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Fujitsu FES-8 Series

Contents

Overview

FES-8 Series is a family of reed and Hall effect switches from Fujitsu. FES-8 Series is described by Kiyoto as a “modernization of the specifications of FES-1/2/5 by reorganizing them”. FES-8 was introduced in the Fujitsu journal in November 1976, and described in extensive detail, mostly in Japanese; this gives an approximate indication of its age.

This series offered both reed and Hall effect sensing; at present only the reed type has been encountered. Fujitsu intended for the alphanumeric keys of a keyboard to use reed switches, in order to lower the overall power consumption, with the “functional” keys being Hall effect in remove the need to deal with contact bounce when directly controlling logic circuitry.

Illumination can be in the form of either an incandescent lamp (fixed to the body of the switch) or by LED, and the LED moves with the keycap to maintain consistent illumination (although an alternative LED type exists that is hard to understand from the magazine illustration). Both upstroke and downstroke damping are provided.

An unidentified overview of FES-8 and FES-9 found on datasheetarchive.com (see Documentation, below) gives the specifications as follows:

Specifications

The following specifications are all taken from the Fujitsu journal article in 1976.

Travel 4.5±0.5 mm
Pretravel 2.8±0.8 mm
Contact bounce 1 ms max (reed types)
Rated lifetime 10 million cycles

The way that force is specified differs between the various switch types:

Switch type Force
Linear 50±15 gf (at full travel)
Tactile 70+30−20 gf (tactile point)
Alternate action 150 gf maximum
LED spot illuminated 90±30 gf (af full travel)

Characteristics

Tactility

In the Fujitsu journal article on FES-8, Fujitsu noted:

押下力特性に関する最近の傾向は,ソフトタッチ指向にある。各メ一力のスナップアクションの押圧力ー変位特性を図-2に示す。FES-8形押釦スイッチの特性は,良好なキータッチのスイッチとして好評を得ているFES-5形押釦スイッチの特性を基に,図-3の特性を設定した。

They indicate that the tactile feedback of FES-8 is based on that of FES-5, which was a well-received design. FES-5 itself was designed to mimic the feel of the IBM Selectric. Figure 2 compared the IBM beam spring (from 1972, around the same age as FES-5 and also believed to be meant to simulate the feel of the Selectric) and Micro Switch SD tactile (from 1976) with FES-5. Figure 3 then depicted the tactile (スナップタイプ, “snap type”) and linear (ノンスナップタイプ, “non-snap type”) force curves of the new FES-8 switch. The diagram below combines figures 2 and 3, and omits FES-8 linear for clarity. FES-8 linear is weighted the same as FES-8 tactile.

Note that there is no single weighting of beam spring. Fujitsu measured the preload at around 34 gf and the tactile point at 64 gf. The patent from 1976 depicts a preload of around 44 gf and a tactile point at 56 gf. Jacob Alexander’s measurements give something like 42 and 60 gf on the IBM 5251 keyboard part 7361073 graph (closer to the patent), and much lower figures of 21 and 45 gf for a new old stock beam spring module. The 7361073 keyboard has over 4 mm of travel, while the NOS switch has around 3.7 mm of travel, closer to Fujitsu’s diagram.

Fujitsu also wrote, “Generally speaking, the letter keys with snap action, and numeric keys with non-snap action tend to be liked.” This can be seen in existing examples, where both tactile and linear switches are used within the same keyboard.

The tactile system appears to be a derivative of that used in Micro Switch SD. Although there are no known patents for the design of either SW or SD switches, Fujitsu took a similar but different approach. In SD, a spring-loaded pin follows a linear cam surface moulded from the shell of the switch. Fujitsu opted for a rigid pin, and a linear cam formed from a leaf spring. The following highly-approximate diagram (based on a Fujitsu journal diagram along with photographs and video footage) depicts the operation:

Initially, the only force acting against the operator’s finger is the return spring. Once the follower pin reaches the angled portion of the leaf spring, the leaf spring exerts additional force on the plunger. This force can be broken down into lateral and vertical components, with the vertical component increasing the resistance of the switch. Once the plunger clears the angled portion of the leaf spring, the force level drops off sharply.

Illumination

Fujitsu offered LED illumination to address the limited lifetime of incandescent bulbs, as well as their susceptibility to damage from vibration. Incandescent lamps may have remained available to give a wider choice of colour than LEDs, including white. LED-illuminated switches could be corner-lit or surface-lit. For the latter, Fujitsu explained that the “LED chips and drive resistors are arranged on the hybrid substrate”. The LED surface illumination system is a rather complex arrangement that is difficult to understand from the poor reproduction quality of the scans made by the Japanese library service of the Fujitsu journal. Someone who can read Japanese would, however, be able to read the explanatory text within the article.

Sensing

Fujitsu offered both reed and Hall effect sensing. In their FES-8 article, they described their intention as follows:

Generally speaking, in most cases, the switches of letter keys send codes through encoders, and those of functional keys transmit on-off signals directly to the system side. For the letter keys, many reed switch elements are used because of low power consumption and high reliability; for the functional keys, hall elements are suitable which can activate directly ICs.

Here, Fujitsu are most likely referring the problem of contact bounce. Although reed switches have very low contact bounce (typically no more than 2 ms), it is still enough to confuse logic circuitry. Thus, Hall effect is a better choice. There is no need to involve the encoder for the special keys.

The Hall sensor block circuit diagram indicates that the sensor is current sinking with dual outputs. There are two “output wave form” diagrams, one captioned “レベル出力” (level output) and the other captioned “パルス出力” (pulse output), suggesting that Fujitsu offered both operating modes. This is corroborated by the block diagram label “MK : Monostable flip-flops (パルス出力のみ)”, with the flip-flops indicated as being for pulse output only.

Variants

Part number Variant Found in
813001 Alternate action, reed? N860-8002-T06501A (1982)
823001 Momentary linear, reed N860-8282-T002 (1980), N860-8002-T06501A (1982)
833001 Momentary tactile, reed N860-8282-T002 (1980), N860-8002-T06501A (1982), Shimadzu FKB-1A

Examples

FES-8 keyboards are rare. The following examples are known:

FES-9

Shortly after FES-8 was introduced, a lower-profile adaptation was created, designated FES-9. For most purposes they are identical.

Documentation

References