begins on one day and ends on
another, an entry at 00:00 is placed in the schedule that is in
effect for the second day, and for any subsequent days of the
event's duration, to ensure the correct result whenever Pre-
sent_Value is calculated.