BECKHOFF EtherCAT Technology Section I - Manual (Page 110)

LED Signals (Indicators)
I-90 Slave Controller – Technology
15.5 Port Error LED (PERR)
Some ESCs support port receive error indicators PERR(x), which display physical layer RX errors.
The PERR(x) LEDs are not part of the ETG.1300 EtherCAT Indicator and Labeling Specification. They
are only intended for testing and debugging. The PERR(x) LEDs flash once if a physical layer RX error
occurs.