Fault:

USB to Serial Converter locks up, and will not work again unless unplugged or windows is restarted

Cause:

Electrical noise has locked the usb device. These devices are intended for office environments only and not when you have a computer located in a control panel.

Solution:

If a desktop computer, use a PCI to serial card.
If a laptop, use a PCMCIA to serial card.

This has been tested with several brands of usb converter and all have found to fail (MOXA, Prolific, USB485, Bafo).