PSK 31 Interface
The simplified diagram above shows three circuits. You will need to isolate your radio from your computer, if not you will probably will produce 60 cycle hum on your signalss and also might also pickup RF and it will drive your computer wild. The transformers provide isolation and also impedance matching. Your transciever and computer have different impedances so it is detrimental to the signals to connect them together. The PTT signal uses an Opto Isolator. This is a common device for isolating signals (discrete signals - On/Off). When the signal is high it will illumunate an LED (inside the IC) and an optical pickup is sensed and it will turn on an output transistor. It isolates the signal, because the only connection is the light path.
You can build your own, or buy a kit, or purchase a commerical unit. I have built one and have also purchased a commerical unit. I built one for my old rig, and I purchased a Tigertronics SL-1+ for my ICOM 746 PRO. The SL-1+ is very small (about the size of a pack of cigarettes) and it also has a built-in PTT sensor so a serial prot is not needed (my laptop don't have a serial port). There is a new SL-1 that has a built-in sound card and has a USB interface, so you don't need any connection to your sound card. It has a special cable to connect to your radio.
Up at my camp house I have an older Ten-Tec (without accessory sockets) so I built my own interface. Building your own interface can be very cost effective. I think I bought all the parts for less than $ 10 and had fun making it.
There are also kits available over the Internet from $ 15 to $ 50. So if you are not quite as adventurous you can get a kit.
If you do a search on the Internet for PSK 31 Interface you will get lots of information. You can buy your parts off the Internet or purchase them at Radio Shack.
Information on PSK 31 Interfaces:
Buxcomm has kits for PSK 31 interfaces and also sells parts for interfaces.