This is also a work in progress. (Read: Its a mess)
I can not honestly recommend buying one of these watches. Timex has responded to every question I have sent them, which is impressive for a company. However, Timex failed to answer every single one of my questions, all they had to say was "we have no plans at this time to make that information available" (paraphrased).
The single bit of useful information I have received from Timex is that the DataLink watch is based on a Motorola 6805 microcontroler. This was mentioned in passing, and in my opinion by accident.
Motorola, on the other hand, has the complete text of their technical reference manuals on WWW, free for the downloading. They also distribute sample assembly, assemblers, and just about anything else you would ever want to know about their products.
When Timex's behavior is changed to be more like that demonstrated by Motorola and HP, then I will consider recommending their product. I have an HP scanner. I asked HP for a manual. They asked "whats your address?". No proprietary junk, no non-disclosure, it was even free. I recommend exclusively HP scanners and printers to my clients. I am also the proud owner of a fully documented HP48 calculator.
This is part of an ongoing effort by several people to, communicate with a Timex DataLink watch, write our own melodies, and write our own "wristapps". Due to lack of information and no cooperation from Timex, we have been forced to reverse-engineer everything. But then again, its been kind of fun too... :-)
Also, since Timex does not support FreeBSD, Linux (or any other unix), there is no other way to use the watch to its advertised capacities.
Email firstname.lastname@example.org if you would like to join us... (not that we have much of an organization.)
There is a counter on the datalink watch library on Karl's page, if you wish to be counted as a supporter of this project.
I also have a file which when assembled by the assembler below produces the sound tester "wristapp". I am not distributing it freely for copyright reasons (it is directly based on timex's file). Email if you want a copy, AND already have a copy of timex's stuff.