Purchase fail-safe (F) digital devices that connect to fail-safe (F) input modules and are monitored by fail-safe (F) logic blocks in fail-safe (F) PLCs.

Or, you can design systems such that you have built-in parity.  For example, you turn something on  or off which provides at least 2 methods of detecting proper response.  For example, you turn on a heater contactor and with 100% command to a heater controller (often a big-heat-sync'd SSR), and you can monitor an aux contact of the contactor back to the PLC, and monitor temperature feedback.  If the contactor is on and the aux contactor fails to indicate the contactor pulled in (which often indicated bad CB or FU) yet the temperature starts to rise, you know there _might_ be a wire-break, but likely the aux contact was inadvertently bumped loose and is dangling in the control cabinet.

However, if you want to ensure those aux contacts always report, you can use special digital devices which - like robot pendant local overrides which have an interesting button which is only on in the middle of its stroke.  Let go and it releases (dropped pendant and ran) or press too hard and it releases (fear? or over-reaction).

It really depends on the risk assessment for each digital input's requirement to function 100% and prevent harm (to humans, process, product, facility, environment, future -- like an undetected failure of a sensor on an x-ray machine could cause future-you from producing offspring).

Brad27243

I apologize if this question is somewhat naive, I'm working with a group to determine diagnostics to be included in a new plant. We'll be using 6ES7521-1BL00-0AB0 digital inputs. My question is, how can digital inputs utilize broken wire diagnostics if the entire concept of a digital signal is to break the curcuit at the low state? Thank you!

Brad27243

We'll be using 6ES7521-1BL00-0AB0 digital inputs. My question is, how can digital inputs utilize broken wire diagnostics if the entire concept of a digital signal is to break the curcuit at the low state? 

Hi Brad27243

the answer is on page 18 of the 6ES7521-1BL00-0AB0 manual and also as per attached pic