OR starts with two inputs. Its properties let you add inputs and remove unconnected ones while keeping at least two. Q is TRUE when any connected input is TRUE. An unconnected input is FALSE, marked off; with no connections Q=FALSE.
Typical FBD symbol
IN1
IN2
OR
Q
Typical FBD symbol for OR
Inputs
Name
Type
Required
Description
IN1
BOOL
No
The first boolean condition.
IN2
BOOL
No
The second boolean condition.
Outputs
Name
Type
Required
Description
Q
BOOL
Yes
The logical OR result.
Stateful
No, the block does not keep internal state.
Internal state allows a block to account for previous input values, elapsed time, or an accumulated result.
Behavior rule
Q = IN1 OR IN2 OR …; unconnected input = FALSE
Truth table
IN1
IN2
Q
FALSE
FALSE
FALSE
FALSE
TRUE
TRUE
TRUE
FALSE
TRUE
TRUE
TRUE
TRUE
Practical use
start from either of two pushbuttons;
alarm from any active condition.
What to watch
Using OR where all conditions must be true. Use AND for that case.
