## 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**

**Advantages:**

**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**. in Safety PLCs of leading brands, the Cpt parameter that is used in the calculation of the Probability of Failure (PFDavg) is very high, between 95-99%.- 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).

**Disadvantages:**

- 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.

**2-Logic Solver with Safety Relays**

**Advantages:**

**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.**

**Disadvantages:**

- 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 PLC with 2oo3D architecture (Triconex)

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