I use a USB external modem. When plugged in, the computer automatically
assigned the modem COM3, no special adapters needed. I would think plugging a
TNC in would have a similar affect.

Then again, I've never heard of a computer without a serial port. That is a bit
strange.
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73! de Andy KC2SSB - WPYI880 (GMRS)
Beachwood, NJ USA! Grid FM29vw
http://vhfradiobuff.tripod.com