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My application is connected to a USB modem which maps to a COM port.
I am trying to find a way to keep my application connected to that same device (via a com port) after a system restart, which - as I obsereved - keeps reassiging different COM port numbers to the devices.

I was looking for an identifier which is constant for that modem type, but different for every physical modem device, but as yet to have found one.

Is there a generic way to accomplish that, or is it driver/manufacturer specific?

Update: The solution I found was hardware related, an update to the modem's PID number via a patch, given by the manufacturer, then this PID can be accessed and linked to the assigned COM# through the registry.

A: 

I was reading through FTDI documents about this the other day, trying to solve the same problem (but as a product manufacturer not an end user).

The problem is related to how the driver identifies the device. You might be able to "hack" it by deleting registry entries at startup - effectively clearing the last known assignment.

Two of their app notes which might help are here and here

peter_mcc