When I'm building a prototype, I have used a cheap USB->RS232 adapter and pulled it apart. Most of them have a Prolific PL2303 (www.prolific.com.tw) chip and a MAXxxx for the ttl to RS232 conversion. Remove the MAXxxx chip and solder wires to the 0V Txd and Rxd signals on the PL2303 chip (use a magnifying glass!!) and you've bypassed the whole RS232 level conversion. I even use the 5V from the USB to power my prototype. Most of the usual tools work without a hitch.

You could also try www.ftdi.com as they have similar devices to the prolific one. The ftdi chip is good for doing RS485 as it has an output that drives the transmit enable on the 75176 485 driver with no extra software. The RS485 works as you would expect.