In any facility, a simple binary logic control system might, for example, back
up analog tank level control to ensure that a tank does not overflow. A complex
binary logic control system might scan for many different process conditions
and equipment states, and take action to protect major equipment
worth perhaps billions of dollars from damage. Some binary logic systems are
software-based and others are hardwired. Simple or complex, every system
needs to be documented. Any proposed binary logic control scheme needs to
be presented for review, discussion and implementation. The proposed controls
interrelationships must be recorded so programming personnel can
understand the system’s intended operation. P&IDs were developed to show
a continuous process, which they do very well. However, an augmented presentation
method may be called for when showing binary logic systems.