serial port and hw handshake?

Hello, we are testing some small device, which is supposed to communicate with PC via serial port, where on PC port we want to use Rebol. The device is supposed to drive small step-engine, and controller we use is not all that fast to be on pair with the speed of PC. So, how should we implement hardware handshake? We simply need our device, while busy, would tell PC to stop sending further data. I am not sure how it should be reflected in Rebol? Maybe "insert port" should return error? Anyone? Thanks a lot, -pekr-