Hi Thomas,
>When using z-modem the carrier gets lost the moment I start the transfer. When I try to close the port, the app hangs.
Have you tried to reduce the baud rate?
If it's a problem with the serial driver in Windows you can use PortMon (
http://www.sysinternals.com/Utilities/Portmon.html) to monitor any activity on the COM port.
--
Christof