Dear Satish,

After posting the preceding entry, I realized that you might take my input as tacit approval of using the voltages present on a DB25 RS 232 port connector as a power source for one of your projects. I would like to set the record straight on this point.

If you are building something for your own use, have fun. If it works on your computer, congratulations. Enjoy it for now and cross your fingers when you replace your computer.

If, however, you are building something for commercial distribution, I would point out that no reputable manufacturer offers RS 232 port-powered devices for sale. Oh, to be sure, you can find such things in various back-of-the-magazine type classified ads (the trade equivalent of those "X-ray Specs" advertised in the backs of comic books), but you won't find them offered by any reputable manufacturers. There's a reason for this.

In the meantime you've already expressed an interest in developing this product for a USB port. My suggestion, for two reasons, would be to forego the RS 232 effort altogether and go straight to the USB implementation. The first reason is everything that has been discussed in this thread. Pins 9 and 10 of an RS 232 port, though they might be sufficient on any particular PC, are just not universally reliable power supplies. USB, on the other hand, has power provision specified in the standard.

But the second reason is even more germane. Have you tried to buy a computer with an RS 232 port lately? They all come with USB now. You might still find an RS 232 serial port on some desktops, but even then there is a significant probability that it will only be a 9 pin, not a 25 pin, port. The fact is that technology is pinching off demand for RS 232 devices. It won't be long before they go the way of 30 pin SIMMs and ISA buses.

Good luck,

Joe