IBM Enhanced Keyboard

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Model M
252309 10151033065251203 1606697737 n.jpg
Part number Various (see below)
Branding IBM, Lexmark, Unicomp, others
Manufacturer IBM, Lexmark, Maxi Switch, Unicomp
Layouts 101/102/104 ANSI, 102/103/105 ISO
Keyswitches Buckling spring over membrane, rubber dome
Interface PS/2, AT, Terminal
Weight 2.0–2.5 kg
Introduced 1985
Price $69–300 (Depending on version, year of production etc)

The Enhanced Keyboard, first produced in 1985, is a 101-105-key keyboard designed for personal computers and terminals. It is commonly known by the manufacturing designation Model M, although it was not the only keyboard with this designation. It was manufactured by IBM at its plants in the US, UK and Mexico; by Lexmark and Maxi Switch; and by Unicomp, who continues to manufacture it to this day. Its key layout became the industry standard, and is still used on most desktop keyboards in an extended 104/105-key form.


The Enhanced Keyboard entered production in mid 1985 and first appeared with the IBM 3161 terminal. It used a new layout, seemingly inspired by the DEC LK201 keyboard, with elements of the 122-key terminal keyboard layout. The original terminal models featured a 102-key (US) or 103-key (rest of the world) layout. The first PC-compatible Enhanced Keyboard was packaged with the IBM 7531 Industrial PC, an industrial version of the PC/AT. Compared with the terminal keyboard, it featured lock lights, a detachable cable, and a 101- or 102-key layout (the numeric keypad having one less button on the former). In 1986, IBM produced variants of the Enhanced Keyboard for certain models of the IBM PC/XT, PC/AT and XT/286. The Enhanced Keyboard and its variants became standard for IBM's PS/2 line of computers in 1987, and an option for various IBM terminals and other computers.

In 1991, IBM divested its Office Products Division, resulting in the formation of Lexmark, who took over most of the of IBM's keyboard production. Lexmark produced the Enhanced Keyboard for IBM and other vendors, and under its own brand. Due to pressures on IBM to produce cheaper computers, the Enhanced Keyboard appeared only on high-end systems after IBM discontinued the PS/2 line in 1994, and as an optional accessory for other systems. Eventually Lexmark could no longer justify producing keyboards, and a staff buyout of their keyboard business resulted in the creation of Unicomp. IBM continued to offer the Enhanced Keyboard, producing them at their Greenock, Scotland plant, as well as outsourcing to Lexmark and Unicomp.

Around 1999, IBM began to discontinue both producing and offering the Enhanced Keyboard. Unicomp continued to produce them for other vendors as well as selling directly to the public. Unicomp's designs have not differed from late IBM ones, save for the introduction of Windows keys, USB connections, and different colour options.

Design details

The Enhanced Keyboard consists of a Model M assembly mounted inside a plastic (usually ABS, or a mixture of PC and ABS) casing. The casing consists of a separate upper and lower part, which clasp together at the front of the keyboard with interlocking tabs, and are secured at the back with four 5.5 mm screws. The controller card is located either under the keyboard assembly or mounted on the assembly itself, above the numeric keypad. On original examples, the cable exits the casing at the center back of the keyboard; on later ones, it exits at the top right. Older style cases have a square gap in the case which allows for either an SDL socket for a removable cable, or a non-removable cable with a square filler. On variants for the PC/XT and terminals, the lock light area above the numeric keypad is blank. The keys on most Enhanced Keyboards consist of a separate base (stem) and a detachable printed cover, but some used single-piece key. Double-piece key eventually became optional, as some customers found the single-piece design less likely to be lost or stolen. Virtually all Enhanced Keyboards have a small speaker grille on their bottom surfaces; however, only a few variants actually include a speaker (the keyboards of the RS/6000 workstations being the primary example).

Distinctive sound

The Model M's buckling-spring key mechanism has a distinctive sound described as "clicky". Most users accepted the sound as inherent part of using the keyboards. Some users, including present-day enthusiasts, consider it a reminder of the Model M's sturdiness compared to other manufacturers' quieter but cheaper and less durable rubber-dome keyboards (many of which were made to resemble the Model M).

Some customers requested quieter keyboards for situations requiring them (e.g. libraries and medical offices). IBM responded by producing Quiet Touch Keyboards with rubber dome switches instead of buckling springs, and with a different style of key stem. While not as prized today as buckling-spring Model M's, many Quiet Touch Keyboards are still in service and are considered among the best rubber dome keyboards made. A later initiative to provide a quieter Model M, without conceding to rubber domes, was the Soft Touch Keyboard made by Lexmark for IBM from 1995–96, whose buckling springs were greased to reduce their volume.

