So because I'm cheap frugal, I'm wanting to get this working with as much of my existing equipment as possible. As mentioned previously, I've got plenty of Raspberry Pi's floating around, and because there are software TNC's like Direwolf, that seems like the way to go for the computer, and hopefully for the TNC as well. I'm also pretty comfortable with either SSH or VNC, to get from any of my other computers (e.g. a Windows 10 laptop) to the RasPi.
From the other end, I've made a homebrew 2m/440 dipole antenna that seems to work fairly well. It's connected via RG8U coax to a Yaesu FT-8900R that I've also been quite pleased with (tons of memory, fun to program, easy to toss into my car with a magmount antenna, etc).
That just leaves one last little problem: how to get the two ends connected. And this is where it seems to get a little bit messy. Yaesu offers a cable, a CT-39A, that goes into the data port on the back of the radio, but then ends in bare wire. The 8900 manual gives hints on connecting that to your TNC, but my TNC is inside a computer, so that's of little help.
My first step was to look around for ... for what? Being new to this, I wasn't even sure what to look for. There are TNCs, like the Mobilink or the MFJ-1270, but again it seems like overkill if I've got all these little $30 boxes and open source software that will do that work for me. So I went to the Direwolf docs and forums, and started reading there.
The forums mentioned soundcard interfaces. Hey, I've got some of them too, already. Little USB plugins that have audio in/out jacks on them; I've even managed to get audio out of SDR# on my laptop to audio in on the RasPi via one of these devices, and seen FT8/JS8 decoded there, just for grins. So they work, at least in one direction; my task is to get them to work in the other direction. And to figure out PTT.
PTT is Push To Talk. It's how the radio knows that you want to transmit, and not receive. When I got APRSDroid working with my Baofeng, I used VOX on the Bao, and let the radio itself realize that there was sound coming from my phone, and start transmitting all by itself (and hence no PTT needed). That "worked" (occasionally), but to my knowledge, the 8900 doesn't do this, and even if it did, I'd like to take advantage of the fact that a) Direwolf knows enough to send PTT info, somehow/somewhere, and b) the 8900 data cable has a PTT line (pin 3). So how do we wire that up?
The Direwolf forum, in discussing the USB soundcards, pointed me to C-Media devices, that apparently have a way to control PTT. Step 1, remove the soundcard case; step 2, microsolder a fine wire to a pin on the SMD chip... okay sorry, when they said "advanced", they meant it; this is over my head/ability level.
But there are products available to help with this. One set of products is available at MastersCommunications.com, e.g. the RA-42, but the price tag of that, assembled and tested and with a case, approaches $100, and scares me off, plus I didn't quite find one that looked like it had the right cables, etc.
Another such product would seem to be the Easy Digi interface. It even lists the 8900 as a supported radio, which is encouraging, and comes with the 6-pin mini-DIN cable that would connect it to the 8900. (I mistakenly thought for some reason that the 8900 used a 9-pin DIN, and asked the Easy Digi guy about a custom wiring; he quickly got back to me, and politely set me right about it working for my application. +1 for support!) If I end up not being able to get my homebrew solution to work, that'll probably be my next path down this road.
The third route, though, that I discovered from the "Raspberry-Pi-APRS" PDF on the Direwolf site, as well as heard mentioned a few times on the forum, was putting together a "transistor circuit" to control the PTT pin. Initially, the idea kinda scared me off; I'm pretty new to cobbling together my own circuits for things. But once I looked over the schematic, and realized what it was doing, it now seems like the way I'll try.
By way of background, I did take a bunch of electronics courses, in high school and college, but that was 30 years (cough) ago, and I've forgotten most of it. (Plus I was way more interested in coding back then, and probably just skated through as much of that as I could. Oops.) In more recent years, I've been reading Make: Electronics (that I got from one of the Humble Bundles), and that's been a BIG help in getting more comfortable with all of this. In fact, I've only just finished the chapter on transistors, and understanding that has the transistor circuit in the above PDF making easy sense, and feeling very doable. So yeah, highly recommended.
Which brings us up to current. I've got most but not all of the parts I need to give it a go, so I'll be putting together a Jameco order, and start putting together some test circuits. More details to come shortly.
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