User Manual

Logical I/O blocks
General functions
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Priority mechanism
Basic function
In order to evaluate the various defined setpoints received from the BACnet command system and via the
data flow connections, the AO, BO, MO, AVAL, BVAL and MVAL blocks each incorporate a priority array
[PrioArr].
All external sources write their defined setpoint and information bit (enable signal) into this [PrioArr]. The
block then evaluates these entries continuously, in order to determine the valid present value [PrVal].
The [PrioArr] holds up to 16 different entries, each consisting of a setpoint definition and the associated
information bit (enable signal). The input number also indicates the priority of the entry, where 1 is the
highest and 16 the lowest priority. Each priority level has a predefined meaning.
Determining [PrVal]
The block continuously evaluates the valid present value at the output [PrVal]. It selects the value that has
the highest priority of those whose information bit (enable signal) is also set. If none of the information bits
is set, the default value [DefVal] is processed.
Structure of the Priority Array [PrioArr]
Each priority level has a predefined meaning.
In the [PrioArr], two adjacent priority levels each are reserved for life safety, manual operation and plant
operation.
● The higher priority (lower number) of each pair is reserved for local control and monitoring, close to the
plant (priority 1, 4, 7 and 15).
● The lower priority (higher number) of each pair is reserved for higher level control and monitoring
(priority 2, 5, 8 and 16).
● Priority level 6 is specifically designed for switch-on and switch-off delays and to maintain minimum
ON and OFF times.
This ensures that, e.g., an on-site EMERGENCY OFF command, initiated at the plant level, takes priority
over a safety function from a higher-level subsystem.