So it turns out that programming cables aren't cheap.  Maybe relatively cheap compared to some ham gear but $20-50 for a cable that you might only use every once in a while (and, even then, it's not necessary since you can program it manually) seems excessive.

The main radio I'm interested in is a Kenwood TM-281A, but being able to do this for other radios is useful as well.  The TM-281A uses a RJ-45 type connector for the radio end of the programming cable.  Schematics I've found indicate that a couple of those pins (2, 5, 4, 7) are used in the factory programming cables (for serial RX, TX, GND and 13.8V).  I'm not sure but I guess the power line might get regulated and be used to run the USB-to-RS232 and/or RS232-to-TTL chips in the cable(?).

I've got computers with physical serial ports as well as USB-to-RS232 cables (most with prolific chipsets) to create virtual serial ports but I think both of those options would require one or both of the chips mentioned above.  I have a few spare MAX232 and MAX3232 chips that I could use to create what I think would be the same as a factory TM-281A cable (KPG-46 or KPG-46U), but I'm wondering if I could forgo some of that and use one of my Raspberry Pi computers to program it directly.

UPDATE
I got CHIRP to run fine on the RPi.  The stable build is in the regular apt-get repository, so I did apt-get install for the stable build and downloaded the daily build to run separately (since that's where the Baofeng support is).  I couldn't add the repository with the daily build listed on the CHIRP website.  Not sure what the problem was.

Anyway, my cable works fine and cost me $0 and about 15 minutes to build.  10 pin connector and ribbon cable on the RPi end, so old head phone plugs and some CAT-5 cable on the other end.  I did have to sacrifice a short network cable for the project, but I think I'll live without it.

I've built some programming cables using the Silicon Labs CP2102 UART-USB chips.

http://www.ebay.com/itm/170895253016

http://www.ebay.com/itm/251039347548

I've used both of these.  The difference is, the red board has flashy red LEDs.  The white board had flashy red AND blue LEDs.  Both work equally well, but the pins marked TxD and RxD are backwards on one or the other.  Maybe it is a matter of TxD (Transmit Data) pin being TO TxData or FROM TxData.  All I know is I have to reverse the two wires to those pins if using one or other other boards.

The three pins used are TxData, RxData, and Gnd.  Other pins are +5v, +3.3v, and RST (Reset?).  They are not used for programming cables.

That's cool; I didn't even think about there being USB-to-TTL chips (I've had experience with SiLabs and Prolific RS232-to-USB as well as Maxim et al RS232-to-TTL chips but haven't done any projects that required USB-to-TTL thus far), but it makes sense to do it directly rather than USB-to-RS232-to-TTL.  I have some RS232-to-TTL converter boards (both pre-made and DIY) and RS232 ports on PCs.  

Regardless of how you get there (USB-to-TTL, RS232-to-TTL, or TTL from Raspberry Pi GPIO header), is it really just a simple as hooking up to the RX/TX/GND pins on the RJ-45 jack and hitting "go"?

Well, I'm still determined to get this going as described in the OP, but I couldn't get CHIRP to build on the Raspberry Pi.  The suggested apt-get repository wasn't found when I tried to add it, so that was a no-go.  I downloaded the tar.gz file but there were a couple libraries missing that wouldn't let it run and I got frustrated and didn't want to keep messing with it.

I did, however, find a work-around.

Using a Prolific chipset USB-to-RS232 adapter cable in my laptop, I plugged that in to one of my MAX3232 RS232-to-TTL boards.  This is where I wished that I had brought out the +5v on the supply I converted in my other thread.  I fired up a Raspberry Pi just to use as a power source from it's 5V GPIO pin to run the MAX3232 board.

I tried to "download from radio" several times and it reported "no response from radio" a few times and a couple other times it said that the virtual COM3 port I was using was unavailable (restarting CHIRP remedied this one).  As mentioned:I had to swap the RX and TX lines.  I think maybe the line designations are just poorly named, because I had to hook them up counter-intuitively (IMO) to get it to work.

Anyway, this solution worked fine for me, I got it to download data from the radio and it worked fine in "Live Mode" to program some local simplex frequencies and repeaters into the radio.

A sacrificed 6' ethernet cable and about 15 minutes sure as hell beats spending $50 on a cable.

$18.95 shipped from amazon
programs mobiles and HT's, including the el-cheapo Chinese ones....

http://www.amazon.com/Kenwood-Programming-Cable-KPG-22-KPG-4/dp/B003DBIS2A/ref=sr_1_9?ie=UTF8&qid=1373902019&sr=8-9&keywords=tm-271+programming+cable

Well, the clusterfk hookup above that worked above for the Kenwood with the RJ-45 jack was a fail boat for the UV-B5.

If I can figure out how to get CHIRP to run on the Raspberry Pi, I'll give that a shot.  If that still doesn't work, I'll program it manually and use the above setup for the TM-281.

Hopefully I'm able to get it running on the RPi and I can build a cable with a connector on one end for the RPi GPIO header and have the other end split off for the RJ-45 and 2.5/3.5mm TRS similar to what is in the cable linked above.  If I get that going, I have a deck-of-cards-sized ham radio transceiver programmer that I can take around with me.

Mission accomplished.

One 10-pin connector, some ribbon cable, salvaged ear bud 3.5mm and 2.5mm plugs and half a network cable later and my deck-of-cards-sized "universal programmer" is rocking and rolling, ready to program radios on the go.

Buying my KPG-46U was well worth the purchase. Not only does it program all the mobiles at work, it also has a use on my Kenwood ham radios at home. And I didn't have to waste any time acquiring the parts and wondering if it will work right off the bat. And since it's a supported factory cable, it's rock solid reliable too. I paid less than $20 for it on Ebay.

Building my cable was well worth it too.  The sum total of time I spent on gathering parts and building the cable was probably on the order of 10 or 15 minutes. Maybe 20 including looking up the pinouts on the radios.  At the end of that time I had a unit in my hand that can program Kenwood (et al) mobiles with the modular connector and Baofeng HTs (et al) with the 2.5/3.5mm jacks.  And I didn't have to wast any time waiting for the delivery man.

Better still, the end product isn't some cable to haul between hooking up work radios to work computers and home radios to home computers.  It is a self contained unit; a computer with CHIRP that runs off of 5V and is the size of a deck of cards.



If the final result was just to end up with just a cable that went from a USB port on a PC to a Kenwood radio, then the Windows laptop stuff I described above with the USB/RS232/TTL conversions would be a pain in the ass.  If that ended up being more trouble or if it was something I needed to do often, buying a cable might look pretty good.  As it is, I didn't build a cable, I built a mobile programmer.

I have two Kenwood 281's... and I can tell you...  I have used that programming cable probably 100 times in the last month.

It's SO MUCH easier to program changes in the radio via software/cable. Takes literally just seconds.

I'll hear guys talking on one repeater... that agree to move to another that I don't have programmed.
If I keep the software open and cable nearby, I can dump a new repeater freq into the radio in less than a minute.

This is why it's handy to setup your radio near a computer.

Holy cow.  I don't think my situation would necessitate programming it anywhere nearly as often as that.  Program in my local stuff and let it be; maybe add repeaters that I look up before I travel for a biz trip or vacation.

Where are you, how are you operating and how much area are you covering to have all a need for all those repeaters (and not have them already programmed in as locals / keep finding new ones)?

With a "cable" like I built, my radio can *always* be next to the computer, no matter where I am.