difference between standard DI and safe DI

Hello hag;

A Safety DI is one necessary element of a Safety PLC, used to implement safe operations in an industrial environment. Also required are a safety analysis to determine the SIL lavel required, the safety hardware (safety relays, safety PLC, Safety IO, Safety networks), specialSafety program blocks, and finally a Safety certification of the system)

Here is a description of a Safety System that could give you a basic understanding:

SIMATIC Industrial Software S7 Distributed Safety - configuring and programming

Principles of Safety Functions in S7 Distributed Safety

Functional safety is implemented principally through safety functions in the software. Safety functions are executed by the S7 Distributed Safety system to place or maintain the system in a safe state in case of a dangerous occurrence. Safety functions are contained mainly in the following components:

● In the safety-related user program (safety program) in the F-CPU

● In the fail-safe inputs and outputs (F-I/O)

The fail-safe I/O ensure safe processing of field information (emergency STOP buttons, light barriers, motor control). They contain all of the required hardware and software components for safe processing in accordance with the required safety class. The user only has to program the user safety function. The safety function for the process can be provided through a user safety function or a fault reaction function. In the event of an error, if the Fsystem can no longer execute its actual user safety function, it executes the fault reaction function; for example, the associated outputs are deactivated, and the F-CPU switches to STOP mode, if necessary.

So the Safety IO (including the Safety DI) contain special electronics and circuitry that allow signal evaluation required for the Safety program in the Safety PLC.Look for further details in the above-mentionned manual, and in the following link:

Hope this helps,

Daniel Chartier