This is absolutely untrue for any PS/2 keyboard, as well as for most pre-PS/2 PC keyboards (incuding AT and XT keyboards). All keyboards have always had a microcontroller in them which is responsible for scanning the key matrix and sending output over the serial link. (In PS/2, it's also responsible for reading input to change the state of keyboard LEDs.) There is nothing analog about them whatsoever.
Here is a set of pictures of an IBM Model M, for instance. The microcontroller is clearly visible in the ceramic DIP40 package.
PS/2 and AT are also bidirectional protocols for controlling the LEDs, key repeat, and various other commands that at a minimum requires some sort of digital state machine to manage the interface.
Here is a set of pictures of an IBM Model M, for instance. The microcontroller is clearly visible in the ceramic DIP40 package.
http://www.clickeykeyboards.com/model-m-gallery/1985-ibm-mod...