Model M Enhanced Keyboard

Also applies to IBM Basic Keyboard, IBM Soft Touch Keyboard, Lexmark Classic, Unicomp Customizer and Unicomp Classic



IBM Enhanced Keyboard<a class='source-link' href='/wiki?id=modelmenhanced#Sources'><sup>[1]</sup></a>
IBM Enhanced Keyboard[1]

The IBM Enhanced Keyboard was IBM's premier keyboard design from the mid-1980s to the mid-1990s. With its distinct style, use of clicky buckling spring key-switches, and far-influencing layout, the Enhanced Keyboard is quite possibly the most famous keyboard of all time and continues to be loved and enjoyed by many. With its many variants for different PCs and terminals and alternate/later branding as the Lexmark Classic and Unicomp Customizer/Classic, the Enhanced Keyboard is also a diverse family of keyboards in its own right.

The Enhanced Keyboard is a member of the Model M keyboard family and is considered to be the definitive member. Model M keyboards were considered to be IBM's flagship keyboards for whatever market the keyboard was intended for, and in that light, the Enhanced Keyboard was IBM's flagship keyboard in the home and office PC, industrial PC and terminal markets during their hey-day. As such, the Enhanced Keyboard enjoys many famous part numbers such as 1388032 (7531 Industrial), 1386303 (3161 terminal), 1390120 (PC/XT), 1390131 (PC/AT), 1391401 (PS/2) and 1392595 (3151 terminal). Including the regional variants, at least 250 part numbers of Enhanced Keyboard exist.


Marketing names
IBM Enhanced Keyboard, IBM Basic Keyboard, IBM Soft Touch Keyboard, Lexmark Classic, Unicomp Customizer, Unicomp Classic
IBM, Lexmark, Maxi Switch, Unicomp
IBM buckling spring (membrane variant) (most), IBM Quiet Touch rubber dome (71Gxxxx)
Dye-sublimated (pearl, most) or pad-printed (black, few) PBT
Case dimensions
48.7cm x 20.7cm x 4.7cm (19.17" x 8.15" x 1.85")
Case material
PVC (most), ABS or PC + ABS (few)
Case colour(s)
off-white (pearl white), black (raven black) or industrial grey
Case screws
5.5mm (7/32") hexagonal
1.67kg (~3.6lbs) to 2.5kg (~5.5lbs) (depending on generation)
101-104 (ANSI) or 102-105 (ISO) key full-size
AT, PS/2, terminal or USB (depending on variant)
IBM Personal Computer/AT Keyboard
IBM/Lenovo Enhanced Performance Keyboard (for IBM & Lenovo), Unicomp New Model M (for Unicomp)


The Enhanced Keyboard was the first discrete (as in, not physically integrated into something) keyboard of the Model M family, only preceded by typewriter keyboard assemblies used on the IBM Wheelwriters 3 and 5 and Quietwriter 7 in late 1984 as the first vessels to bear the membrane variant of IBM buckling spring key-switches. The first system to adopt the Enhanced Keyboard was the IBM 7531 Industrial Computer that was announced in May 1985[2], a system known for using keyboard P/Ns 1388032 (US ANSI variant) and P/N 1388076 (UK ISO). This was quickly followed by June 1985's IBM 3161 ASCII Display Station with keyboard P/N 1386303[3].

IBM Industrial Keyboard, the genesis of the Enhanced Keyboard<a class='source-link' href='/wiki?id=modelmenhanced#Sources'><sup>[4]</sup></a>
IBM Industrial Keyboard, the genesis of the Enhanced Keyboard[4]

The Enhanced Keyboard was made available for the wider office and home sectors in April 1986, firstly being introduced with the IBM 5170 PC/AT model 339 at the start of the month[5], then made available as an option for S-models of the IBM PC/XT, and IBM PC/AT Extended by the end of the month[6]. The first new non-industrial PC system to ship with a PC-compatible Enhanced Keyboard was the IBM 5162 Personal Computer XT 286 announced September 1986[7]. Enhanced Keyboards P/N 1390120, P/N 1390131 and regional derivatives of both with similar part numbers are associated with the first home and office Enhanced Keyboards. During this time, IBM also made the Enhanced Keyboard available for its first commercial RISC-based computer, the IBM RT PC[8], namely P/N 1392366. The Enhanced Keyboard got its big break in April 1987 when IBM released the Personal System/2 series of PCs[9]. PS/2 Enhanced Keyboards constitute the Model M's most famous variants with part numbers like 13914xx becoming the de facto Model Ms in the eyes of many. Whilst the legacy of IBM PS/2 systems is mixed, the PS/2 Enhanced Keyboard went on to become one of the most famous and revered keyboards of all time and the PS/2 keyboard (and mouse) port also become industry standard. By June 1987, another major Enhanced Keyboard variant was released for the IBM 3151 ASCII Display Station, P/N 1392595[10]. 1989 also saw the launch of the IBM InfoWindow series of Display Stations, starting with the IBM 3471 that sported the common P/N 1394204 (FRU 1394802)[11] and would become the most common terminals with Enhanced Keyboards.

