User Manual

Logical I/O blocks
General functions
19
236 | 353 CM110664en_07
19.1 General functions
This section describes the general functional scope shared by many of the I/O
blocks. Each subsection includes a list of the blocks to which that subsection
applies. Any block-specific details which are not shared by other blocks are
described together with the block concerned.
Priority mechanism
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.
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.
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.
Blocks: AO, BO, MO,
AVAL, BVAL, MVAL
Basic function
Determining [PrVal]
Structure of the Priority
Array [PrioArr]