Casio always had the edge with touchscreen technology in their watches and the TC-600 was no exception.
There were three similar touchscreen calculator watches in this series, sharing two modules  and .
The TC-50 was the plastic/resin watch which had the  module (NB the only difference between the modules was that  didn’t have a light). The TC-500 was a chrome-plated version with resin/plastic bezel which used the  module, and the TC-600 which again was chrome-plated but had no bezel and was probably the best looking of the three also using the  module.
The crystal is divided into sections that react when pressed with a fingertip (but not with a stylus – sound familiar…like an iPhone) so this innocuous looking watch becomes a fully-featured eight-digit calculator.
This kind of technology springboarded Casio into creating their flagship touchscreen products, the AT-550 and DB-1000 but even on its own, this technology was incredibly impressive for the early 1980s.
Add to this all the other features of the watch, including three individual daily alarms, countdown timer, stopwatch etc and you realise the Casio programmers were wringing as much as they could out of the small chip that resided inside the  module!
The downside to this cutting edge technology is that it can break, and break annoyingly at that. Most ‘untested’ TC-600s you see on eBay will have one or more ‘keys’ that don’t work in calculator mode. Having a calculator with a broken key is next to useless so finding a fully-working one is gold! Obviously there just aren’t the parts around for these watches anymore, and I’ve never heard of anyone who has been able to repair a faulty touchscreen.
So keep an eye out for one of these – it is a great watch – but get a guarantee from the seller that it works 100% in calculator mode otherwise let some other sucker grab it instead.