The start of the 1990s saw the formation and sale of IBM Information Products Corporation in August 1990 and March 1991 respectively to facilitate the divesture of IBM’s Lexington, KT and Boulder, CO typewriter and keyboard manufacturing operations[12]. This resulted in the formation of Lexmark International, which IBM subsequently started marketing its products with IBM branding (including Enhanced Keyboards) for the next five years[13]. Pretty much all US English Enhanced Keyboard production fell to Lexmark, although IBM US produced occasional examples as late as 1994[14]. IBM Mexico also produced some P/N 1391401 keyboards during this time alongside their usual workload of producing South American models. IBM UK largely continued producing Europe, the Middle East and Africa (EMEA) models by themselves. During the Lexmark era, several new part numbers derived from 1391401 were introduced such as P/Ns 52G7980, 52G9658 (RS/6000 US ANSI but destined for Korean markets[15]), 52G9700, 60G3571 (IBM Easy OPTIONS[16]), 82G2383 (RS/6000 US ANSI but destined for Chinese markets[15]) and 92G7453 (OPTIONS by IBM[17]). Lexmark also introduced their own branded version of the Enhanced Keyboard such as P/N 1398601, which they branded invariably as the Lexmark Classic Touch Keyboard or simply Lexmark Enhanced 101-key Keyboard. Lexmark started selling their own-branded keyboards on 1st April 1992[18].

IBM Enhanced Keyboard in PC Mag, February 1992<a class='source-link' href='/wiki?id=modelmenhanced#Sources'><sup>[19]</sup></a>
IBM Enhanced Keyboard in PC Mag, February 1992[19]

Lexmark and IBM expanded the Enhanced Keyboard's reach to other companies as well. Lexmark made versions for the Enhanced Keyboard for CompuAdd (P/Ns 1369167 and 1397771), Cube Computer Corporation (P/N 1378207), Dell (P/Ns 1397651 and 1369050), GTSI (P/N 70G8638) and Reply (P/N 1395100). By 2nd August 1993, IBM created Ambra Computer Corporation as a wholly-owned subsidiary to market low-cost PCs[20] and shipped some of these systems with reskinned Enhanced Keyboards largely made by IBM UK. IBM UK's Ambra keyboards were available in two series; AMB100x and AMB200x. AMB100x are coloured blue-grey, have keycaps with Apple-style bottom-left aligned italicised legends and have a unique lock-light overlay with circular view-holes. AMB200x is a more conservative version that simply has a different oval badge and circular view-hole overlay - their case colour and keycaps are identical to standard Enhanced Keyboards. Lexmark additionally produced an Ambra keyboard for the US market that had an isosceles trapezium-shaped logo but otherwise was a standard Enhanced Keyboard (P/N 1378160). By October 1993, IBM introduced what is technically the most unique Enhanced Keyboard variant in the form of a Quiet Touch rubber dome model called the IBM Basic Keyboard (P/N 71Gxxxx). The IBM Basic Keyboard was solely produced by Lexmark (IBM themselves seemingly never continued production after April 1996) and was available as an option for the IBM ValuePoint workstations for AS/400 midrange computers[21] and later IBM PC 300[22] and 700[23] series computers.

At some point in 1995, Lexmark and IBM UK introduced their final revision of the Enhanced Keyboard design. P/N 42H1292 for the US market (EMEA/SA versions retained their P/N 13914xx nomenclature) represented the largest change in the keyboard's design by changing the cable position and introducing a new type of pressure-fit controller around the lock-light area. The new controller importantly reduces the part count over the previous Triomate-style controller, however, the position of the lock-lights was changed from bottom-left to roughly middle. Despite this, some keyboards based on the older designs were still produced in 1995 and 1996. By April 1996, Lexmark exited the keyboard market after its five-year contract with its former parent company was over[24]. As such, IBM UK took on the immediate mantle of producing Enhanced Keyboards based on the 42H1292 design and continued producing Model Ms in-house until the end of 1999.

However, 1996 saw former Lexmark (and by extension, IBM) employees form Unicomp, Inc. who purchased IP and tooling from Lexmark to restart Lexington-based keyboard production. Whilst Unicomp could produce some keyboards based on the pre-42H1292 design during its opening years, their products are largely based on 42H1292. By late 1998, Unicomp had established its online presence and was marketing the Enhanced Keyboard as the Unicomp Customizer[25]. The first major change to the keyboard design was the addition of Windows keys by October 2000, allowing for 104 (ANSI) and 105 (ISO) key layouts[26]. In 2006, Unicomp made a USB version of the keyboard. In early 2012, Unicomp renamed the Customizer to Classic[27], beginning the present period in the Enhanced Keyboard history.

Customizer in Unicomp's Product Catalog, 2000<a class='source-link' href='/wiki?id=modelmenhanced#Sources'><sup>[28]</sup></a>
Customizer in Unicomp's Product Catalog, 2000[28]

Whilst the keyboard continues to see production as of 2022, time has certainly taken its toll on the moulds and, unfortunately, production defects are becoming ever increasingly more common. As such, since 2021, Unicomp discontinued most SKUs of the Classic to preserve the life of the moulds as much as possible and advises customers to purchase the Unicomp New Model M instead. Unicomp has recently started the process of retooling, but it's presently unclear if they will introduce entirely new moulds for the Classic. For now, the Classic still endures and Unicomp has made a business for itself making these keyboards for other companies in much the same way as Lexmark in the first half of the 1990s. In particular, Unicomp has produced General Electric Marquette and Healthcare branded keyboards since 2000 and continues to do so as of 2020[29].


The pricing of the Enhanced Keyboard and its derivatives has fluctuated over the years. All inflation adjustments were made with US Inflation Calculator.