International Enhanced Keyboards

In contrast to IBM's previous keyboards, the Enhanced Keyboard was produced with two different physical key arrangements: one for US English and another for Europe, the Mideast and Africa. (Far Eastern IBM keyboards, with unique designs, were manufactured by IBM Japan.) This likely mirrored tastes in typewriter layouts: Non-U.S. keyboards often included dedicated keys for accented characters in other languages; the US market preferred large shift and return keys, considered easier for typing. For the international market, IBM had a 102-key layout (later known as the ISO layout) seemingly inspired by the 5251's key layout. These keyboards were produced in IBM's plant in Greenock, Scotland (where they also made PCs, terminals and laptops for the non-US/non-Japanese markets). The standard Enhanced Keyboard had the necessary membrane contacts to support both ANSI and ISO layouts, so the only physical difference aside from the different keycaps was the location of the spring/hammer assemblies to accommodate the appropriate keys.

Unlike in the US, the Greenock plant remained under IBM's control after the Lexmark divestiture. IBM continued to manufacture ISO layout Model M's, but followed the trend of Lexmark's keyboards and adopted the 'blue' logo and drainage holes. Unlike their American keyboards, IBM changed these designs without changing their part numbers, meaning that for each ISO part number there are three distinct variations. Around 1994 they switched from a 1391401-based design to a 52G9xxx-based one. By 1995 or 1996, they had changed to a 42H1292-style design. Due to more stringent regulations in the EU/UK compared to the US, many Greenock-made Model M's also have electrically-grounded space bars made out of ABS (often easily recognisable by yellowing) to prevent static electricity (often caused by monitors, then based on cathode ray tube technology) from building up and discharging into the user. Furthermore, Greenock-made M's with drainages holes do not have drainage channels inside.

In addition to the usual 13914xx part numbers, IBM also manufactured keyboards under a wide variety of seemingly random part numbers. They also made some rubber-dome variants, most of whose part numbers begin with 71G.

US-layout keyboards were also made in Greenock. All 42H1292s made after 1996 were produced there. Earlier variants were also produced in Greenock, sometimes with the same part number, but there were some Greenock-specific US keyboards; for example, the 1396790 was a standard 101-key variant based either on the 1391401 or 52G9658, depending on its year of manufacture. The different part number seems to have indicated a US layout keyboard for customers in Europe, the Middle East and Africa (EMEA). The 1396790 is common in the Netherlands and other countries in the EMEA where the US layout is popular. The 1394950 was an Industrial model made from 1996 on, and was largely identical to the earlier 1394946 with one distinguishing feature: Instead of the hard plastic label with raised silvered letters, it has a simple flat sticker.

Starting in 1986, IBM also made keyboards at their Mexican PC manufacturing center in Guadalajara, Mexico. These keyboards were largely intended for the Latin American market, although there were also some for the French Canadian and US markets. Lexmark eventually took control of keyboard production there, producing a number of Mexican-made Lexmark Model M's. Production in Guadalajara likely ceased in 1996 when Lexmark stopped making keyboards. Unlike Greenock Model M's, Mexico-made Model M's have no defining characteristics from IBM US/Lexmark Model M's. Mexico-made Model M's should not be confused with the Maxi Switch–made IBM Model M13 derivatives made in a different plant in a different part of the country.

As with US Model M's, international layout Model M's left production in 1999. For most Model M sub-variants (with the main exception of the M15) there were ISO versions, although they are generally rarer.



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 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 stabilizer bars, whereas horizontal ones used a stabilizer 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.

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. Stabilizer inserts were used for vertical keys as well as horizontal ones (using a different type of insert for each).

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

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.

Noteworthy part numbers

IBM produced Enhanced Keyboards under a wide variety of part numbers, many of which are the same. Below is a list of part numbers that either stand out for one reason or another, or are particularly common and thus warrant a mention. Almost all other Enhanced Keyboards can be considered variations of those below.


The first Enhanced Keyboard was associated with the 3161 ASCII terminal, to which it was connected by a 240-degree DIN-5 connector on a non-detachable cable. Its 102/103 terminal layout differed from the more common 101/102-key layout by having an extra key on the numeric keypad (two normal-sized keys in place of the double-sized "+" key.)


The first PC-compatible Enhanced Keyboard, shipped with the 7531 Industrial PC. It was in IBM's industrial grey colour, designed to make grime less obvious in non-office environments, and was attached to the PC with a removable AT connector cable. It is easily identifiable by the unique black square logo at its top right.


Entering production in January 1986, this keyboard was associated with certain later models of the IBM PC XT that were announced in April 1986. Its most distinguishing characteristic is its lack of lock lights (the original PC/XT keyboard lacked them too). It attached to the XT with the same sort of detachable DIN 5 connector cable as the 1388032. Whilst designed to be compatible with the XT, it was also compatible with the AT (and PS/2) interface, and it is believed that many of the early AT keyboards could interface with the same XT models with which the 1390120 was designed to work.


Appearing at the same time as the 1390120, the 1390131 was associated with certain variants of the PC AT, as well as the XT 286. It differed from the 1390120 by having lock lights. It could be considered the same as the 1388032 except for its standard white colour and metallic logo.


The original keyboard option of the IBM PC RT 6150, a Unix workstation launched in 1986 and powered by a predecessor of IBM's POWER RISC architecture. It was similar to the 1390131, with slightly different key legends (in particular, right Ctrl was marked Action), an internal speaker, and a non-removable cable with a bulky 6-pin AMP connector. Relatively few were produced before they were replaced by rubber-dome keyboards made by NMB (possibly as a cost-cutting measure when the $20,000 RT failed to sell); consequently, 1392366's are very rare.


