Or by using a passive USB to PS/2 adapter. USB and PS/2 are both serial protocols with similar wiring, so a USB keyboard that supports passive adapters can dete

Or by using a passive USB to PS/2 adapter. USB and PS/2 are both serial protocols with similar wiring, so a USB keyboard that supports passive adapters can detect what kind of port it is plugged into when powered on, then send unmodified PS/2 through the USB cable. For this reason, there are a few mechanical keyboards that only support N-key rollover in PS/2 mode, dropping down to 10-key rollover in USB-HID mode (it's cheaper and less likely to cause bugs than presenting as multiple HIDs, and anyone that cares about N-key rollover probably has a PS/2 port anyway).