Original price
2021 price
IBM 3161 terminal, P/N 1386303
IBM 3164 terminal, P/N 1386304
IBM 3191 terminal, P/N 1390123
IBM 3151 terminal, P/N 1392595
IBM 3471 terminal, P/N 1394204
IBM PS/2, likely P/N 1391401
Unicomp PS/2
Unicomp PS/2, P/N UNI0416


More information: Model M keyboards


More information: IBM buckling spring#Membrane

Animation of a buckling spring<a class='source-link' href='/wiki?id=modelmenhanced#Sources'><sup>[34]</sup></a>
Animation of a buckling spring[34]

The most famous feature of the Enhanced Keyboard is its employment of a buckling spring over membrane switch design. Such a switch is comprised of a coil spring, rocker (also known as a flipper or hammer) and the membrane circuitry underneath - pressing down on a buckling spring key causes the coil spring to compress and eventually buckle, which then rocks the flipper attached to the bottom of the spring thus actuating the membrane[35]. The mechanism rocks the membrane before its fully buckled (bottomed out), allowing for part-way actuation.

By 1993, a rubber dome version of the Enhanced Keyboard appeared for those wanting the same keyboard layout, style and construction but quieter. These were marketed as a Quiet Touch version of the Enhanced Keyboard (although named as the IBM Basic Keyboard) and can typically be identified by their part number nomenclature of 71Gxxxx. Quiet Touch keyboards also do not possess part-way actuation and their keycaps are not compatible with buckling spring keyboards. Unicomp still offers rubber dome versions of its keyboards and still refers to them as Quiet Touch keyboards. Another more conservative take on a quieter version of the Enhanced Keyboard is the Soft Touch version, which features greased springs designed to suppress spring ping.

Outer case

The Enhanced Keyboard can be easily identified by its unique physical design. Most notably, the keyboard case has a distinct wedge shape seldom copied by other manufacturers and they have a large space above the F-keys where the keyboard's branding typically resides. The case is typically made from PVC (especially during the IBM years) and it does not yellow with UV exposure, however, some Enhanced Keyboards have been invariably known to use ABS or PC + ABS cases that could potentially yellow. The keyboard's inner assembly is curved and partially swoops up above the F-keys and the case is designed to accommodate this, allowing for the space above it to hold items such as pens or pencils.

Internal assembly

The aforementioned curved assembly is comprised of three layers; the barrel plate, the membranes and the backplate. The barrel plate sits top facing and is used to guide the individual switch components at their correct position above the membranes' contact points. The barrel plate design has a degree of redundancy in the number of barrels available, allowing for a 'one size fit all' design for ANSI and ISO keyboards and terminal versions of both. Barrel plates made by IBM UK can be made of Noryl SPN422L (an alloy of PPE, PS and PTFE)[36]. The membranes are a part of the key-switch system used as the circuitry to be actuated by the buckling springs (or rubber domes on Quiet Touch models) and facilitate a 16x8 matrix. The membrane sheets are usually atopped by a rubber or latex sheet called a "blanket" to dampen the stress exerted by the actuators. The backplate provides rigidity. The assembly is held together by many plastic rivets designed to provide the tension needed for the buckling springs to operate properly. Unfortunately, these rivets are the single largest flaw in the Model M design as they can weaken and break with age. Such an issue can be permanently solved with a bolt or screw mod.

Internal assembly<a class='source-link' href='/wiki?id=modelmenhanced#Sources'><sup>[ASK]</sup></a>
Internal assembly[ASK]


The keycaps are usually IBM's famed PBT dye-sublimated variety. Versus the most common keycap material, ABS[37], PBT is more durable, does not degrade/yellow with age, UV or heat exposure, and will keep its texture for longer without shining[38]. Dye-sublimation is also a very durable text printing method that sinks dye material into the keycap's plastic itself, meaning there is nothing to quickly wear off as would be the case with pad-printing or silk screening. This means many vintage Model M keycaps will still be in good condition, requiring at most a good cleaning. Enhanced Keyboard keycaps are also typically two-piece that when present allows you to swap the keytop featuring the text legends instead of having to pull the entire keycap out of the barrel.


Through the years, the Enhanced Keyboard used two main types of controller boards; the Triomate-based membrane dual or triple flex cable connection controller (first through third generation Enhanced Keyboards) and the pressure-based membrane connection "overnumpad" controller (fourth-generation Enhanced Keyboards).

Triomate-based controller

The most familiar Enhanced Keyboard controller PCB is dubbed Triomate-based, named after the type of connector the membrane flex cables use - TE Connectivity's Triomate, pitched 2.54mm. Enhanced Keyboards with this style of controller that also have lock-lights with have their LEDs placed on a separate daughterboard. The number of flex cables could differ:

  1. Very early PC-compatible controllers (~1985-1987) could have two Triomates; 16-pin matrix connection and 8-pin matrix connection. If the host keyboard has lock-lights, the LED daughterboard is hooked up with a 4-pin Berg connector.
  2. Very early terminal-compatible controllers (~1985-1987) could have three Triomates; 16-pin matrix connection, 8-pin matrix connection and 13-pin shorted to PCB GND[39].
  3. Most PS/2-compatible controllers (~1987-1991) could have three Triomates; 16-pin matrix connection, 8-pin matrix connection and 4-pin LED lock-lights connection.
  4. Later PS/2-compatible controllers (~1992-1996) could have two Triomates; 16-pin matrix connection and 12-pin combined matrix and LED lock-lights connection.

Aside from the number of Triomate sockets, the next big variance is cabling. Enhanced Keyboards were made with both fixed cables and modular shielded data link (SDL) cables. Terminal Enhanced Keyboards with fixed cables typically had a 2x6 pin cable header whereas PC-compatible Enhanced Keyboards of the 1990s typically had a 4-pin JST header. SDL sockets remained the same throughout the years with only cosmetic/shell colour variance. One thing that didn't change for Triomate-based controllers is power consumption - Model Ms with them can draw between 90 and 130 milliamps depending on the status of the LED lock-lights, which is significantly more than most standard keyboards today[40].

