computer controlled power switch

Problem: DSL modem locks up periodically, usually when I'm away.
Cheap solution: Cold boot modem.

Since my computer polls the modem already, if it detects the modem is knackered, I'd like to have the computer cold boot the modem. I figured this would be easy, but existing products (server power controls, X10 etc) cost more than a new modem.

I have RS-232 and USB available. After considering several alternatives, I figured the cheapest and easiest for a noobie like me (very limited digital electronics fooling around ~20 years ago) would be to have an RS-232 controlled relay to switch off the power in the 5V output of the modem's switching power supply wall-wart.

So I dreamed up RS-232 on computer to a little box next to modem (in another room near phone line entering house) containing:

- MAX232 for RS-232 to TTL conversion
- something
- 74xx107 configured as a toggle
- toggle to drive a transistor
- transistor drives NC relay to open the circuit
- relay controls power in 5V line.

I send two signals - the first to shut off power, the second to power up.

My problem is - what is the easiest "something". I just need a single signal to the toggle to change the state of the switch. A UART like 8250 is a tad too complicated. I found a post on this forum showing a 74HC299 instead - I could use the high bit to toggle. Any other ideas? Better solutions?

Mike

How about this.
The following circuit is more complex but it shows what transistor and discretes are needed to drive a relay from RS232.
A Hardware watchdog and shutdown button

That looks like it.

I was so focussed on the use of the data transmit line that I forgot I could mess with the flow control lines.

Thanks!