Topic:

The "EPOS4 Module 60/20 STO" (P/N: 894249) and "EPOS4 Compact 60/20 CAN STO" (P/N 833726) report a "STO error" (0x8A88) although the required STO signals had been already applied at start-up.  This does not happen in case of other EPOS4 product types. 

• What is the cause for "STO error" states of the "EPOS4 ... STO" although STO signals are applied?

Solution:

The certified STO of "EPOS4 Module 60/20 STO" (P/N: 894249) and "EPOS4 Compact 60/20 CAN STO" (P/N 833726) demand for a STO signal start-up sequence to ensure proper STO wiring and functionality.

The differential STO signal inputs must be at "low" (i.e. connected to GND, short-circuited, or below 5 V DC) initially after power-up. After the EPOS4 and STO card have started up, a voltage of 15 V DC or higher (e.g. typical I/O voltage of 24 VDC) must be applied to both differential STO inputs between "INP+" and "INP-".

Typical sequence:

1. Connect "Logic supply" (X2) and "Power supply" (X1) to a supply voltage.
   Cross reference: "EPOS4 Module / Compact 60/20": Power & Logic supply
2. Control and power stage has to be kept in "Disable" state.
3. A "Low" signal level has to be applied to both differential STO inputs, i.e. the "INP1+" and "INP2+" can be connected to GND or "INP+" and "INP-" of both differential STO inputs can be short-circuited.
4. If an error state is present, try to clear the error state.
5. Wait some seconds.
   • One LED of the "Safety card STO" should turn green.
   • Object "STO card status" (0x3202/0x02) should hold 0x33 or 0x13 then.
6. Connect the required "High" signal levels (of above 15V DC, e.g. logic supply) to both differential signal "INP+" inputs. "INP-" must be connected the GND of the "INP+" supply.
7. Wait some seconds resp. until there is just a green LED of the "Safety card STO" active.
8. Enable control and power stage.
9. If there is still an error state reported, please try to clear the error state. The final result should be then:
   • Just a green LED of the "Safety Card STO" is active.
   • Object "Functional safety" (0x3202) should finally report the following state:
      
10. If there is still any "STO error" state reported, please ...
    • ... ensure that STO wiring and signal levels are correct. Use a voltmeter to measure and confirm that a voltage of 15 V DC is present in between "INP+" and "INP-" of both differential STO inputs.
    • ... start at point 1 again.

Technical background:

The "Safety Card STO" (P/N 833720) which is in use by the "EPOS4 Module 60/20 STO" (P/N: 894249) and "EPOS4 Compact 60/20 CAN STO" (P/N 833726), is a certified STO. This also means that there are higher standards fulfilled than by the (not certified) STO of all other EPOS4 product types.

The certified STO demands for applying the required STO signal states after power-on, i.e. initially the STO signal wires have to be low. This procedure ensures that there is no STO signal present "by accident" (e.g. in case of some mismatching wiring or short circuit to supply). 

Please find more information about required state changes in chapter "5.5 Status indicators" of the "EPOS4 Safety Card STO User Manual.pdf":

Hint:

The STO input states and STO card status can be checked via object "Functional safety" (0x3202):


Please check the "EPOS4 Firmware Specification" for details. 

Important: 

The signals at the STO inputs have to actively be changed from Low (i.e. below 5V DC) to a voltage level of above 15V DC.

Please note the specified voltage levels in chapter "6.2 STO (X9)" of the "EPOS4 Safety Card STO User Manual.pdf":

Cross Reference:

• Please read the "EPOS4 Safety Card STO User Manual.pdf" closely!
• If the error state persists even although the STO signals have been applied based on the described sequence, please check the add-on information provided in this Support Center document:
  • EPOS4: Causes of "STO error" (0x8A88)