What is Logic 4, why does it exist, and how would you use it? These are all questions I had when I joined
the Computrols Team, so let us get right into it.

Logic levels in CBAS consist of four levels. Level 1 has the lowest priority, and Level 4 has the highest
priority. When any Logic is programmed, the default priority level is Logic 1. The reason three other
priority levels exist is that there are situations where a sequence of operations will need to control one
or more devices during different scenarios. Let us talk about one example.

The standard design approach to protecting a chill water coil is to place a low limit cut out switch or two
in an AHU on the discharge side of the preheat coil. This switch would be a hard wire interlock to a safety
circuit, and when tripped, the safety circuit would open, causing the AHU to shut down. What happens if
the low-limit detector fails and does not open the circuit?

When I designed systems and software for critical environments, I always focused on ways not to have a
single point of failure. By using Logic 4 statements, you can help prevent a single point of failure.
When writing the sequences for this air handler, I would use the appropriate temperature sensor to act
as a backup safety switch. In this example, the preheat temperature sensor would trip a software safety
point that would turn off the AHU start-stop point and send the speed signal to zero. This logic would be
programmed at Logic 4 compared to the default programming, which is at Logic 1.

You can also find additional information about Logic Programming starting on Pg 117 of the CBAS User
Manual.

Logic 4 priority is intended to be used sparingly throughout your sequences because it is so powerful.
When you have those critical sequences or scenarios, the use of Logic 4 will be there as another tool for
you to provide a great product for your customers.