Pressure-based controller

In 1995, the introduction of P/N 42H1292 and its EMEA/SA counterparts saw the move from Triomate-based controllers to a smaller, simpler card that is connected to the membranes' traces by pressure alone. This 25-pin pressure-based controller is sometimes referred to as the "overnumpad controller" due to the fact it's positioned just above the numeric keypad keys. Due to this, the LED lock-lights were placed on the controller itself, eliminating the daughterboard. The LEDs are also placed different compared to the old daughterboard due to the position of the membrane connection pins and the large IC, which is what gives fourth-generation Model Ms the characteristic lights that are aligned in the middle of their external overlay instead of bottom left. The change in controller also reduced the part count and power consumption significantly, with Unicomps circa 2001 drawing at most ~30 milliamps with LED lock-lights turned on[40].

The physical size of the overnumpad controller has not significantly changed, although there is some variation in mounting and IC. The main difference over the years is the increase in mounting hole size - Unicomp presently uses 4.3mm mounting holes, however, Lexmark/IBM UK/earlier Unicomp overnumpad controllers had 3.2mm mounting holes[41]. Additionally, Unicomp has moved to mounting the controller with screws instead of relying on a plastic clip (visible in the photo above, left side of the board right next to the capacitor). In 2006, Unicomp introduced USB versions of its keyboards which required a change in IC (new PS/2 keyboards still used the old larger IC though). In 2020, Unicomp introduced blue LEDs for most of its keyboards. Today, the USB overnumpad controller sports a small Cypress IC and surface-mount components.

Speaker grille

Perhaps one of the most common questions people may have about Enhanced Keyboards is the grille on the back of them. This grille is in fact for an internal speaker, but in practice, very few Model Ms populated this and only first to third-generation Enhanced Keyboards have the grille. The speaker was only available on keyboards for IBM workstations of the RT PC and RS/6000 family, and the Soft Touch version of the Enhanced Keyboard (P/N 8184692). The most well known Enhanced Keyboard part numbers are P/N 1392366 (RT PC), 1394540 and 51G8572 (RS/6000). The former has a unique and bulky 6-pin AMP connector on its cable, however, the latter (and the aforementioned Soft Touch Keyboard) use a PS/2 mini-DIN plug with the typically unused pins in the connector used for speaker (since standard keyboard connectivity only requires 4 of the 6 pins on PS/2 mini-DIN).

When present, the device is typically a P/N 1392326 8Ω, 0.2W speaker[44]. The speaker was used for providing a system audible queue. For RS/6000 keyboards, you would have been required to purchase and fit the IBM Optional Keyboard Cable with Speaker (feature code 6599, FRU 93H8878)[45]. For fourth-generation Enhanced Keyboards, the speaker grille was removed. However, the artefacts from Lexmark/IBM UK modifying the case moulding to fill in the grille is visually present, even on brand new Unicomp Classics

Flip-out feet

All Enhanced Keyboards have single setting flip-out plastic feet. In most cases, they're relatively stubby feet that have a characteristic hollowed-out space in between the two clips that hold the feet in place. However, for some IBM InfoWindow Enhanced Keyboards, an elongated foot variant is known and is likely borrowed from the Type III 122-key Model M design.


Most Enhanced Keyboards are proudly adorned with the branding of IBM, Lexmark or Unicomp to the point most of the badge styles have become iconic (or infamous) in their own right. These badge styles also prove useful for quickly identifying the approximate period and generation a given keyboard is from. For IBM, the two main styles of badges were a square logo in the top-right corner of the keyboards and an oval logo in the top-left corner of the keyboards.

The black-square logo of early industrial models was technically the first branding Enhanced Keyboards had, however, the silver-square logo first-generation terminal and non-industrial PC-compatible Enhanced Keyboards had is more famous. Square-badge Enhanced Keyboards are typically belonging to the first generation. An oval logo typically means the keyboard is from the second to fourth generations produced between 1987 and 1999. The oval logo has a further three variants; oval with grey text (1987-1992), oval with blue text (1992-1999) and black oval that was used on various industrial Enhanced Keyboards since 1987.

Self-branded Lexmark Classic keyboards featured a rectangle logo in the top-left corner with the Lexmark black and red logo printed on an off-white background matching the rest of the case. Unicomp's self-branded Customizer and Classic keyboards typically feature Unicomp's logo printed on the lock-light overlay or simply lack any branding at all on the top case. Very late keyboards produced by Unicomp for IBM could have the IBM logo printed on the lock-light overlay. Enhanced Keyboards produced by IBM, Lexmark or Unicomp for other companies such as Affirmative, Ambra, Dell or General Electric Healthcare/Marquette could feature a plethora of logo styles in oval, rectangle or lock-light form.


The Enhanced Keyboard underwent various changes since its inception in 1985, mostly in the name of cost and weight saving required for the keyboard to be a competitive sell in a market increasingly saturated by cheaper PC clones and keyboards in the late 1980s and throughout the entire 1990s. Generally, four distinct generations are recognised by Model M enthusiasts and the following properties are true for IBM UK, IBM US, Lexmark and Unicomp produced Enhanced Keyboards[48]:

IBM branding
Controller style
IBM silver-square
1.3mm, rainbow
Triomate connectors for membrane, Berg connectors for lock-lights
Typically early industrial and terminal, and IBM PC/XT and PC/AT, keyboards
IBM oval with grey text
1.0mm, grey
All triomate connectors
Typically PS/2 PC, 3151 terminal, and most industrial keyboards
IBM oval with blue text
0.8mm, grey
All triomate connectors
Typically late PS/2 PC and terminals and OPTIONS by IBM keyboards
2kg (1.67kg post-1999)
IBM oval with blue text, Unicomp logo on lock-lights
0.8mm, grey
Pressure-based "overnumpad"
Typically OPTIONS by IBM and all Unicomp keyboards

Note that this is a rule of thumb set of information - there may be exceptions. This also doesn't take into account any possible case, assembly or controller swapping that might result in keyboards made during one generation exhibiting properties of another.


