IBM keyboards of old were distinguished by their superior tactile feedback.
In fact, some of the old mainframe terminals, e.g., the 3270†, were capable of physically locking the keyboard when the program was not expecting input. When the keyboard was locked, you couldn't type — the keys would not depress. When the system was ready for input, it would send a keyboard restore order, you would hear a faint click from within the keyboard, and the keys would then go down when pressed.
In fact, some of the old mainframe terminals, e.g., the 3270†, were capable of physically locking the keyboard when the program was not expecting input. When the keyboard was locked, you couldn't type — the keys would not depress. When the system was ready for input, it would send a keyboard restore order, you would hear a faint click from within the keyboard, and the keys would then go down when pressed.
†http://en.wikipedia.org/wiki/IBM_3270