I can't explain it yet. Flashed a Modbus firmware and connected Tx - Rx and D7 pins of Photon to the ports of a MAX485 transceiver. MAX485 is mounted on a separate board and supplied by a NCP1117-5V. The NCP is powered from the auxiliary 12V output of a Modbus RTU unit. I plugged the USB cable to PC, without firstly powering the Modbus unit, Photon was powered-up and its tiny voltage regulator ended-up through smoke. Desoldered the regulator, but checking with multimeter +3.3V and GND it shows short circuit. Desoldered and capacitors and short circuit remains. So the P0 of Photon is also damaged and probably caused the short circuit that damaged the voltage regulator.Photon's P0 I/O pins are 5V tolerant.