I'm sorry for the confusion; I guess it's easy for me to be cryptic when I am close to an issue.
My issue is this: I want to make voltage measurements every 100 usec, which are triggered by a timer interrupt. I also want to be able to communicate with a PC over the USB port. So, I want to be able to send data strings while not interfering with the on-going, 100-usec measurement intervals. The worst-case scenario would be sending these strings every 100 msec at 9600 baud. This would be equivalent to sending up to 96 bytes every 100 msec.
Thanks.