At minimum, just a USB sound card ($5 US) and some way to key up your radio. The Direwolf UserGuide details all this for you. On a Raspberry Pi, a simple transistor circuit and a GPIO pin off the Pi is all you need.

For software guys, it is now possible to make receive-only installations with even less hardware.

On our co-channel diversity repeater system we have sites where an RTL DVB-T stick is used with SVXlink "remotetrx" configured to provide a number of virtual NBFM receivers where some of them are connected to the SVXlink master site to receive voice to be transmitted over the repeater, and other virtual receivers are connected via a loopback audio device to a direwolf process used to gateway the received APRS packets to the aprs2.net

As the APRS frequency on 70cm is in the process of being changed, we receive on both the old and new frequency and also on 2 different repeater input frequencies, all with a single stick (and a bandpass in front of it, of course).

I have even tested an SSB input to the repeater, which worked fine too. It is possible to add receivers and connect them to some application without touching the soldering iron, without even going to the site.

It is really wonderful what software can do today...

Rob