Safety Programmable Logic Controller (PLC) is a special purpose machine that is used to provide critical control and safety applications for automation users. The controller is usually an integral part of the security instrument Systems (SIS) used to detect potentially dangerous situations that process.
A safety PLC is specifically designed to achieve two important goals:
1. Do not fail (redundancy that works well) but if that cannot be avoided.
2. Only fail safe manner predicted.
Many special design considerations taken into account. A safety PLC will emphasize internal diagnostics, the combination of hardware and software that enable machines to detect improper operation in it. A safety PLC will have software that uses some special techniques to ensure the reliability of software. A safety PLC will have the redundancy to continue their activities even when parts fail. A safety PLC will have extra security on every read and write through a digital communication port.
A safety PLC is also different from a conventional PLC in a safety PLC is typically certified by a third party to meet rigid safety and reliability requirements of international standards. Rigorous, systematic methods should be applied to the design and testing of the safety PLC. A third party expert to provide independent validation and verification of PLC design and testing procedures.