EPOS4: Causes of "STO error" (0x8A88)

Topic:

• What can be the cause of a "STO error" (0x8A88) reported by the EPOS4?

Solution:

1.) Basics

The "STO - Safe Torque Off" is a feature which blocks the power stage on hardware level directly in case of missing STO input signals. If there is no sufficient signal voltage present at one or both STO inputs (-> EPOS4 connector X9), the EPOS4 reports the "STO error" 0x8A88 when the power stage is enabled.

Remark:
Please note that EPOS4's STO is not safety certified.

Cross Reference:

Please find more details about STO in these documents:

• Support Center document "What is the meaning of STO?"
• Manual: "EPOS4 Application Notes Collection.pdf":
  Chapter "9 Safe Torque Off (STO) Functionality"
  (Please find this chapter attached as a PDF extract)
• EPOS4's "Hardware Reference":
  Chapter "Setup / STO (X9)"

2.) Causes of EPOS4's "STO error", 0x8A88

EPOS4 Firmware specification:

"EPOS4 Firmware specification.pdf" states the following in chapter "7 Error Handling":

Practical aspects:

If one or both of the STO inputs signals are not present, the "STO error" is reported and it is impossible to enable the power stage. It is also impossible to clear this error state as long as the STO signals are not properly applied, i.e. no voltage level of 4.5V up to max. 30V DC in between 'STO-IN1+' and 'STO-IN1-" as well as in between 'STO-IN2+' and 'STO-IN2-".

Wiring:

• 'STO-IN1+' and 'STO-IN2+' have to be connected to a positive signal voltage (typ. 5V or 24V).
• 'STO-IN1-' and 'STO-IN2-' have to be connected to GND of the supply voltage.

Example: maxon's "STO Idle Connector", (P/N: 520860):

Analysis of possible STO error's causes:

If a "STO error" is reported, you should check the following points independently if maxon's "STO Idle Connector" is in use or the required signals are provided by some external cabling:

1. Is the STO connector properly plugged in (at EPOS4 -> X9)?
2. Are all wires and signal states properly present?
   • Check by EPOS4's object "STO input states" (0x3202/01) which of the STO input signals is not  active?
   • Note that the STO inputs are differential, i.e. take care that not just a voltage is connected to both 'STO-IN+' but also GND to both 'STO-IN-'.
   • Ensure by a voltmeter that at least 4.5V is present in between the following contacts:
     • 'STO-IN1+' and 'STO-IN1-"
     • 'STO-IN2+' and 'STO-IN2-"

Additional note:

• If the "STO Idle connector" is in use and its wiring seems to be o.k but the "STO error" is still reported and both STO inputs seems to be inactive (-> Object 0x3202/01), please check if the 5V supply might be overloaded by some sensors or actuators connected to the 5V supply output of another connector.

  • The "EPOS4 50/5" (P/N: 546047) and "EPOS4 70/15" (P/N: 594385) uses the same internal 5V supply for Vsto of "STO" connector X9 and Vaux of "Digital I/O" connector X7 and "Sensor" connector X6. If the Vaux is overloaded (e.g. by a sensor or actuator connected to X6 or X7), the 5V will be shut down (due to overload protection), the STO error will be triggered then due to missing STO input signals (based on shut down 5V).
• If some external STO wiring is in use, ensure that none of external switches, emergency buttons, or relays are currently in a state which interrupts STO signal lines or their supply voltage.

Add-on Support Center documents about STO basics:

• What is the meaning of STO?
• EPOS4, STO, ISO 13849: Safety certification of systems?