I bought a USB to serial cable converter from e-gizmo and one of their engineers engaged me in a conversation regarding some problems they encountered with this device. This may be of interest to you since a lot of us experimenters got a need for this type of cable, and you may already have one with similar problem (not necessarily from e-gizmo).
Apparently, when e-gizmo shopped for this cable, they had several choices coming from different manufacturers. They picked the one with the higher price tag for the following reasons- It employs the immensely popular prolific chipset PL2303, not some chip hidden under a black epoxy blob; better overall workmanship, and perhaps a very important criteria, it is serviceable.
The case can be opened without destroying anything, and the PCB is constructed in a familiar (SMD components) way. According to e-gizmo, the samples they had work splendidly during their tests.
But reports of troubles started to pour in as soon as a few dozens were sold; troubles that range from dropping connections, to no functions at all. E-gizmo engineers took a second look at the guts of the cable and found something they missed on the initial evaluations: the USB cables used CERAMIC RESONATOR instead of crystal!!!
Ceramic resonators makes cheaper alternatives for crystal, but has wide tolerance spread and mediocre tempco. That makes them unsuitable for circuits requiring tight tolerance timings, such as this USB to serial bridge.
Fortunately, the fix to this problem is simple and straightforward- replace the 12MHz ceramic resonator with 12MHz crystal. The USB to serial cable worked flawlessly after the fix was made.
Worth noting at this point: I also had a conversation with Prof. Mike Abundo of U.P. IRC lab about a week later and learned they encountered similar problem, solved with exactly the same fix.
So if you have a USB to serial cable you are about to trash, Huwag muna! If it can be opened for servicing, this fix may work for your cable.
P.S. Kung bibili kayo ng USB to serial cable sa egizmo, tanong nyo muna kung napalitan na ng crystal