COM Ports and USB Serial devices

We have some STARCOM USB Serial dongles ( They use the Prolific chipset )and discovered some issues when using them with the RS232 output of Datawell RX-D receivers.

We setup one dongle and got data coming in fine, to Datawell’s RFBuoy application but on connecting the second device we would get a message from RFBuoy saying the second wfd ( Reciever setup file ) was invalid or both RFBuoy apps would hang. The strange thing was if you connected Teraterm to the dongle com port you could see the Hex data being transmitted on both.

After spending some time diagnosing this problem it became clear it was worth assigning specific COM Ports to the dongles and choosing low ones, I selected 4,5,6 as they are low but above the usual standard serial ports of 1,2 and 3 which is usually assigned to a modem if one is present. I also disabled any existing devices in device manager than might attempt to use those ports ( bluetooth devices are an example ).

After assigning the com ports I found restarting the machine was necessary to get RFBuoy to recognise all wfd as valid, it is sometimes necessary to write the com port manually in the wfd as the RFBuoy setup doesn’t always see the correct ports.

It is also handy to label the dongles, to aid testing, I labelled each dongle with the com port I had assigned it and the last three digits of its Device Instance ID or Device Instance Path in Windows 7. So you know which dongle is which COM Port.

After assigning the com ports each dongle appeared to reliably retain its own com port on XP, Vista and 7. If a dongle was removed and reinserted, the corresponding RFBuoy app would need to be closed and reopened to resume data reception.

If you have many phantom COM Ports in use, it is worth getting rid of them to have a blank slate to work from, There are details below on removing these, mostly changes to the registry, worth a restart afterwards and uninstalling your dongles from device manager if they already been given high com ports.