The IBM Numeric Keypad for IBM Personal System/2 L40SX was a 17-key numeric keypad option available for the eponymous IBM PS/2 L40SX portable computer. Designated Model M3, this keypad was the first major deviation from buckling spring keyboards in the Model M family and the genesis of input devices using IBM buckling rubber sleeve switches. Like its host portable computer, the Model M3 numeric keypad became available on 26th March 19912. The most common part numbers are 1396199 (US English version) and 1396572 (German) and most examples use a specialised modular 8P8C to PS/2 cable with an unusual pinout.
Not much is known about the keypad's history itself, but some details can be inferred from the IBM PS/2 L40SX's design history. Development for the laptop started in February 1990, when IBM was already behind in the 386 laptop market. Looking to cut down development time for the machine, the buckling sleeves keyboard design (from which this keypad was derived) skipped the usual mock-up and prototype stages and was quickly developed and pushed to production. IBM Information Products Corporation in Lexington, Kentucky under CEO Tom Hancock was responsible for the keyboard design. Leap Technologies of Otsego, Michigan was given the task of producing the plastics for these keyboards3.
The L40SX was withdrawn from marketing on 9th July 19924. It is likely the M3 numeric keypad was withdrawn from marketing at the same time since examples of this keypad made after the first half of 1992 have yet to be observed.
The Model M3 numeric keypad is an off-white cigar box style device with a folding cover piece. The keypad is branded atop with an IBM grey oval badge with the "IBM" text embossed and painted silver. Like its host machine, this keypad sports dye-sublimated PBT keycaps. The keypad itself is believed to be made from a PC + ABS blend, which whilst for the most part is robust, the screw sockets inside can break with time.
Around the rear are two ports; on the left is a passthrough PS/2 port for connecting a mouse to the host machine through the keypad and on the right is a modular 8P8C (ethernet/RJ-45 like) jack for the modular keypad to host cable. It should be noted that despite its appearance, the PS/2 plug on the end of the cable is not a standard keyboard connectivity plug. Instead, it acts as a PS/2 mouse plug with the typically-unused pins 2 and 6 used for keypad connectivity as shown on the host-side pinout diagram below. Alternatively, a DE-9 serial-based variant of this keypad is also known albeit somewhat more uncommon.
As aforementioned, this keypad uses IBM buckling rubber sleeve switches. Specifically, it uses the original rod-actuated variant that is referred to as M3/M4/M4-1 type. Buckling rubber sleeves were IBM's primary portable computer switch technology in the first half of the 1990s. Unlike generic rubber dome switches or scissor-switches as you might expect from a device like this, the rubber component in buckling sleeves plays no part in actually pressing down on the membrane, instead offloading this duty to rods on the keycaps that descend through the keypad's barrels. This aims to reduce the mushy feeling of bottoming out on rubber dome keyboards since the actuation interface is more solid, whilst still keeping the design tactile and quiet.
The internal keypad assembly is similar to most other Model Ms - three layers (the barrel plate, membrane and metal backplate) are held together with melted plastic rivets. The membrane's flex cables also socket into 2.54mm pitch TRIOMATE connectors. Unlike their larger brethren, however, this keypad's rivets seem to be stronger. The entire assembly design was later reused for the IBM Model M4/M4-1 Space Saver Numeric Keypad, with the only difference being the PCB the membranes connect to; on the M3, there is a PS/2 compatible controller, whereas on the M4/M4-1 keypad, the connections are instead piped directly to the host (a Model M4 or M4-1 keyboard) through a 10P10C jack which instead takes on the controller duties7.