USB Virtual Serial Port

[Warren Hill]

Hello,

We develop a range of test equipment much of which when plugged in appears as a serial com port.

This uses the Microsoft "usbser.sys" file and the device is USB 2.0 Full Speed implementing the CDC class. The problem I have is that on slow PCs plugging one of our devices in causes the CPU usage to increase significantly -- The PC is running XP, I don't know if the problem exists on other operating systems.

On investigation this appears to be because windows is talking to the device every millisecond even when no application has opened the port. If the port is not open I can see no advantage in this.

I have looked at a number of other devices which similarly appear as virtual COM ports but do not use "usbser.sys" but a driver supplied with the device (one example is the FTDI USB to serial ICs). These only communicate when an application has opened the port.

Is there a way - either by changing our ".inf" file or changing the devices USB descriptors - to get windows to only poll the port when an application opens it?

I know this must possible if I develop my own driver but we are a device manufacturer and would not want to get involved with writing device drivers for each version of Windows + Linux + MAC etc.

Any help would be appreciated.

[Solomon]

A file named OEM*.inf (where * is a number) is created when Windows enumerates your device and an assosiated OEM*.pnf file in the Windows\inf folder (hidden by default). So it will be good if you can delete these when developing descriptors and re-enumerating as this is where most source of system confusion lies as Windows unsurprisingly tries to do the right thing but gets tangled up from the developers point of view. A simple batch file during this phase simplifies matters.