Festo — CMMT-ST-C8-1C-...-S0 — 2021-04b
Product overview
9
If the safety classification of SIL 3, cat. 3, Pl e is required for operation with an EC motor, the diagnostic
contact STA must be evaluated.
The diagnostic feedback signal STA has been designed as a potential-free contact (opto relay). The
diagnostic contact STA is only closed (conductive) when STO is requested on 2 channels via the control
inputs #STO-A and #STO-B. If the contact is closed (conductive), the STO state has been achieved (safe
state).
#STO-A
#STO-B
STA-C1/C2
Low level
Low level
Diagnostic contact closed (low resistance)
Low level
High level
Diagnostic contact open (high resistance)
High level
Low level
High level
High level
Tab. 2: State of the diagnostic contact STA
If the safety sub-function STO is not requested but protective functions are triggered simultaneously
on both channels (#STO-A and #STO-B), e.g. if the voltage at #STO-A and #STO-B is too high, the
internal protective functions switch off the channels as if STO had been requested and the diagnostic
contact STA is closed (safe state feedback).
Recommendation: The safety relay unit should check the state of the diagnostic contact whenever
there is an STO request. The state of STA must change according to the logictable. The safety relay unit
can cyclically test the #STO-A and #STO-B signals for high level with low testpulses and for low level
with high testpulses, within the specified limits
The diagnostic contact STA is not evaluated in terms of safety engineering.
