Operation Techniques
87
The release condition for logical triggers is always the reversed operate condition
(see
Table 4-4
). The numeric limits are not applicable.
NOTE
Pulse triggers and interval timers may not be shared by different setpoints . The first
setpoint that checks such a trigger will clear it, so that following setpoints will never be
operated.
Logical Operations on Triggers
The
PM295
allows the trigger conditions to be combined by logical operations
OR/AND
.
Logical operations can be used in any sequence. They have no specific priority or
precedence rules that are common for such operations, so the only rule applied to
logical operations is their virtual precedence in the logical expression. The full
expression is always evaluated in the direction from the left to right. This means that
any operation affects all the conditions evaluated before it when both
OR
and
AND
operations are combined in one expression.
As example, any trigger condition bound with an
OR
operation and being evaluated
to
true
will override any preceded condition evaluated to
false
. Similarly, any trigger
condition evaluated to
false
and bound with an
AND
operation will override any
condition evaluated before it to
true
. Hence, to avoid confusion, it is not
recommended to alternate different logical operations in one expression. Instead,
when you are intending to use both logical operations, bring all conditions with the
same operation together at one side of the expression, and the others - at the opposite
side.
If you want to explicitly override all other conditions with the critical trigger, put it
to the end of the expression with an
OR
operation if the setpoint is to be operated
when the trigger event is asserted, and with an
AND
operation, if the setpoint should
be operated when the trigger event is released. This is used, for example, to provide
manual forced setpoint operations.
