Model M keyboard

Model M designates a group of computer keyboards designed and manufactured by IBM starting in 1985, and later by Lexmark International, Maxi Switch, and Unicomp. The keyboard's many variations have their own distinct characteristics, with the vast majority having a buckling-spring key design and swappable keycaps. Model M keyboards have been praised by computer enthusiasts and frequent typists due to their durability and consistency, and the tactile and auditory feedback they provide.
The popularity of the IBM PC and its successors made the Model M's design tremendously influential, to the point where almost all later general-purpose computer keyboards would mimic not only its key layout but other aspects of its ergonomics as well. That layout would become standardized by ISO in 1994 and ANSI in 1998; with minor additions, notably of the Windows key and Menu key, it continues to dominate today.
The Model M is also regarded as a timeless and durable piece of hardware. Although the computers and computer peripherals produced concurrently with the Model M are considered obsolete, many Model M keyboards are still in use due to their physical durability and the continued validity of their ANSI 101-key and ISO 102-key layouts, through the use of a PS/2 female to USB male adapter with a built-in level converter. Since their original popularity, new generations have rediscovered their unique functionality and aesthetics. 
It is estimated that during the IBM/Lexmark years over 10 million Model Ms were shipped, but the keyboard's mass-market success ended in the 1990s amidst an industry-wide switchover to lower-cost dome-switch devices. After the end of its product lifetime it became the focus of an enthusiast community that puts a remarkable amount of effort into salvaging vintage model Ms, restoring them, and adapting them for use with modern computers.
The Kentucky-based company Unicomp continues to manufacture and sell new Model M keyboards as an upgrade to customers who require ruggedness, long-term reliability, and the ergonomic benefits of tactile feedback.
History.
The Model M keyboard was designed to be more cost effective than the Model F keyboard it replaced. Principal design work was done at IBM in 1983-1984 and drew upon a wide range of user feedback, ergonomic studies, and examination of competing products. The keyboard layout, significantly different from that of the Model F, owed much (including notably the inverted-T arrangement of its arrow keys) to the LK-201 keyboard shipped with the VT220 serial terminal.
Production of model M keyboards began in 1985, and they were often bundled with new IBM computers. While today primarily associated with the IBM PC and its successors, it actually first shipped with the 3161 terminal and was deployed across several other IBM product lines as well, notably including the 5250 terminal and the RS/6000.
They were produced at IBM plants in Lexington, Kentucky, Greenock, Scotland, and Guadalajara, Mexico. The most common variant is the IBM Enhanced Keyboard identified by IBM assembly part number 1391401, the U.S. English layout keyboard bundled with the IBM Personal System/2. Until around 1993, most Model Ms included a sturdy, coiled, detachable cable, with either an AT (pre-1987) or PS/2 connector, in 5- and 10-foot lengths (1.5 and 3 metres). From about 1994 onwards, flat non-detachable cables were used to reduce manufacturing costs; however, IBM retained its 101-key layout, never implementing the Microsoft Windows keys common on other keyboards from that time. Unicomp later designed a 104-key Model M with Windows keys.
On March 27, 1991, IBM divested a number of its hardware manufacturing operations, including keyboard production, forming Lexmark International. Lexmark continued manufacturing model M keyboards in the United States, United Kingdom, and Mexico, with IBM being Lexmark's major customer. Many of these keyboards are identified by IBM assembly part numbers 52G9658, 52G9700, 71G4644, 82G2383, and 42H1292, which were bundled with IBM PS/ValuePoint and IBM PC Series. Because of pricing pressures, these keyboards were manufactured with a new, lower-cost design, including lighter plastic, an integrated cable, and a single color for key legends.
During the Lexmark years a few Model M variants were manufactured with rubber-dome keyswitches rather than buckling springs. Due to the comparatively short lifetime of these switches not many have survived. Despite their rarity, today's enthusiasts and collectors do not value them anywhere near as highly as the more common buckling-spring variants.
A five-year agreement obligating IBM to purchase nearly all of its keyboards from Lexmark expired on March 27, 1996. Lexmark exited the keyboard business, selling related assets to IBM and Maxi Switch. When Lexmark discontinued keyboard production in April 1996, IBM continued producing buckling-spring model M keyboards in Scotland until 1999. Maxi Switch purchased assets for rubber-dome keyboards and the Lexmark Select-Ease Keyboard (model M15), including a buckling-spring switch patent. Maxi Switch continued to manufacture the IBM Enhanced Keyboard with TrackPoint II (model M13) in Mexico until 1998.
Some of Lexmark's keyboard manufacturing assets were also sold to a group of Lexmark employees, who formed Unicomp. Unicomp's basic version of the Model M was similar to part number 42H1292 but first renamed 42H1292U and subsequently the "Customizer". There have been other configurations including updated 104- and 105-key layouts; a Unix layout (where the Ctrl and Caps Lock keys and Esc and tilde keys are transposed); models with integrated pointing sticks or trackballs; and POS-specific models such as those with built-in magstripe readers.
