USB to Serial Adapter Nightmare
Okay, I have had a living nightmare trying to get a Prolific USB-RS232 Serial adapter working with my Labview program. I just solved the problem and thought I would share my experience in case it may help others.
The story begins when I bought a Trendnet TU-S9 USB adapter that uses the Prolific PL2303 driver. When I first installed the USB-Serial adapter, it showed up as COM3 an everything worked great. My labview program was able to communicate properly with a syringe pump I was controlling.
I made the silly mistake of unplugging the USB to serial adapter and plugging it back in. This is when everything went to hell. Everytime I would plug in the adapter the COM port number changed. My Labview program could no longer access the device, even when I programatically entered the correct COM number. I kept getting an error in labview indicating that the VISA resource is valid but the program cannot access the resource. I was not aware of any other programs utilizing the COM ports.
I tried connecting to the COM using hyperterminal and kept getting an error stating " Another program is using the selected telephony device. Try again after the program completes ". I was not able to open the COM port with Hyperterminal.
After some intense web searching, I came across a lot of posts about how these drivers are complete crap and they are very finicky about how they are installed. I tried every version of the driver I could find and still no luck.
Fustrated, I ended up buying a FTDI based USB-Serial adapter cable (Model USA-FTDI-A12). Still no luck. I got the same error in Hyperterminal. Clearly this was not a hardware or driver issue.
I even downloaded a different emulator program (Teraterm) which was unable connect to the COM ports (prolific or FTDI adapters). I downloaded another program called Portmon ( a serial port monitoring program). This program was not able to open the COM ports either, indicating they are in use. I started to suspect that there actually was another program that was disrupting the USB communications or hogging the COM port.
Next, I downloaded a program called Process Explorer to try and find out what was using my COM port. At first I could not find the specific virtual COM ports or Devices when searching handles in process explorer. Out of curiosity, i searched simply for "USB". Process explorer brought up a process called SacReminder.exe that seemed to be utilizing almost 10 of the USB ports. This process is for a Clickfree backup hard drive that I use. I killed the process and tried Portmon and Teraterm again.
Hazzaaah !! It finally worked and I can use my syringe pump. Everything is back to normal and I have a newfound hatred for Clickfree. Unfortunatley, i do not know enough about computers to understand why things suddenly went wrong, but this might give some idea to others about how to approach problems with these USB-serial adapters. Anyone else having problems with their USB-Serial, I would definetly figure out how to use Portman and Process explorer to determine what 3rd party programs might be interfering with your adapters. Thanks to many numerous people on the internet for their suggestions (too many to name).
Sasas
