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:
Quoted:
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.
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.