Unicomp continued to use the original IBM machinery to produce Model Ms, leading to a gradual decline in build quality as the tooling became increasingly worn. This, and various problems with their USB controllers helped keep a market for vintage Model Ms thriving. In 2020 Unicomp replaced its tooling and shipped a "New Model M" with noticeably improved build quality that more resembles the classic 1391401 (though with a 104- or 103-key layout and USB); many older variants are no longer sold on Unicomp's website and some still on sale have been deprecated.
Due to their extreme durability and now-classic styling, IBM and Lexmark original production models retain their value well among collectors and computer enthusiasts, often selling for as much as their original sticker price or (for a few rare variants) even more.
Variations.
The Model M's numerous variations (referred to as "part numbers") incorporated alternative features and/or colors. One of the most popular variants is the Space Saving Keyboard, which integrates the number pad into the keyboard's main section, substantially reducing its width.
IBM released the standard and Space Saving Model M's in an alternative 'gray/pebble' colour for use with their Industrial computers; this darker color was designed to conceal discoloration from handling in production environments. Other variable features include a grounded spacebar and, on some later models, drainage holes to deter damage from spilled drinks.
The M2 was a late Lexmark variation issued under cost pressure from competing rubber-dome keyboards. It retained buckling springs but had a much thinner, lighter case and discarded the metal backplate. It can easily be distinguished from the Model M proper by the flat, unsculpted front case section; also the manufacturing label, if present, will say "M2" rather than "M". M2s were poorly fabricated and notoriously unreliable; comparatively few survived into the 21st century, and Model M enthusiasts do not value them.
The M2 should in turn be distinguished from the M5, another Lexmark variant which returned to the rugged Model M case/backplate construction but added a built-in trackball; and those two from the M13, which was also built like rugged Model Ms but featured a pointing stick. The M5 and M13 designs are still carried by Unicomp in 2021 under the names "Trackball Classic" and "Endura Pro".
Design.
The variant most commonly referred to as "Model M" is Part No. 1391401, on which many other variants were based. This model, known as the Enhanced Keyboard, included IBM's patented buckling spring design and swappable keycaps.
The Model M's design has been widely praised as durable and reliable, and has remained basically the same since the 1980s, while virtually all other computing hardware, from PCs to monitors to mice, has changed dramatically. The M's sturdy design, including its heavy steel backplate and strong plastic frame, has allowed even the most abused examples to survive for years.
The Model M's buckling spring key design gives it a unique feel and sound. Unlike more common and cheaper dome switch designs, buckling springs give users unmistakable tactile and auditory feedback. Because of its more defined touch, some users report they can type faster and more accurately on the Model M than on other keyboards. Additionally, many model M enthusiasts believe that tactile-feedback keyboards like the model M reduce stress on the hands, preventing or even reversing Repetitive Strain Injury.
Unlike competitors such as Cherry and Alps-style keyswitches, buckling-spring keyswitches don't have a plunger part with sliding surfaces that can be fouled by contaminants. This makes model Ms much more resistant to dirt, dust, and grit, and is a significant factor in their long service lifetimes and appeal in challenging industrial environments.
Until the late 4th-generation variants, most Model Ms have a 1.25" slotted, circular speaker grille in their bottom surfaces. Relatively few contain an actual speaker, however, which was useful only for sounding beep codes on older terminal systems. The most common P/Ns with speakers are 1394540 and 51G872, made for RS/6000 UNIX workstations.
Model Ms have been manufactured to quite a number of different interface and connector standards, some of which (such as the 5-pin DIN used on 5250 terminals) are poorly documented and have had to be reverse-engineered by enthusiasts. Early variants shipped with the PC XT and AT used connectors specific to those systems. After the introduction of the PS/2 most shipped with a connector for a PS/2 port; these included the 1391401. Unicomp introduced support for USB.
Older model Ms used a detachable cable with an SDL connector on the keyboard, later Lexmark and Unicomp variants used a fixed cable, and one very recent variant from Unicomp has a detachable cable with a rare locking variant of the USB Type A jack on the keyboard.
There are some drawbacks to the Model M design:
Identification.
All Model M keyboards made by IBM, Lexmark and Unicomp have an ID label on the underside indicating the assembly part number, individual serial number, and manufacturing date. The general period in which a Model M was made can also often be distinguished by the type of logo "badge" above its keys. The first model Ms (part numbers 1390120 or 1390131) have a square aluminum logo badge in the top right corner. Part number 1391401, and most variants based on it, has a gray IBM logo in a recessed oval at the board's upper left. Later IBM-made Model Ms, and variants subsequently made by Lexmark in the early 1990s (part numbers 1370477, 52G9658, 52G9700, 59G7980, 92G7453, 82G2383, 42H1292, etc.), have a similar oval badge area with a blue logo. Unicomp Model Ms have a blue and red Unicomp logo on the lock-light panel.
Features by part number.
Layout types:
Logo position legend:
Click [show] to display the table's contents.
Note: Manufacture dates are approximate.