Model M keyboard

Keycap in a French Model M

The Model M keyboard was designed to be more cost effective than the Model F keyboard it replaced. Production of model M keyboards began in 1985, and they were often bundled with new IBM computers. 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.[8][9][10][11] Lexmark continued manufacturing model M keyboards in the United States, United Kingdom, and Mexico, with IBM being Lexmark's major customer.[12] 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,[13] including lighter plastic, an integrated cable, and a single color for key legends.

A five-year agreement obligating IBM to purchase nearly all of its keyboards from Lexmark expired on March 27, 1996.[14] Lexmark exited the keyboard business, selling related assets to IBM and Maxi Switch.[15] 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.[16] 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.[1] Unicomp still manufactures its version of the Model M (similar to part number 42H1292 but first renamed 42H1292U and subsequently the "Customizer"), as well as 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. Although current Unicomp models may still be purchased, original production models retain their value well among collectors and computer enthusiasts.

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.

• 
  IBM model M Space Saving keyboard Industrial variant
• 
  IBM Model M13 'black' variant featuring the optional black trackpoint cap

• 
  IBM Model M5-2, featuring a purple trackball and alternative 'RGB' keys from Unicomp
• 
  IBM Model M2, Part no. 1395713, manufactured 1992

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[17] 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.[18][19]

The Model M is also less susceptible to dirt and wear. Dirt can interfere with the proper operation of most other keyboard technologies; however, the buckling spring switch's design is such that any dirt that falls between the keys is unlikely to enter the switch mechanism, which is covered by the key. Even if dirt were to enter a mechanism, a large amount would be necessary to prevent it from operating.

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.

There are some drawbacks to the Model M design:

• Because they are so large and heavy (over 1.5 kg, heavier than some modern laptops) they are not as portable as more modern keyboards.
• Their buckling-spring keys are noisy enough to be inappropriate in quiet locations such as libraries and medical facilities.
• Liquids spilled on most Model Ms do not drain out, and remain in the keyboard with potential to cause a short circuit. The later 42H1292 and 59G780 designs, as well as post-1993 1370477s and 1391401s made mostly by Lexmark and Unicomp, have drainage channels designed to avoid this problem.
• Unicomp currently produces Model Ms with USB connectors that can be connected directly to modern PCs. However, earlier Model Ms have either AT or PS/2 connectors, requiring adapters for PCs lacking those capabilities. (PS/2-to-USB adapters exist for computers without PS/2 ports and additional AT-to-PS/2 adapters, or SDL-to-PS/2 cables, for computers without AT ports.) Some PS/2-to-USB adapters are unreliable with early Model Ms that require more power than the adapters can provide.[20]

The square aluminium badge on a 1390131 series keyboard compared to other variants.

All Model M keyboards made by IBM and Lexmark 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.

Logo position legend:

Click [show] to display the table's contents.

Note: Manufacture dates are approximate.

• 
  1995 IBM Model M (P/N 1391403) (German)
• 
  1994 IBM Model M (P/N 1394542) label
• 
  1987 IBM Model M Space Saving Keyboard P/N 1391472 (no numpad) with SDL to USB adapter
• 
  Steel backplate of the 1987 Space Saving Keyboard
• 
  IBM & Unicomp Model M variants
  Top to bottom:
  • 1986 IBM (P/N 1390131, square logo)
  • 2011 Unicomp Spacesaver M (P/N UNIZPHA)
  • 1991 IBM Space Saving Keyboard (P/N 1392934)
• 
  2012 Unicomp Classic 104 (P/N UNI044A)

• IBM PC keyboard
• Model F keyboard
• List of mechanical keyboards

Wikimedia Commons has media related to IBM Model M Keyboard.

• IBM Enhanced Keyboard (Deskthority.net Keyboard Wiki) – another guide to Model M history and variants
• Model M Troubleshooting FAQ ( Eric S. Raymond) – Interoperability problems with modern hardware.
• PC keyboard, Unicomp – current Model M manufacturer
• Clicky keyboards – distributor of refurbished and never used Model M keyboards
• "Inside the World's Greatest Keyboard", PC World
• Care and feeding of an IBM Model M keyboard, Preater – a Model M restoration guide
• Product Review: Unicomp Ultra Classic keyboard, or, the IBM Model M reborn, The Story's Story – Comprehensive review