Balancing programming costs and complexity for logic solvers

The most common determining factors in choosing which logic solver best fits a safety application in a SIS (safety instrumented system) are loop and point counts and the complexity of logic required for safety mitigation.

When dealing with high-density point and loop counts, a larger, more complex logic solver such as a safety PLC may be the best choice. Larger systems enable complex control logic and reliable voting schemes across multiple loops and points. However, the purchase, programming and installation costs for such systems can be notably high, not to mention their high life cycle costs.

For scenarios where only one or a few points need to be monitored in a single SIF loop, a much less expensive and complex single-loop logic solver can be a very effective choice. Single-loop logic solvers, or alarm trips, have advanced significantly in recent years. They now feature programmable inputs, safe password protection, and comprehensive internal, input and sensor diagnostics. Standalone models are less expensive and simpler to program but are limited in handling multiple loops, accepting multiple inputs, performing logic or internal voting, or providing digital communication with a process control system or other host system.

For applications that require higher-density loop and point counts with more advanced logic, and loop monitoring of only a few points or loops, multiloop logic solvers are often a better fit for many SIFs. They can handle multiple I/O points, one to three loops and logic equations, and offer significant flexibility for voting architectures at lower cost and complexity compared to larger safety systems and PLCs. Multiloop logic solvers combine the simplicity of single-loop logic solvers with the advanced functionality of larger systems and PLCs.

Programming cost and ease of use

When choosing a logic solver, consider whether you need custom or licensed software, then consider its ease of programming to meet your SIS requirements. Many safety PLCs and systems offer proprietary software with high costs and annual licences. Alternatively, there are affordable single and multiloop logic solvers with licence-free programming software, including Field Device Technology (FDT) compliant interfaces.

The rigor of programming and long-term maintenance should always be a key consideration when choosing a suitable logic solver. Implementing logic routines in these safety systems will require someone with significant knowledge of the programming language being used, whether it be ladder logic, function block diagram or structured text. This creates the need for programming expertise and adds additional time and expense to day-to-day operations and long-term maintenance of your safety solution. The litmus test may be to examine how long it would take to reprogram your current 1oo3 voting safety function to a 2oo3 voting scheme. If it takes someone who is not familiar with the logic solver software more than 10 minutes, you may want to reconsider your logic solver choice, especially for such a straightforward application.

Single or multiloop logic solvers offer much more straightforward programming options with easy-to-understand dropdown menus, checkboxes, radio buttons, prebuilt common control functions and equation generation utilising Excel-like formulas. While they may have limitations on I/O count, you may be surprised to find out how powerful but easy to program these logic solvers have become.

For more information on multiloop logic solvers, click here.