How does a USB to DB25 parallel cable work?
I want to program a parallel port to control a USB device


Unfortunately, such adapters will not help you do that as your requirement is the wrong way around for them.

The adapters are devices that connect to a USB host port on a PC, and then appear to the operating system of the PC as limited feature virtual parallel ports.

You cannot use them to connect a USB device (e.g. mouse, memory stick) to a parallel port.



If you need to do things the other way around, you would need an entirely different and much more custom device. Some programmable circuit that features a USB host port, along with IO that can be used for parallel communications.

The communication between the custom device and PC would then have to be some custom protocol wrapped in driver software to allow your device to be emulated at the other side of the link. This would be a very complex and in depth system design task.