Hi Dave, There is a page on the Icom FAQ site, with a lot of information including links to various keying-interface sites: http://www.qsl.net/icom/auxrelay.html I use a solid-state amplifier (Yaesu Quadra), which has a TTL-level keying line. Its PTT line connects directly to the Pro3 Send jack. Cheers for now, 73, Adam VA7OJ/AB4OJ