Safe Decentralization and Enable Principle

As explained already, in many cases safety technology follows the developments made in standard control technology. The benefits from transferring the input/output level to the field via decentralization have resulted in the same process being applied to safety related inputs and outputs. This was followed by the development of a safety bus system, which not only allows field inputs and outputs but also a safety related connection between safety control systems.

The diagram below illustrates a typical application in which the enable principle has been implemented.

Circuit diagram for the enable principle

The safety control system switches the safety-related outputs, and the standard PLC transfers the switch command for the corresponding output to the safety control system via fieldbus.

Essentially it is a really simple principle, if you ignore the disadvantage that the switch command from the standard control system must be considered in the program for the safety control system. Graphically speaking the situation is this: The standard control system must place the switch command on the fieldbus, from where the failsafe control system retrieves it before inserting it into the output's control program as an AND function.

Programming becomes unclear, because the control task and safety function are mixed within the safety control system. A further development of the field transfer principle helps to simplify this case.

The diagram below illustrates the extension of the enable principle. The enable for the control command from the standard control system now takes place directly at input/output level. Handling is simplified tremendously as a result; both control systems can be programmed and tested independently. Performing the enable in the I/O system means there are no delay times from processing within the safety control system, and it's no longer necessary to pass on the control commands via the fieldbus.

Extending the enable principle