You can use the Alarm Detection function block to check the outputs of two different blocks and set an alarm on a deviation range.
As an example, we have 2 motors that are operated at slightly different speeds. Therefore, two Bias/Gain blocks are needed, one for each motor. However, the difference between the two motors should never exceed a certain value. If it does, an alarm should trigger. Since the alarm is on the difference, a Subtraction block is used to calculate the difference between Motor 1 and Motor 2 and that output is evaluated by the Alarm Detection function block. When the OUT value from the Subtraction block is outside the limits set in the Alarm Detection block, an alarm triggers.
Figure: Use Example for Alarm Detection function block