The eponymous "Enhanced layout" is perhaps the single biggest impact on the computing industry that the Enhanced Keyboard made. The layout can be best described as a fusion of the layouts previously used on the IBM 4704 62/77/107-key Model Fs and IBM 3270 PC 122-key Model Fs, and DEC's LK201 terminal keyboard that was responsible for introducing the inverse-T style cursor keys[46]. This resulted in a marked difference over the previous IBM PC, PC/XT and PC/AT keyboards, including separated Esc and navigation keys, two new F keys, duplicated Ctrl and Alt keys for ambidextrous access[1] and enlarged backspace key. A change in philosophy was also adopted, with slightly different physical arrangements introduced for US English and rest-of-world versions of the Enhanced Keyboard.

The US English version of the Enhanced layout is now referred to as the ANSI layout (named after the American National Standards Institute) and the European, Middle Eastern and African (EMEA) and South American (SA) versions are now referred to as the ISO layout (named after the International Organization for Standardization). The EMEA/SA keyboard also has many variants within like previous PC keyboards, including versions with different symbol sets for different languages and even different alphabetical key arrangements such as typical QWERTY, QWERTZ (German) and AZERTY (French). ISO keyboards also have an AltGr (Alt Graph) key instead of a right Alt key as another way to access additional and typically uncommon characters for the given host region. A select few EMEA/SA countries such as The Netherlands have since adopted ANSI as their national/preferred layout. Multilingual countries like Canada have adopted both ANSI (English) and ISO (Canadian bilingual or English-French/French-Canadian) as possible national layouts.

The Enhanced layout remains the primary layout of computer keyboards, with even Apple adopting a similar physical layout starting with the Apple Extended Keyboard. Minor alterations have been attempted with various levels of success. Most attempts usually end up as a quirk of the manufacturer that tried to introduce it, however, Microsoft's addition of Windows keys supported by the release of Windows 95 resulted in widespread adoption thus most PC keyboards today have them[47]. Smaller form-factor variants of the Enhanced Keyboard have also materialised such as the tenkeyless layout introduced by the Model M-based IBM Space Saving Keyboard.


More information: Keyboard Connections topic

The Enhanced Keyboard is best known for using a 6-pin modular AMP shielded data link (SDL) port and a detachable cable. This allowed the keyboard to be uncoupled from its cable as needed. The cable was typically a high-quality grey and thick coiled cable that could stretch as needed, however, black SDL cables are known to exist for early PC/XT and PC/AT Model Ms.

Terminal variants always had fixed cables from the very beginning, however, most later PC Enhanced Keyboards eventually received fixed cables by the mid-1990s that were at first coiled but later straightened.

In terms of protocol, PC-compatible Enhanced Keyboards typically communicated with IBM scancode set 2 (aka, "AT") protocol through an AT (5-pin DIN in 180-degree pin arrangement) or a PS/2 plug (6-pin mini-DIN) on the end of their cables. PC/XT and PC/AT Model Ms could also communicate via IBM scancode set 1 ("XT"), being able to auto-sense what the host computer expects (more info below). Terminal versions spoke IBM scancode set 3 instead, which was either carried via DIN connectors with a 240-degree PIN arrangement or modular 8P5C "RJ-45" jacks.


Early industrial, and (as aforementioned) PC/XT (eg, P/N 1390120) and PC/AT (P/N 1390131) Enhanced Keyboards, could communicate via all three IBM scancode sets. These keyboards supported a "Mode 1" and "Mode 2" communication that allows for set 1 and set 2 communication respectively. Mode 2 can further be used to access set 3 or go back to set 1. The modes are selected via an auto-sense routine that figures out what the host system wants based on the status of the 'clock' line. This mode selection can only be achieved immediately after the keyboard's power-on reset (POR) and the host system must be compliant otherwise the keyboard will essentially default to Mode 2. The July 1985 IBM 7531/7532 Industrial Computer Technical Reference System Unit describes the procedure:

If the 'clock' line is active (high) immediately after POR, the keyboard sets up for Mode 1 operation. It then waits for the 'clock' line to become inactive (low), executes the basic assurance test (BAT), and returns the completion code.

If the 'clock' line is inactive (low) immediately after POR, the keyboard executes the BAT, waits for the 'clock' line to become active (high), and sends the completion code in Mode 2 protocol. If the system has not made the 'data' line inactive within 40 microseconds, Mode 2 operation is established. If the 'data' line has become inactive within this time, Mode 1 is established.

Mode 1 uses scan code set 1 only. Mode 2 uses scan code set 2, but can be switched to scan code set 1 or scan code set 3 using the Select Alternate Scan Codes command.[49]

The March 1986 revised IBM Personal Computer XT & Portable Personal Computer Technical Reference mentions the following and gives a table of scancodes for Enhanced Keyboards that appears to be set 1, confirming the procedure applies to them and that the Enhanced Keyboards of the time indeed support set 1.

