RAFI RC 72

Contents

• Overview
• Design
• Specifications
• Product range
  • RC 72 C
    • Models
  • RC 72 M
    • Models
  • RC 72 L
    • Models
  • RC 72 N
    • Models
  • Keylock switches
• Keyboards
  • Express-2 keyboard
• Documentation

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.

The Hall effect switches have a high longevity, being rated for more than 100 million cycles; this is the same rating as RS 76 C. The mechanical switches however are rated for only one million cycles.

The Hall type is covered in German patent 2256360A1 filed in November 1972. This is also available in English as US patent 3882337 from November 1973.

RC 72 appears to have been found favourable by TKI in Hungary, who manufactured keyboards based around RAFI parts.

Design

RC 72 switches fit into crenellated rails with the same rail spacing as used with East German RFT switches. The RFT design appears to be a copy of RAFI’s. RAFI’s design appears to be a copy of the mounting bars from 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.

Specifications

Force and travel are given as “Operating stroke” and “Operating force”, so the forces given appear to be the total travel force. Jacob Alexander’s force graph correlates with this.

Product range

All official information is based on the 1983 catalogue. At that point, RC 72 comprised four series, detailed below. The series are:

• RC 72 C: “contactlos” (with a “c”) Hall effect switches, in momentary and alternate action form, with optional illumination
• RC 72 M: “mechanische Kontakte” (with a “k”) mechanical switches, also in momentary and alternate action form with optional illumination
• RC 72 L: dummy switches, with optional illumination
• RC 72 N: mains power switches (220 V, 2 A)

The part number prefixes in 1983 were as follows:

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 original design uses a vertically-arranged circular magnet, perpendicular to the Hall sensor, with an air gap where it passes over the sensor. This seems to have been changed to a pair of very small magnets in the 1980s.

The 1983 specifications 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.

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 for the dynamic switches is given as approximately 20 µs, which corresponds with that of Siemens dynamic Hall sensors, whose pulse duration 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:

• RC 72 C non-illuminated force curve
• RC 72 C illuminated force curve

Models

RC 72 M

RC 72 M switches are mechanical. These are all DPST-NO-NC (form 1A+1B). From the catalogue diagram, the implementation appears to be bridge contact, with a sliding contact leaf attached to the plunger bridging one pair of stationary contacts at a time (break before make).

The contacts are described as “silver-gilded” (silber-vergoldet/en argent, doré): this appears to indicate that they made of silver for good conductivity, then gold plated for protection. These switches are rated for 100 mA at 60 V DC, and 200 mA at 60 V AC, comparatively high for a mechanical keyboard switch.

Models

RC 72 L

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.

Models

RC 72 N

RC 72 N contains mains power switches. These are rated for 220 volts and 2 amps.

Models

Keylock switches

RAFI also provided keylock switches in the range. These are rated for 200 mA at 60 V AC, and 100 mA at 60 V DC, just as with RC 72 M. These may fall under RC 72 N, but the catalogue is not clear. There are ten models, 3.11998.001–010, and these may be ten different key fittings: the explanation is given in English, French and German and still remains confusing.

Keyboards

So far, only three keyboards with these switches have been discovered.

Express-2 keyboard

The Express-2 railway station terminal keyboard is fitted with RC 72 C contactless switches and RC 72 L dummy switches for space bar, and a keylock switch that may also be RC 72. 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−” (V_CC, 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.

These switches appear to be fitted with damping prongs, similar to those in some Clare low-profile reed/Clare-Pendar S880 reed switches.

Documentation

• RAFI-System 72 full catalogue, supplied by RAFI, freshly scanned (October 1983)
• RAFI RC 72 ordering reference supplied by RAFI, freshly scanned