Why are my 2 USB connected devices, using separate FTDI Virtual COM Ports (COM3 and COM2), communicating over the same COM port?

Question

I have a program written in Visual Basic (.NET Framework 3.0) that communicates with a power supply via a D2XX driver (over COM3) and a pump via a VCP driver (over COM2). Currently, if both USB devices are plugged in when I start the program, it tries to communicate with the pump, (COM2) when trying to communicate with the power supply (COM3). I found a temporary solution to this issue by only have the power supply connected when starting the program. Then I initialize communication with the power supply, and afterwards plug in the pump; this allows the program to communicate with both separately for the rest of the time running the program. But this process is tedious and I am hoping there is an alternative solution where I can keep both units connected via USB and the computer communicates separately. What is confusing is that this communication issue was not always the case, a week ago it worked without having to disconnect the pump (all drivers are updated, and on the device manager they are listed as their respective COM ports as well) ; I am struggling to find some correlation between what is the underlying issue.

I tried to delete other hidden additional COM ports. I made sure all drivers were updated. When the pump is communicating via the same port as the power supply, and then I try to communicate with the pump, I receive this error:

System.UnauthorizedAccessException
  HResult=0x80070005
  Message=Access to the port 'COM2' is denied.
  Source=<Cannot evaluate the exception source>
  StackTrace:
<Cannot evaluate the exception stack trace>

But again, If I start the program with only the power supply plugged in and initialize communication with the power supply, and THEN plug in the pump, this issue is evaded. I hope someone can give some sort of direction at all troubleshooting this issue because I am completely stumped. Thank you!

Answer 1

Most likely your program implicitly relied on the correct enumeration order. If you use devices with an FTDI product with internal flash or with dedicated eeprom, you may be lucky: Many can configured via FT_PROG to be visible only for D2XX and not as VCP. This way you can avoid the problem. Alternatively, you can try to modify your program so that it accesses D2XX first. Make sure to filter out the correct device using device informations. E.g. the serial number or the VID:PID if the device manufacturer has made the effort to aquire its own IDs. Check 3.4 FT_GetDeviceInfoList of D2XX Programmer's Guide for details.