Editing IBM Enhanced Keyboard (section)

==Generations==

Various changes were made to the Enhanced Keyboard's construction over its long production life. A number of distinct design phases have been noted, as described below. In addition to various visible changes, the tolerances of the springs used for the buckling-spring switches varied, leading to slight variations in key feel and stiffness (although a keyboard's condition can be a more significant factor).

===First generation===

At 2.5 kg, the original Enhanced Keyboards were somewhat heavier than the later ones, due to their thick, brass-colored steel backplate similar to those of [[IBM Model F|Model F]] keyboards. The lock lights in first-generation Enhanced Keyboards were connected to their controller cards with a Berg connector cable with yellow wires, like those used in the AT Model F. Vertical largest keys used stabiliser bars, whereas horizontal ones used a stabiliser insert. Some very old Enhanced Keyboards have been seen with a shaped piece of polyester in the middle of the case, apparently to provide extra support for the keyboard assembly. It is unclear how long this practice continued.

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File:Support for Keyboard Assembly Model M 1390120 UK 1986-09-29 1920x1080.jpg | First generation early model support for keyboard assembly
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There was some overlap in the production of first- and second-generation designs: keyboards with first-generation features continued to appear as late as 1989; second-generation versions started appearing between 1987 and 1988.

===Second generation===

The second generation consisted of some relatively subtle changes, mainly internal. A thinner steel backplate was used, resulting in a drop of weight to 2.2 kg. The lock light LEDs were connected to the controller board by a thin ribbon cable. The backplate's twisted-cable grounding wire was replaced with a simpler one. Stabiliser inserts were used for vertical keys as well as horizontal ones (using a different type of insert for each).

Second-generation features began appearing around 1987, and were replaced by the third generation in 1992.

<gallery widths=250 heights=187>
File:IBM Model M -- late 1G numpad stabiliser.jpg | First generation vertical keycap stabiliser
File:IBM Model M -- 2G numpad stabiliser.jpg      | Second generation vertical keycap stabiliser
File:IBM Model M -- 4G numpad stabiliser.jpg      | Fourth generation vertical keycap stabiliser
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===Third generation===

The third generation is most commonly associated with Lexmark-made Model M's, indicated most obviously by a change to a blue IBM logo. The primary difference was the addition of a drainage system to direct spilled liquids away from the keyboard's components, and consisting of holes in the case's bottom connected to interior channels in the upper part of the keyboard assembly. For unknown reasons, Greenock-made Enhanced Keyboards had holes but no channels; one theory is that Lexmark made the cases, but IBM produced the keyboard assemblies and did not upgrade their tooling. The lock-light cable was consolidated with the ribbon cable attaching the keyboard membrane to the controller card. While SDL cables were sometimes used, third-generation keyboards usually had a flat, coiled, non-detachable cable.

Third-generation keyboards began appearing in 1992 and continued to be produced until 1999, although by 1996 they had been largely supplanted by fourth-generation models.

===Fourth generation===

Fourth-generation Enhanced Keyboards were considerably modernized and are quite different from prior generations. The case was redesigned, eliminating the speaker grille, and the non-removable cable exited through a small hole at the rear of the keyboard's right side. The controller card was mounted above the numeric keypad, with the lock lights integrated into it, eliminating the need for separate PCBs. Consequentially, fourth-generation controller boards and membranes are not interchangeable with those of prior generations. Due to the slightly different placement of the lock-light LEDs, fourth-generation lock-light labels are not usable with older Enhanced Keyboards or vice-versa. Fourth-generation cables are straight, with no coils like prior cables. A thinner backplate was also used, reducing weight to 2.0kg.

Fourth-generation Enhanced Keyboards appeared in 1995, and are produced to this day by Unicomp.