The guide explains that single-function safety relays do offer a low-cost solution in limited applications involving a limited number of functions/devices, such as:

• stopping movement in a controlled and safe manner.
• monitoring the position of movable guards.
• interrupting a closing movement during access.
• emergency off/stop.

However, it also addresses the typical shortcomings of single-function safety relays, which include:

• difficulty troubleshooting due to complex wiring between each device.
• the significant amount of engineering required for system modifications or upgrades.
• the inability to operate in mixed modes (safety and standard).
• the tediousness of functionality changes because of each relay's defined purpose.
• the need for additional safety relays for additional safety functionality.

The guide describes how multi-functional relays can control between two to three safety functions and typically offer a smaller footprint and reduced, simplified wiring - depending on the safety functionality required.  They come in a number of styles and configurations.  Typical applications include:

• A simple base unit can handle a variety of inputs.
• Most require no software for configuration or operation. Typical setup and maintenance is simple (but with no flexibility).
• Multi-function safety relays are often a choice when single-function relays can’t meet the safety functionality. 
• Suitable for systems with more-complex requirements such as machine tools or robots with many I/O points or a high number of safety-related tasks

The typical shortcomings addressed by the guide, include:

• The cost and complexity of hard-wiring control devices.
• The difficulty involved in expanding or modifying the control system.
• The time-consuming processes involved in troubleshooting faults.
• The limited safety logic due to predefined safety relay functionality.

The guide explains how safety PLCs address the need to monitor a much larger number of input and output (I/O) points for today's increasingly complex machine tools, automation systems, and process facilities.  

As the guide points out, safety PLCs offer important advantages:

• MEETS SAFETY STANDARDS - These PLCs meet the required safety standards, able to achieve up to PL e as per ISO 13849-1 or SIL 3 as per IEC 61508.
  
  
• PROGRAMMABLE - Safety-functionality changes can be easily done via programming rather than device replacement and related rewiring.
  
  
• SAFETY-FUNCTION LIBRARIES - TÜV-certified, safety-function libraries are available, significantly reducing the design time and increasing flexibility.