RAFI RS 74 and RS 76
RAFI RS 74 and 76 C may be the oldest production keyboard switch type in the world. RS 74 switches were introduced around 1975, offering 2.5 mm travel in an ultra low profile, with both Hall effect and mechanical sensing offered. The series included illuminated switches as well as single and double-throw alternate action.
Shortly afterwards, RS 76 was introduced in 1976: the general design was the same, but travel was extended to the full 4 mm. RS 76 mechanical switches included the option of an integrated diode. The general product range was as follows:
|RS 74 C||Hall effect||2.5 mm|
|RS 74 M||Mechanical|
|RS 76 C||Hall effect||4 mm|
|RS 76 M||Mechanical|
See the Full-Travel Key Switches article for a broad overview of these switches.
Normally the mechanical types and Hall types are used exclusively, but an Italdata keyboard exists with both types together (possibly from 1984). Most keys are mechanical, but a small number of keys are Hall effect. This keyboard also features static dummy switches (no movable switch contact and presumably a locked plunger). The mechanical switches have at least three different metals or metal finishes for the movable contact: silver-coloured, copper-coloured and red.
Tentatively, it appears that the series numbers (74 and 76) may correspond with the years in which the switches were conceived.
German patent 2504091A1 filed in January 1975 covers RS 74 Hall effect and mechanical switches. The mechanical type is loosely depicted as the double-throw type: the top view shows one normally-open contact and one normally-closed contact, but the side view shows the normally-closed stationary contact only (rather than the normally-open contact). “The new Rafi RS74 keyboard system” was mentioned in Engineering Materials and Design 19 in 1975. RS 74 C was announced in Elteknik med aktuell elektronik 18 from 1975.
RS 76 followed in 1976. The following mention was made in Electronic Design magazine in October of that year:
Hall-effect keyswitches give bounceless output
RAFI-Raimund Finsterholzl, Elektrotechnische Spezialfabrik, D-798 Ravensburg-Berg, Ravensburger Strasse 128-134, Postfach 2050, West Germany
A new switching technique for keyboard system RS 76 uses a Hall-effect IC to ensure that switching is bouncefree and reliable. Two-shot molded keys guarantee wear-resistant legends. Complete keyboards or single keyswitches allow for the manufacture of standard or special configurations. Keyboard height is 15 mm. Operating strokes of 4 mm allow control of force-displacement characteristics of the keys to provide a particularly comfortable typewriting feel. The keyswitches are also available in a mechanical version, and both types can be illuminated or non-illuminated.
The magazine entry is curiously worded, as though Hall effect is new to RAFI, despite RS 74 C having been introduced the previous year.
It seems that the basic RS 76 switches are covered by the same patent as RS 74, as there are no significant differences between the basic switches of each family. RS 74 C has the Hall IC placed horizontally to save space, and detects the proximity of the magnet, while RS 76 C detects the magnet’s lateral motion. There is however a separate patent for the subfamily of RS 76 M with diodes, specifically German patent 3037448A1 filed in October 1980.
In April 1979, RAFI also filed German patent 2914954A1 for sealed keyboards. Both RS 74 and RS 76 support sealed keyboard assemblies.
RS 76 is still in production. RS 74 C has never knowingly been observed, and no specifications or part numbers are known. Information from RAFI regarding RS 74 M is unclear. It seems that most of the series was terminated in 2005, and it is no longer advertised, but as of May 2019, two RS 74 M models remain in production:
- 3.12.961.001/0000: momentary, with increased ESD strength
- 3.12.960.101/0000: alternate action
There are many RS 74 and RS 76 variants; see the RS 74 and RS 76 types page for a table of known part numbers. The general characteristics and feature list is as follows:
- Mechanical and Hall effect contactless sensing
- 2.5 mm and 4 mm key stroke
- SPST-NO and SPDT forms (mechanical only)
- Downstroke damping (RS 76 C non-illuminated only)
- IP 54 (RS 74) and IP 65 (RS 76) protection (using a special keyboard assembly)
- Momentary and alternate action
- Single and dual LED illumination
- Internal diode (only known from RS 76 M)
The IP 65 seal system can be observed in a Deutsche Aerospace Bedienfeld BF 1246 keyboard from circa 1994.
The full extent of the product range is not known, as various models including static dummy and SPDT were either never listed in catalogues, or went end-of-life before the publication of the known catalogues, which are all much more recent. There are various anomalies in the product range that are difficult to explain from the limited number of examples. In this Rohde & Schwarz keyboard, the alternate action switch is mint green, which is unusual. The Dr. Böhm Digital Drums II machine uses RS 74 M, in both covered and uncovered forms, with the covered type being the less common type with red covers. The reason for this is not known, but the different switch types also use different-coloured keycaps. (It is not even known whether red and clear covers are interchangeable, or whether the cover colour was significant, just as there is no confirmed reason for orange versus pink plungers for the RS 76 M switches with integrated diodes.)
By 2001, all Hall effect switches were single output with enable input, with a narrow 4.75–5.25 V supply voltage range. These are found with East German HFO B 461 G Hall sensors. Comparing the markings on the ICs to those on the switches, and working on the basis that production had ended by the 2000s, it would appear that RAFI bought up and stockpiled a large number of ex-GDR Hall ICs. Previously, it appears that RAFI offered additional output options, as they did with RC 72. A photograph of an unidentified RS 76 C switch shows what appears to be a Siemens SAS 251 S5 Hall IC, which is dual output with a supply voltage range of 4.75–18 V. As RS 76 C was introduced around 1976 and HFO did not introduce their B 4xx Hall ICs until 1981, older RS 76 C batches would have had to use Siemens chips. The list of known RS 76 C part numbers leaves many of them unidentified, and these likely represented the other output types.
- RS 74 M SPDT latching (3.12.960.905)
Documentation and literature
- RAFI RS 76 announcement in Electronic Design Vol. 24, No. 22, October 25, 1976 (transcribed above)
- RAFI full-travel key switch section of the Deskthority wiki for further details including illustrated lists of common types