There are no packets in traditional RS232 communication, let alone device descriptors. Neither the computer nor the device inherently know anything about each other.
Usually, the device has fixed characteristics; sometimes some of these characteristics can be modified by setting switches, moving links, choosing an appropriate cable, or judicious use of a soldering iron.
These characteristics have to be accurately matched at the computer end, when the device is installed, typically by the user reading the device manual, and modifying options, perhaps in a .ini file, or on the command line of the communications program.
Failure to communicate accurately results in an iterative process of trying different options, scratching one's head, and (often) rebooting the peripheral to clear out garbage (like thirty thousand characters misinterpreted as form feeds aka "new page" in its print buffer)
Some of the variable characteristics are:
1) baud rate. Failure to agree on this causes complete gibberish
2) Control flow. This can be :
- none - transmit regardless of whether the other end is receiving or processing previous data
- Software : listen for "stop" and "start" characters when transmitting. Note that failure to agree on baud rate renders this unsuccessful.
- Hardware : Watch CTS and/or DTR signal lines for permission to transmit; assert RTS and/or DSR to signify readiness to receive. Note that the cable you grabbed only wires up CTS and RTS, so your equipment will be waiting for DTR for a very long time. Note also the Diablo daisywheel printer which wires one of these signals to Pin 11 unlike everything else in existence...
3) Variations in what a simple byte looks like : 7 or 8 bits, parity or not, odd or even parity (if present), 1, 1.5 or 2 stop bits afterwards.
4) Standard 25-pin D-type connectors with an apparently arbitrary mix of male and female connectors at each end of the cable, and an assortment of non-standard connectors like 9-pin D-types...
All this variability used to support an entire industry of RS232 testers and adapters...
And I haven't even mentioned the liberties sometimes taken with voltage levels.
Sometimes it feels like a miracle that anything RS232 ever worked at all...
There are plenty of low-power FPGAs. Actel (now Microsemi), in particular, has some nice ones.
I once used one of their products (sorry, I forget precisely which) in a battery-operated headset project that needed to funnel several channels of digital audio between multiple ADCs and DACs and a DSP chip. The client was quite happy with how it worked out.
Best Answer
For under $50, you could buy an SPP bluetooth module (and a microcontroller dev board if you need one)
For $50 you can buy a complete WiFi module with a serial interface.
If you use the headphone jack, investigate Frequency Shift Keying. FSK is a modulation scheme used in audio modems.
Here's a couple of relevant projects for pointers:
On the Blackberry side, you could either do the same thing, or play back pre-canned .wav files.