Associated with IBM's PS/2 line of computers, the 1391401 appeared in early 1987. It was mass produced until 1994, with some made as late as 1996. Due to the PS/2's high sales, the 1391401 is quite common; serial numbers indicate more than 10 million were produced by IBM and Lexmark. Effectively the same as the earlier 1390131, it had a plastic ovular label at its left, and usually shipped with a removable PS/2 cable, although an AT cable could also be used.


The 1394618 was the keyboard of the IBM Personal Typing System, a PS/2 Model 30 dedicated to word processing. It was otherwise identical to the 1391401 except for custom keycaps for word-processing software.


The 1393464 was a version of the 1391401 with custom keycaps for airline reservation software. United Airlines bought a large quantity of PS/2 computers in 1987, so it is quite likely that this variant was initially made for them. The Sabre Corporation, which continues to produce reservation systems for airlines, hotels and other travel-related services, also ordered many 1393464s, some of which have a red Sabre logo in place of IBM's.


A relatively rare industrial keyboard from the late 80s, it had the same metallic IBM logo as the early non-industrial Enhanced Keyboards.


The 1394946 is the most common industrial Enhanced Keyboard, made from the late '80s through 1996. It could be considered an industrial-colour version of the 1391401, with a black ovular logo with raised silver lettering.


The 1394540 appeared in the early '90s with the first generation of IBM RS/6000 workstations. It was similar to the 1391401 with two major differences: the right Ctrl key was marked "Ctrl/Act", and it had an inbuilt speaker. While other Enhanced Keyboards adopted a blue-lettered logo in 1992, the 1394540 retained the older black-on-white logo.


Effectively the same as the 1394540, the 51G8572 seems to have been produced longer. Later examples from 1995 on have the blue IBM logo used on fourth-generation Enhanced Keyboards.


Known as the Soft Touch Keyboard, the 8184692 was another 1394540-style workstation keyboard, but was unique in that the buckling springs were greased to make it quieter. They are quite rare, and were manufactured in US layout only by Lexmark between 1994 and 1996. They should not be confused with the rubber-dome Quiet Touch keyboards.


An early Lexmark-specific model, it replaced the detachable SDL cable with an attached flat PS/2 cable. Many featured single-piece keys; otherwise it was similar to the contemporary 1391401.


Similar to the 52G9658, with one-piece keys.


Similar to the 52G9658, it was packaged with an IBM mouse. It is unclear whether these were retail products or were distributed with IBM computers.


Otherwise similar to the 52G9658, but sold directly to retail customers.


This and keyboards with similar part numbers were, like the 52G9658 and 42H1292, sold as part of IBM's Options scheme, where parts were sold directly customers rather than as part of a computer system. Some variants featured permanently affixed AT cables instead of PS/2 cables.


A rubber dome keyboard, externally identical to the 52G9658.


The part number associated with fourth-generation Enhanced Keyboards. Originally the Unicomp Customizer was marketed as the 42H1292U.


The 102-key US English keyboard available as an option on IBM's 3270-compatible Info Window terminals.


The 102-key US English keyboard available as an option on IBM's 5250-compatible Info Window terminals.




Lexmark, named after its location in Lexington KY, was formed in 1991 when IBM sold its keyboard manufacturing operations to an investment group. Lexmark continued to make Model M's for IBM, its primary customer. They also made some with their own branding, and custom-branded Model M's for other companies.


Lexmark manufactured several models of custom-branded Model M's for US based PC maker CompuAdd.



Main article: Ambra

These keyboards were built by ICPI, a subsidiary of IBM Greenock.


  • Sabre – Some 1393464 keyboards, with special keycaps for airline reservation systems, were made expressly for Sabre Corporation with the red Sabre logo in place of the IBM logo.
  • Wang – Made during IBM's 1991–92 partnership with Wang Laboratories, these keyboards (p/n 1397721) have a black WANG logo in place of the IBM logo. They are otherwise similar to 1391401 keyboards produced at that time. Most 1397721s are presumed to have been destroyed following Wang's transition to newer data systems, as few remaining examples are known.



Hexagonal nut securing the case

Model M keyboard cases are secured with 5.5 mm (7/32") hexagonal screws, which in Europe are a non-standard size.

Bolt mod

A bolt mod, or nut-and-bolt mod, refers to a repair process for Model M keyboards. To reduce production costs, IBM used plastic rivets to attach the barrel plate (the plastic plate providing the shafts for the sliders) to the steel backplate. These rivets may break over time from age and/or vibration. A bolt mod is the replacement of these plastic rivets with nuts and bolts, to restabilize the keyboard and keep its parts aligned for proper functioning. Screws (a "screw mod") may be used when only a few rivets need replacement, as they do not require the keyboard to be fully disassembled.



Greenock-made fourth generation Model M in UK ISO layout.

See also

  • IBM part numbers - WIP of a list of known Model M keyboards and some other stuff from IBM, sorted by P/N

External links