Some EPROMs, due to the voltage needed might require an external power supply. If you burn out your USB port, don't go blaming the programmer. ;) The USB port is only 5V and 500 mA. Trying to bump that up to 21 V will draw a LOT of current from the USB port, The GQ-3X also has a power port, but is only needed for the EPROMS that require a higher voltage to program, as well.