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RAFI RC 72

Contents

Overview

RAFI RC 72 comprises the high-profile Hall effect and mechanical switches. RAFI report that the series is “obsolate since 1993” and “obsalate since 1973, September” [sic]. It appears that RC 72 was introduced in September 1973, and discontinued in 1993. This would seem reasonable: each part number range is one position below those of RS 74. RAFI themselves state on their website that they produced the first RAFI keyboard in 1970. If the numbers in RAFI switch series indicate the year of announcement, this would also put RC 72 before RS 74.

Design

RC 72 switches fit into crenellated rails with the same rail spacing as used with East German RFT switches; from the apparent age of the RAFI switches, it looks like the crenellated rail design may have originated with RAFI, as an alignment aid derived from the mounting bars of Micro Switch KB and related series. German patent 6805719U from 1968 depicts a taller version of the mounting bar system, but not clearly what switch type was used.

A front metal bar on the illuminated switches appears to be heat staked via protruding plastic tubes.This suggests how Omron B2R might work, as those do have unexplained short protruding tubes.

Each switch has a pair of slots at the front, to hold a stabiliser wire. As with the later series, some types bear a 1 cm ruler in 2 mm increments.

Product range

RC 72 comprises three series, detailed below. The series are:

The part number prefixes are as follows:

Prefix Range
3.119… RC 72 non-illuminated
3.11900 RC 72 L
3.11906 RC 72 C non-illuminated Dynamic, 4.75–5.5 V
3.11918 Static, 4.75–18 V
3.11919 Dynamic, 4.75–18 V
3.11930 Static, 4.75–27 V
3.11960 RC 72 M non-illuminated
3.169… RC 72 illuminated¹
3.16900 RC 72 L illuminated¹
3.16906 RC 72 C illuminated¹ Dynamic, 4.75–5.5 V
3.16918 Static, 4.75–18 V
3.16919 Dynamic, 4.75–18 V
3.16930 Static, 4.75–27 V
3.16960 RC 72 M illuminated¹
  1. These ranges include non-illuminated types that accept illuminated keycaps

RC 72 C

RC 72 C switches are contactless, using a Hall sensor inside each switch. The specifications provided by RAFI imply that they were using Siemens Hall sensors. In the only known instance of RC 72 C being discovered, it was found with HFO B 461 G Hall ICs, as seen in other RAFI Hall effect switches. HFO Hall ICs are derived from those of Siemens, but only offer a subset of Siemens’s total product range.

In the catalogue excerpt provided by RAFI, four output types were offered. Three of these correspond to later Siemens Hall ICs. There are no details obtained yet of a Siemens Hall IC with 4.75–5.5 V output; SAS 241 S4 is close at 4.75–5.25 V.

Output Supply voltage Possible Hall IC
Static 4.75–18 V Siemens SAS 251 S5, possibly also a SAS 211 variant
4.75–27 V Siemens SAS 251, possibly also a SAS 211 variant
Dynamic 4.75–5.5 V SAS 201 variant?
4.75–18 V Siemens SAS 241, possibly also a SAS 201 variant

Static and dynamic correspond to Micro Switch’s level and pulse switches respectively: static switches continue to draw current so long as the key is held, while dynamic switches draw only a brief pulse of current. The pulse length of the Siemens dynamic switches ranges from 15–40 µs, with a 20 µs being typical. Micro Switch contactless and mechanical pulse switches achieve a form of N-key rollover by preventing two or more keys from conducting simultaneously (ensuring that no ghosting can occur), and the same is likely to be true in the case of RC 72.

The Hall IC for static output across 4.75–18 V could be either SAS 251 S5 (dual open collector outputs) or SAS 261 (single open collector output with enable input), but in the absence of any indication that the outputs differ by model, SAS 251 S5 seems more likely, although RAFI did switch to the latter arrangement with the HFO ICs.

It should be noted that Siemens SAS 251 was introduced in 1977, and SAS 241 seemingly in 1979 or 1980, so the output options may have changed along with the Hall IC product range. By 1985, it seems that only SAS 231 and SAS 251 were still current products, which may be why RAFI switched to HFO Hall ICs (the RC 72 C example with HFO ICs is from 1986).

Two force curves have been measured:

Models

Part number Type Plunger type Output type Max. voltage
3.11918.001 Momentary Static 18 V
3.11930.001 27 V
3.11906.001 Dynamic 5.5 V
3.11919.001 18 V
3.11918.101 Alternate action Static 18 V
3.11930.101 27 V
3.11918.002 Momentary 13° Static 18 V
3.11930.002 27 V
3.11906.002 Dynamic 5.5 V
3.11919.002 18 V
3.11918.102 Alternate action Static 18 V
3.11930.102 27 V
3.16918.901 Momentary 0° illuminated (no lamp) Static 18 V
3.16930.901 27 V
3.16906.901 Dynamic 5.5 V
3.16919.901 18 V
3.16918.905 Alternate action Static 18 V
3.16930.905 18 V
3.16918.001 Momentary illuminated 0° illuminated Static 18 V
3.16930.001 27 V
3.16906.001 Dynamic 5.5 V
3.16916.001 18 V
3.16918.101 Alternate action illuminated Static 18 V
3.16930.101 27 V

RC 72 M

RC 72 C switches are mechanical. No further details are presently available, and these have yet to be observed.

Models

Part number Type Plunger type
3.11960.001 Momentary
3.11960.101 Alternate action
3.16960.901 Momentary 0° illuminated
3.16960.904 Alternate action
3.16960.001 Momentary illuminated
3.16960.101 Alternate action illuminated

RC 72 L

Models

RC 72 L contains dummy switches. “L” denotes “Leertaster”, meaning “empty button” (plural “Leertaster”), as distinct from “Leertaste” (“space bar”, plural “Leertasten”). These switches can be illuminated.

Part number Plunger type Type
3.11900.001
3.11900.002 13°
3.11900.003 For two-part keycaps
3.16900.001 0° illuminated
3.16900.003 For two-part keycaps

Keyboards

There is only one known keyboard with these switches, fitted with RC 72 C contactless switches and RC 72 L dummy switches for space bar. This is the Express-2 railway station terminal keyboard. This keyboard has German inscriptions internally and Russian text externally, and was made in 1986. The keyboard assembly is branded “TKI” and has a type number of “K2677ol”, which depending on the equipment used, could instead be “K2677o1” or “K267701”. The PCB itself is marked “Vilati K2677”, where Vilati is the Hungarian manufacturer.

The switch bases read “+QQ−” (VCC, output, output, GND), but the Hall sensor found inside one of them is a B 461 G, which is single output with enable input (Freigabeeingang), which would be represented as “+FQ−”. It looks like the switch shell moulds were not retooled to accommodate the change of Hall sensor.

Documentation