Sounds like you have small packets. Why not just send the same packet three times? If the PC receives the same packet three times, it sends an ACK back to the device. If any of the three packets are not the same it sends a NAK and the device resends all three packets again.

This isn't very efficient but since it appears you'll be handling small packets and there won't be contention on the communications line, it's probably sufficient and is very straight-forward. No need to complicate things needlessly.

Craig Steiner