Logic configuration of redundant safety integrated systems is a component or group of components that receives inputs from sensors, performs a predetermined decision-making function, causes final elements to assume a protective position, and provides alarms.  Technologies used to implement logic include:

• Pneumatic logic (e.g., pneumatic relays)
• Electrical logic (e.g., electromechanical relays)
• Electronic logic (e.g., solid-state devices)
• Programmable Electronic Systems (PES) (e.g., Microprocessor based devices such as Programmable Logic Controllers (PLCs)).  Some of the PES logic configurations typically used are:
  1. 1oo2D redundant:
     This uses a dual processor with diagnostics to achieve fault tolerance.  Either processor channel can initiate a trip however a diagnosed failure of one processor channel allows the other channel to continue to protect the process for a period of time until the faulty channel is repaired.  The required availability and the safety certification of the system set the length of this period.  If the faulty channel is not repaired and this period elapses the system initiates a trip.
  2. Triple Modular Redundant (TMR):
     The TMR system uses three parallel processors to achieve fault tolerance and to execute a 2oo3 function on the output states.  Similar considerations apply to the TMR system when it degrades to 1oo2 operation as apply to the 1oo2D redundant system.