A passive converter refers to a simple device designed to merely change the port of a keyboard's cable. This excludes the translation of the keyboard's protocol, which is a task usually done by an active converter instead.
Thanks to their simplicity, passive converters are often more affordable than active converters, but their practical uses can be very limited.
Although backwards compatibility is rare among protocols, the PS/2 protocol stands out as a prevalent example as it can interpret an AT keyboard. Furthermore, an active PS/2-to-USB converter can be used afterwards to practically convert an AT keyboard into USB.