Theoretically it can be used to program or computer-control any amateur radio that is programmable or controllable by TTL-level signals. I use it to program a Yaesu FT-7900R dual-band FM transceiver and an ICOM IC-T7H HT using adapters described below. I also use it out of the box for controlling an ICOM IC-746 using Ham Radio Deluxe. Note that you do have to install the USB driver on the CD that comes with the cable.
Unfortunately, Radio Shack has discontinued this particularly cable and replaced it with a new model. You can still (early 2012) sometimes find the 20-047 on closeout at some Radio Shack stores. I have no idea whether the new model also works for the applications described below. If anyone tries the new model, I would be interested in hearing of the results. You can e-mail me at k1oc at arrl.net.
The Radio Shack cable is capable of duplex communication, TX on the tip and RX on the ring of the stereo mini-phone plug. The Yaesu data port, on the other hand, is capable of simplex only, using pin 3 (PTT) for programming TX and RX. (Here, TX and RX refer to uploading programming data to and downloading it from the radio, not transmitting and receiving on the radio itself.)
So, if you connect the tip and ring of the mini-phone plug to pin 3 of the mini-DIN, and the mini-phone ground to pin 2 of the mini-DIN, you can use the cables to program the radio.
I did that by connecting the six pigtails from the CT-39 cable to a female 6-pin Molex connector, effectively giving me a breakout box for the mini-DIN so I can use it for other applications. I set the pins up in the connector body to correspond to the pin locations in the mini-DIN to make it easier to figure out what to connect to where. If you look at the Molex connector plug-end on and orient it long sides up, then pins 1-3-5 are down the left side, and pins 2-4-6 are down the right side. Here is a picture.
Then, I took a stereo mini-phone jack and wired the terminals for the tip and ring together and connected them to the appropriate pin in a male Molex connector so they would connect to pin 3 of the mini-DIN. Then I wired the ground connection of the mini-phone jack to the Molex plug so it would connect to pin 2 (ground) of the mini-DIN. Here is a picture.
You will need software to program the radio, and this cable works very nicely with Bob Freeth's (G4HFQ) excellent and inexpensive FTB7900 software.
If you decide to make one of these up for yourself, double-check the pinouts in your manual. And then triple-check that you've wired it up correctly.
The T7H is programmed via the ring on a 1/8" stereo mini-phone plugged into the earphone/speaker jack. (Seems like an odd way to do it, but OK.)
So, I made up a short cable with a 1/8" stereo mini-phone jack at one end and a 1/8" stereo mini-phone plug at the other. The ring and tip of the jack are connected together and then to the ring of the mini-phone plug. (Similar to the Yaesu, the T7H uses the same connection for RX and TX, so you have to wire the separate TX (tip) and RX (ring) outputs of the Radio Shack cable together.) The grounds of the jack and plug are connected to each other. Here is a picture.
The mini-phone end of the Radio Shack cable gets plugged into the jack, and the mini-phone end of the adapter cable (with only the ring and ground wired) gets plugged into the earphone/speaker jack on top of the radio.