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NMB desktop membrane keyboards

NMB are notable for a widely-used style of membrane switch using a proprietary-mount plunger over rubber dome. In time, the use of a separate plunger was phased out. Production of desktop membrane keyboards ended in 2015 when NMB left the keyboard market.



Travel for the membrane types is given as 0.140±0.020″ (3.56±0.5 mm), which is exactly the same as the figure given for the Series 725 mechanical series RT-8200W.

Tentatively it appears that keyboard lifetime was dropped from 20 million to 10 million cycles between 1997 and 1999 for lower-end models:

Model Lifetime (Feb. 1997) Lifetime (Feb. 1999)
RT-4900W 20 million
RT-6600W 20 million 20 million
RT-6800W 20 million 20 million
RT-9100W 20 million 20 million
RT-9500W 20 million
RT-2200W 10 million
RT-2700W 10 million
RT-2900W 10 million
RT-5100W 20 million
RT-9200W 10 million
RT-9600W 20 million

The general specifications are the same across all models, although rated lifetime varies by model:

Model RT-9100W RT-2200W RT-2700W RT-2900W
Full travel 0.140±0.020″ (3.56±0.5 mm)
Pretravel 0.090±0.020″ (2.29±0.5 mm)
Operating force 2.0 oz nominal (55 gf)
Rated lifetime 20 million cycles 10 million cycles
Legend printing Unspecified Laser etched Laser marked
Switch type With discrete plunger Not depicted
Contact material Unspecified Mylar with silver–carbon overlay


The same model numbers—RT-101, RT-102 etc—were used by both the Series 725 and rubber dome over membrane models. Later, to avoid confusion, separate model numbers were created for the mechanical and membrane types.

arapineau on Geekhack provided detailed disassembly photos of an RT-102 membrane keyboard. This is a standard three-layer membrane type. The membranes are marked with the code 8770-0025 REV.B, and appear to be dated July 1991; the controller appears to be dated week 17 1991.

NMB also made custom keyboards. Model WMF-4E1Q2 is a custom assembly produced for the Commodore Amiga 500. Here, conductive domes are used, together with what appears to be a two-layer membrane assembly. The exact formation of the membrane assembly is not clear. The membrane code here is 8770-0056 REV.E, and it appears to be dated November 1992. The upper moulding is marked October 1992.

Date markings

The date markings on the membranes appear to be subtractive: they start out with every year and month visible, and each month is deleted as it passes, and likewise each year is deleted as it passes. Presumably when a new year commences, a fresh set of month numbers 1–12 is restored to the printing screen. Another example is the AppleDesign Keyboard, which can be seen with membrane markings suggesting May 1998. This matches the dates on the case and controller. The membrane code here is 8770-5021 REV.B.


The documents are mostly marked with a code such as “KB10/98”, which is taken to be the month and year that it was written. The PDFs themselves were archived by the Wayback Machine in later years.

A PDF datasheet for RT-6800W also exists on the Wayback Machine, but it is corrupt and presently nothing seems able to open it.