The keyboard has 101 keys (102 in countries outside the U. s.). At system power-on, the keyboard monitors the signals on the 'clock' and 'data' lines and establishes its line protocol.[50]

The keyboard sections of earlier technical references like IBM PC 5150 (August 1981) and IBM PC/XT 5160 (April 1983) do not mention of clock becoming high immediately after POR[51][52], which means this mode selection is unlikely to work for earlier systems. If an auto-sensing Enhanced Keyboard is hooked up to an incompliant system, it will either not work or produces random characters at best and proves ultimately unreliable[53]. It's uncertain if BIOS upgrades were available that specifically addressed this - some have reported these keyboards working on 5150 or earlier 5160s, which might confirm the issue can be addressed without a motherboards swap. The following systems have been said to ship with an Enhanced Keyboard or had an Enhanced Keyboard added later as an option:

It should also be noted that some later PC-compatible Enhanced Keyboards have invariably been said to support mode selection and thus scancode set 1 (and 3), including P/N 1391401 and P/N 52G9658[55], when hooked up to a compliant system.


Industrial Keyboard (1985)

eg, P/Ns 1388032, 1390653, 1394946, 1394950

The Industrial Keyboard was technically the first Enhanced Keyboard, launched in May 1985 for the IBM 7531 Industrial Computer and beating out the first terminal Enhanced Keyboard by a month. Thus, these have the honour of introducing the 101-key (ANSI) and 102-key (ISO) Enhanced layout. The original black-square badge Industrial Keyboards had a part number within range of 1388032. Oval-badge Industrial Keyboards soon followed in 1987 and beyond, with P/N 1394946 being the most common part number and largely sporting a black oval badge with raised silver "IBM" text. Industrial Keyboards continued to be produced for IBM by Lexmark and then Unicomp until the end of the 1990s, by which point Unicomp has switched to using a blue text oval badge for them instead. All Enhanced Keyboard-style Industrial Keyboards from IBM were AT compatible. Compared to the later off-white Enhanced Keyboards, Industrial Keyboards are essentially the same underneath but feature a characteristic grey-coloured case designed to hide the dirt and damage expected to be inflicted upon the device within an industrial environment.

DIN terminal Enhanced Keyboard (1985)

eg, P/Ns 1386303, 1386304, 1390123, 1390636, 1390766

The first Enhanced Keyboard available in significant numbers and can be relatively easily sourced today are the terminal Enhanced Keyboards for the IBM 316x family ASCII Display Stations. The IBM 3161 ASCII Display Station Keyboard such as P/N 1386303 came first in June 1985 with the IBM 3164 ASCII Color Display Station Keyboards such as P/N 1386304 dropping in February 1986. Also in 1986, this keyboard design was made available for the IBM 319x family Display Stations as well such as P/Ns 1390123 (IBM 3191), 1390636 (3196) and 1390766 (3192). All such early terminal Enhanced Keyboards were known to sport silver-square IBM badges and featured 240-degree pin arranged DIN plugs much like their 122-key counterparts of the era. In terms of layout, terminal Enhanced Keyboards generally follow the Enhanced layout with only functional differences rather than physical, however, one exception is what would be the upper vertical 2u key on the rightmost column used for numeric keypad "+" is split into two 1u keys on terminal variants. This results in a 102-key ANSI and 103-key ISO layout.

PC/XT Enhanced Keyboard (1986)

eg, P/N 1390120

The PC/XT Enhanced Keyboard was launched at the same time as the PC/AT Enhanced Keyboard in April 1986 and was originally intended to be used with S-model IBM PC/XT systems. Despite this, PC/XT Enhanced Keyboards have the ability to change between IBM scancode set 1 and 2 automatically, however, incompatibilities with the IBM 5150 PC (which uses the same protocol as XT-class computers) have been reported. Later in 1986, the PC/XT Enhanced Keyboard started shipping with the IBM 5162 PC/XT 286. They were also shipped with IBM 5160 PC/XT models 089, 268, and 278[54]. Due to its intended use with XT-class computers, these omit lock-lights much like their terminal counterparts. PC/XT Enhanced Keyboards typically have a part number within range of 1390120. PC/XT Enhanced Keyboards were only made to first or second-generation standards and always retained an IBM silver-square badge. PC/XT Enhanced Keyboards originally came with a black SDL cable with a full-size 5-pin DIN connector, but a more common SDL to PS/2 cable could also be used.

PC/AT Enhanced Keyboard (1986)

eg, P/N 1390131

The PC/AT Enhanced Keyboard was launched at the same time as the PC/XT Enhanced Keyboard and was intended to be used with IBM PC/AT Extended systems. Apart from the inclusion of lock-lights, these are essentially the same as the PC/XT Enhanced Keyboard, including having the ability to automatically change between IBM scancode set 1 and 2. PC/XT Enhanced Keyboards typically have a part number within range of 1390131. PC/AT Enhanced Keyboards were only made to first or second-generation standards and always retained an IBM silver-square badge. PC/AT Enhanced Keyboards originally came with a black SDL cable with a full-size 5-pin DIN connector, but a more common SDL to PS/2 cable could also be used.

ALA DIN terminal Enhanced Keyboard (1986)

P/N 1390680

The ALA Keyboard was a version of the DIN plug based terminal Enhanced Keyboard that has numerous diacritics keys to support American Library Association (ALA) character entry. It was intended for IBM 3163 and 3164 ASCII Display Station models 860 and 861, which were specifically designed for supporting these characters for library environments. It was made available in September 1986 and only for US English users[60].

