Logic Solver (PLC versus relays)

The Logic Solver is the SIS subsystem where the logic of the safety functions resides, that is, what are the interlocks of the sensors and how they act on the final elements. We do not always refer to a Safety PLC when we talk about the Logic Solver, because the logic can also be carried out with safety relays or even with hardwired logic.

There are several criteria for selecting Logic Solver technology, of which perhaps the two most important are the number of Safety Instrumented Functions (SIF), and the Software.

The advantages and disadvantages of the PLC compared to the relay-based system can be summarized as follows.

1-Safety PLC


  • Easy to modify
  • Communication capability of the Logic Solver with other systems.
  • It is easy to program any logic, even in complex SIFs with architectures of type 2oo3, 2oo5, 2oo24, …, or combinations between groups (e.g.: 2oo2 of 2oo(1oo2), 1oo2, 1oo1).
  • Supports all kinds of digital or analog signals.
  • High degree of internal fault diagnostics. Self-diagnostics of a safety PLC of leading brands detect a very high percentage of internal faults, between 95 and 99%.
  • It is easy to implement diagnostics of dangerous failures of sensors in order not to trip the process when required.
  • It is easy to implement application diagnostics in the PLC, either for sensors (for example, diagnostic by comparison) or for final elements (for example, PVST partial stroke test).


  • The application software of the Logic Solver must be developed, tested and maintained.
  • You have to manage the firmware such as patches and updates.
  • Cybersecurity measures must be implemented and maintained.
  • The life cycle of the Safety PLC requires more qualified personnel than if the Logic Solver is based on safety relays.

logic solver

2-Logic Solver with Safety Relays


  • Easy to implement when the SIF is simple (1oo1, 1oo2, 2oo2).
  • There is no software (we avoid potential risks of cyber attacks, software corruption due to electrical interferences or other reasons, and systematic errors in logic).
  • High response speed (only critical in certain applications).
  • The cost of the Logic Solver is lower when the number of SIFs is small.
  • In existing installations without Safety PLCs that require few SIFs, the solution based on safety relays may be the most suitable in many cases.

logic solver


  • The Logic Solver has a high cost of implementation and maintenance when there are more than 5-10 SIFs.
  • It is not recommended when the architecture of the sensors or final elements is complex, or when the SIF has start-stop permissives, or operating sequences. In these cases, the logic of the Logic Solver can be almost impossible to implement with relays.
  • If we use transmitters it will be necessary to include analog-digital converters.
  • The modifications of the Logic Solver are more complicated (the wiring must be changed).
  • There is no ability to communicate with other systems.
  • The diagnostic capacity is small or zero. It is not possible to implement application diagnostics in the sensor and actuator subsystems.

Related links:

Example of safety relay

Example of Safety PLC with 2oo3D architecture (Triconex)

Example of Safety PLC with 1oo2D/2oo4D architecture (HiMax)