[candydance]

Since a USB device gets paired with a driver through a VID/PID pair, buying a single VID allows you access to 65536 PIDs for that VID. They give out the PIDs for free but hold on to the VID for themselves.

FTDI own a bunch of VIDs which are associated with "FTDI" devices. If you ask FTDI they'll reserve a small block of PIDs for you to use for your own products that use the FTDI USB chips. 8 PIDs cut out of a 65536 block is not a significant cost.

I'm surprised that VTM still allows them to do it though, maybe it's a special case because you're using their IC.

The problem is that VTM maintains an "official" list of VID/PID owners, so that they can keep drivers on seperate IDs and prevent clashes for users. Most hobbyists just pick a random VID/PID for their personal projects and hope for the best, but for small scale hobby releases it's problematic since "technically" they are going against the terms of service for the USB protocol. Sometimes there will be clashes if people pick random numbers, and VTM hates that.

http://www.ftdichip.com/Support/Knowledgebase/index.html?can...

[ajross]

USB devices are supposed to be detected via the class (es) declared in the descriptor actually. This allows for e.g. a single HID driver to work with input devices from all manufacturers. Linux drivers almost all work this way, using the vid/pid for quirk detection only. The windows world tends to be less clean.

[jmaygarden]

That falls flat for custom/vendor classes. It's a fairly convenient route for a device that doesn't fit the other molds using libusb/WinUSB drivers.