RT PC & RS/6000 Enhanced Keyboard (1986)

eg, P/Ns 1392366, 1394540, 51G8572

IBM made the Enhanced Keyboard available for its efforts in RISC computing, starting with its first RISC-based commercial computer - the IBM RISC Technology Personal Computer (RT PC). Whilst the RT PC itself wasn't a commercial success[8], its silver-square badge P/N 1392366 keyboard became the first Enhanced Keyboard to have an internal speaker. IBM's February 1990[43] launching IBM RISC System/6000 (RS/6000) family become the largest adopter of Model Ms with speakers. The early and most common RS/6000 Enhanced Keyboards had part numbers in range of 1394540, however, some new Lexmark-era part numbers such as 51G8572 were introduced later on. The early RT PC versions of this keyboard used a unique and bulky 6-pin AMP connector on its cable and weren't completely natively compatible with PS/2 compatible PCs, however, it can be converted with Soarer's Converter. The RS/6000 era versions reverted to PS/2 and can easily be used on PS/2 compatible PCs. On PS/2 plug versions, the speakers are wired to use the typically unused pins on the PS/2 plug, however, no standard PS/2 compatible PC or port is configured to be able to operate these pins and thus the speakers.

PS/2 Enhanced Keyboard (1987)

eg, P/Ns 13914xx, 52G7980, 52G9658, 52G9700, 60G3571, 82G2383, 92G7453

The second and third-generation PS/2 Enhanced Keyboards are the most common and recognisable. The design is an evolution from the PC/AT Enhanced Keyboards, now sporting an oval-shaped badge with grey or blue text depending on generation and a controller PCB that ditched Berg-style connectors for the lock-lights in favour of a membrane flex cable. PS/2 Enhanced Keyboards consistently retained SDL cables until 1993, after which numerous variants produced by Lexmark appeared that invariably dropped SDL for fixed cables whilst not altering the internal design significantly. In 1995, these were succeeded by the fourth-generation design dubbed "Late PS/2 Enhanced Keyboards". Second and third generation PS/2 Enhanced Keyboards commonly have a part number within range of 1391401, although the Lexmark era saw PS/2 Enhanced Keyboards in the 5xxxxxx, 6xxxxxx, 8xxxxxx and 9xxxxxx range introduced.

8P5C terminal Enhanced Keyboard (1987)

eg, P/Ns 1392595, 1393705, 1394204, 1395162

The terminal Enhanced Keyboards with 8-pin modular (ie, same style of jack as RJ-45/ethernet) connectors are the most common terminal Enhanced Keyboards. This style was introduced with the IBM 3151 ASCII Display Station with its P/Ns 1392595 (ASCII-style) and later 1395162 (PC-style) keyboards. The keyboard design was also widely adopted by the IBM InfoWindow family of Display Stations first seen in 1989 with the IBM 3471 and its P/N 1394204 keyboard. One interesting thing to note with 1394204 and some other InfoWindow Enhanced Keyboards is that they could sometimes be seen with longer than usual flip-out feet and even a cable router, something reminiscent of the Type III 122-key Model M also introduced in 1989. These inherit the 102/103 key physical layout of its DIN plug predecessors.

Airline reservation system PS/2 Enhanced Keyboard (1987)

P/N 1393464

P/N 1393464 is a version of the standard PS/2 Enhanced Keyboard with unique keyboards for airline reservation software. This keyboard today is widely associated with Sabre Corporation with various examples of P/N 1393464 with Sabre branding stickers observed. However, they were not the only company to utilise this keyboard. By October 1987, United Airlines (and to a lesser degree Delta Airlines) committed to purchasing a considerable number of IBM PS/2s for their airline reservation systems[63] and it's believed this keyboard served their systems too.

Dell-branded PS/2 Enhanced Keyboard (1991)

P/Ns 1397651, 1369050

Almost immediately after Lexmark's founding, IBM's then-rival Dell started putting their name to Enhanced Keyboards. P/N 1397651 was the first to appear, which was simply a standard PS/2 Enhanced Keyboard with modular SDL to PS/2 cable. This part number originally featured a rectangle old-style minimalistic Dell logo, although by 1993, they started featuring Dell's more familiar and current logo with a rotated "E". It should be noted that P/N 1397651s with Honeywell branding have also been observed, however, it was theorised that Dell sub-contracted for Honeywell due to the fact the observed example of a Honeywell P/N 1397651 also had a "Dell Field Service" sticker on it[65]. P/N 1369050 appeared by 1994 and was a fixed-cabled PS/2 Enhanced Keyboard with the more familiar Dell logo. Other than their branding, these keyboards are unremarkable.

CompuAdd-branded AT Enhanced Keyboard (1993)

P/Ns 1369167, 1397771

CompuAdd, a PC manufacturer from Texas, also put their name to Enhanced Keyboards produced by Lexmark and they appeared not long before their 1994 bankruptcy. CompuAdd Enhanced Keyboards used fixed grey cables with an AT-style full-size 5-pin DIN plug and could come with a standard ISO layout (for example, the Arabic P/N 1369167) or a so-called "Asian 101" layout (the more well-known US English P/N 1397771). The latter layout has a 1-unit backspace and a backwards-L shaped (aka, "big-ass") enter key just like IBM's PC/AT Keyboard layout.

ICPI Ambra PS/2 Enhanced Keyboard (1993)

P/Ns AMB100x, AMB200x, 1378160

Ambra Computer Corporation and its parent ICPI were IBM subsidiaries that marketed low-cost PCs, some of which included third-generation Enhanced Keyboard derivatives. AMB100x and AMB200x are the European versions made by IBM UK, and P/N 1378160 is a singular US ANSI version based by Lexmark. AMB100x featured an all grey-blue case, an oval badge with the Ambra logo inside, bottom-left aligned italicised legends on its keycaps and circular view-holes for the lock-lights. AMB200x by contrast is a more conservative take featuring only an oval badge with the Ambra logo inside and circular view-holes for the lock-lights, otherwise, the case and keycap style is the same as a standard Enhanced Keyboard. P/N 1378160 is simply a standard Enhanced Keyboard with an isosceles trapezium-shaped Ambra logo. All Ambra-branded Enhanced Keyboards have fixed cables.

