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Difference between a PLC and a Safety PLC

What is the difference between a normal PLC and a SAFETY PLC?

The need for flexible and scaleable safety applications has driven the development of safety PLC's/controllers. Programmable safety controllers provide users the same level of control flexibility in a safety application that they are accustomed to with standard programmable controllers. However, there are extensive differences between standard and safety PLCs. Safety PLCs, come in various platforms to accommodate the scalability, functional and integration requirements of the more complex safety systems.

Where as most PLC's use one main processor, Safety PLC's integrate multiple microprocessors to process their I/O, memory, and safe communications. Watchdog circuits perform diagnostic analysis. This type of construction is known as 1oo2D, because either one of the two microprocessors can perform the safety function, and extensive diagnostics are performed to ensure that both microprocessors are operating in sync. Input circuits are internally tested many times each second to make sure they are operating correctly. You may only hit an E-Stop once a month; but when you do, the circuit has been continuously tested so that the E-Stop will be sensed correctly internal to the safety PLC.

Safety PLCs program very much like standard PLC's do. All of the additional diagnostics and error checking mentioned earlier is done by the operating system, so the programmer is not even aware that it is happening. Most safety PLC's will have special instructions used to write the program for the safety system, and these instructions tend to mimic the function of their safety relay counterparts. Though the logic behind each of these instructions is complex, the safety programs look relatively simple because the programmer simply connects these blocks together. These instructions, along with other logical, math, data manipulation, etc. instructions are certified by a third party to ensure their operation is consistent with the applicable standards.