Safe or Non-Safe Input for Diagnostic Feedback

[ryanb013]

Hi,
I'm trying to understand a couple of things with a feedback signals that are used in a SRP/CS that I'm working with. Quick disclaimer: I'm somewhat of a newbie in this area.

The use case I'm working on is a (servo) motor drive which is interfaced to a safety PLC. The motor drive has (2) STO inputs and (1) output which reports status of the STO function. The output is not a safety output from the servo drive.

My PLr is PLd and the servo drive's STO function can achieve this when the STO status output is connected to and used by the Safety PLC. I believe a CAT 3 architecture would be needed to achieve this

Does the STO status output from the servo drive need to be connected to a safe digital input on the safety PLC or can it be connected to a standard digital input?

 

[mk42]

My PLr is PLd and the servo drive's STO function can achieve this when the STO status output is connected to and used by the Safety PLC.

Where did you get that information? it's an awfully specific statement. If it it was an app example from the manufacturer, I'd expect it to come with a wiring diagram of some kind.


Regardless, though, I'd ask the drive manufacturer for clarification. If the drive was certified as going back to a safety input from it's standard output, even if it's OK according to some standard to use a standard input instead, it might not be OK because you're outside of the drives safety certification.


For better or worse, Safety is as much about the paperwork as it is actually not hurting people. It's kinda like how we all KNOW that 1+1 = 2, but actually proving it with math theory is a lot less trivial.

 

[d x l]

Good question and I'm confused about it myself. Do you have plenty of spare safety inputs? Were you hoping to daisy-chain the status outputs with another drive or two?

Previously I had always thought feedback could go to a standard input, but I have a servo amp here for which the manual says

"Connect the monitor signal as a feedback signal, e.g., to the Safety Unit. Note: To meet performance level e ...... the EDM1 signal must be monitored by the host controller. If the EDM1 signal is not monitored by the host controller, the level will be safety performance level and SIL 1."

This is difficult to interpret because I want to use relays rather than a safety PLC. In your case, if you have spare safety inputs, you could just connect there and move on. But it would be nice to know the right answer.