Quiet Touch PS/2 Enhanced Keyboard (1993)

eg, P/N 71G46xx

The IBM Basic Keyboard was a version of the third-generation PS/2 Enhanced Keyboard with Quiet Touch key-switches. It was likely named "Basic Keyboard" to avoid any potential confusion as the keyboard design from the outside is identical to other fixed-cable third-generation Enhanced Keyboards. The Basic Keyboard retains high component commonality with the Enhanced Keyboard with the only differences being the membrane's actuator (rubber dome instead buckling spring) and the keycap mount. As such, it's technically possible to convert a Basic Keyboard to use buckling springs. Most IBM-branded Basic Keyboards have a part number like 71Gxxxx, which is the easiest way to identify one of these from photos. To this day, Unicomp continues to sell and refer to rubber dome versions of its Model Ms as Quiet Touch or Q/T for short.

Soft Touch PS/2 Enhanced Keyboard (1994)

P/N 8184692

The IBM Soft Touch Keyboard was a version of the PS/2 compatible RS/6000 Enhanced Keyboard that featured greased buckling springs. The grease is used to make the keyboard queiter by muting the spring's ping (high-pitched resonation after clicking) but doesn't completely mute the keyboard.

Late PS/2 Enhanced Keyboard (1995)

eg, P/N 42H1292, late 13914xx

This represents the final iteration of the Enhanced Keyboard design that involved Lexmark or IBM - the fourth generation. Indeed, these introduced the biggest change to the Model M design by changing the cable position and introducing a new type of pressure-fit controller around the lock-light area. Now, the (always non-coiled) cable sprouts from the right-hand side of the keyboard. The new controller importantly reduces the part count and power consumption over the previous Triomate-style controllers, however, the position of the lock-lights was changed from bottom-left to roughly middle. This means fourth-generation Enhanced Keyboards can be easily identified by the new lock-light overlay. P/N 42H1292 (the US ANSI model) is most commonly associated with this keyboard; whilst P/N 1391401 (former US ANSI part number) was not carried forward, EMEA/SA models still retained the old 13914xx style part numbers.

Unicomp Customizer/Classic (1998)

eg, P/N 42H1292U, UNI041A

The Unicomp Classic (originally called Customizer) is the continuation of fourth-generation Enhanced Keyboard production under Unicomp. At first, the keyboard design and quality remained about the same as its immediate Lexmark and IBM UK predecessors. However, Unicomp soon started altering the design. In October 2000, Unicomp first made Windows keys available as an option[26]. In 2006, Unicomp made a USB version of the keyboard available. In July 2013, Unicomp revised its Windows keys layout designs to more closely follow the traditional Enhanced layout, whilst also releasing an entirely new 103 (ANSI) and 104 (ISO) key design that retained the 7u spacebar and filled in the blanks between the Ctrl and Alt keys[68] (aka, a Tsangan bottom row design). Quality remained consistent until the late 2000s when time began to take its toll on the keyboard's moulds. Today, Unicomp Classics may have noticeable blemishes and dimples on the exterior, although they are still mechanically sound. In 2020, Unicomp released the New Model M to succeed most of its old product lines and now recommends its customers purchase that keyboard instead. Unicomp has also discontinued many of its Classic SKUs, however, a small selection remains on sale (including the classic off-white and 101/102 version) for those who still want one.

Galactech SmarTrex POTS Keyboard (1999)


Galatech Corporation's SmarTrex Keyboard is a heavily modified Unicomp Customizer with Quiet Touch rubber dome key-switches. The most obvious modification is the telephone cradle and phone jack, which supports a plain old telephone system (POTS) analogue line. It's also unique for being built to almost third-generation Model M standards, featuring a cable spouting from the classic (left-side) position and a Triomate-based controller. They're very uncommon and only examples produced in 1999 have been observed thus far.

Florida Gators & Kentucky Wildcats Keyboards (1999)


From 1999 until the end of the 2000s, Unicomp produced themed versions of the Customizer with Florida Gators and Kentucky Wildcats American football team styling. They had various orange and white or blue and white keycaps respectively as well as a unique squircle-shaped badge in the place where most Enhanced Keyboards have an oval or rectangle shaped badge. Otherwise, these are fairly standard PS/2 Model Ms of their period.

General Electric Healthcare & Marquette Keyboard (2000)

eg, P/Ns UNZ4416, UNZ3416, 2045802-001, 2045802-002, 2054858-001, 2054858-002, 2054858-003

Unicomp Customizers and Classics produced for General Electric Marquette and General Electric Healthcare are the most numerous non-IBM, non-Lemark or non-Unicomp branded Model M known. Unicomp and General Electric have been partners since at least 2000, and the relationship endures as of 2020[29]. The typical (but not always universal) customisations are extensive but purely cosmetic; the F-keys and numeric keypad legends are entirely replaced with specialised legends, most of the alphanumeric keys have secondary and tertiary legends, the Alt keys replaced with "Action" keys, the Tab and Enter keys lack their usual symbols, the Tab key is also capitalised, and the numeric keypad largely has bright yellow keycaps. Early GE Model Ms were typically branded GE Marquette, but most latest ones are branded GE Healthcare. GE Healthcare ones could be intended for GE Healthcare's CardioLab Electrophysiology Recording System, Mac-Lab Hemodynamic Recording System, or both. All known GE Model Ms can be used on normal PCs without modification.

Further reading & resources


ASK. Admiral Shark's Keyboards original content. License/note: CC BY-NC-SA 4.0.

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