It has an open drain to ground for activating the Tx key line on an external amplifier. This is in the form of a 2.5mm diameter TRS connector. It may be used byt the 100 watt amplifier for the K3, so you might have to be a bit clever in applying it to your PW1... 73, Lyle KK7P > Does the KX3 has a TR out to key my ICOM PW1 Amplifier? >