KW11-L Line Time Clock
The programming interface is extremely simple; there is one register, with two bits; one to enable interrupts, and one which is set for every clock cycle (and causes an interrupt when that happens, if the other bit is set).
Physically, the original KW11-L is an extended-length single-height board (M787), which plugs into a dedicated slot in the CPU's backplane. Only the PDP-11/20, PDP-11/40, PDP-11/45 and PDP-11/70 support the original card, although later PDP-11's generally provided a program compatible equivalent. E.g. on later UNIBUS machines (e.g. the PDP-11/04 and PDP-11/34), the functionality was included in the DL11-W.