Omron B3G-S series

Here follows a few notes on Omron B3G-S series mechanical keyboard switches and B35 series keycaps. Some of these notes relate to discussions with Sandy in 2015.

Contents

• Specification
• Models
• “Flat” switches
• Keycaps
  • B34 series
  • B35 series
• Changes
  • “JAPAN” marking
• Discontinuations
  • PRDN 1106
  • PRDN 1777
  • PRDN 1779
  • PRDN 2620
• Moving click leaf patent

Specification

In addition to a B3G catalogue page, I also received a B3G-S catalogue page (from a catalogue reportedly dated “1986/7”) from Omron via Quest Components:

• B3G-S series catalogue entry (1986)

形B3G-Sフルキーボード用キースイッチ(ロープロフィール形)
DIN規格に適した全高18.1mmのロープロタイプ

• 小型、ローコストにもかかわらず、高級機なみの操作感触が味わえます。
• リーフ・ツイン・クロスバ接点機構の採用により無接点形に匹敵する信頼性を実現。
• ジャンパー端子（2個のNO端子）構造ですから、基板上での配線が容易。

This translates to:

Key switch for B3G-S full keyboard (low profile type)
Low-profile type with an overall height of 18.1 mm suitable for DIN standards

• Despite its small size and low cost, it can provide the same feel as a high-end machine.
• Achieves reliability comparable to non-contact type by adopting dual-leaf crossbar contact mechanism.
• The jumper terminal structure (2 NO terminals) makes wiring on the board easy.

This two-finger contact approach was previously introduced by companies such as Marquardt and Mitsumi. The switch is specifically noted as being DIN-compliant, which is likely to be one of the reasons for the redesign.

Until I get around to posting a transcription of the whole document, here a couple of notes:

The page above only covers B3G-S100, B3G-S200 and B3G-S400 series. B3G-S300 series, which was not retained in B3G-S*N, is notably absent, and this possibly represents the alternate action variant, since Sandy notes that the later design no longer supports the latching guide frame. The higher numbers listed below (500 to 700) are also not covered.

The second digit covers “LEDスポット照光” (the presence of an integrated LED), being 0 (no LED), 1 (赤, red LED), or 2 (緑, green LED). For example, B3G-S220 would be S200 with a green LED.

Models

The following models were listed in the 1986 catalogue entry. B3G-S4□□ is described as “感触つき” (with feeling, or with touch), which is presumably the common tactile type that is sold as “B3G-S100N”. The close correspondence between the operating force and total travel force of the other models seems odd, but some linear Alps force curves display the same correspondence. However, B3G-S4□□ has exactly the same operating force and total travel force, which highly implies tactility.

The catalogue pages supplied omit B3G-S3□□ (suspected to be alternate action) and higher-numbered models, which may be the click types.

“Flat” switches

“Flat” is Sandy’s nickname for the original design of B3G-S switch: the slider is a plain rectangle with notches in the corners, as seen on the 1986 catalogue page:

“Flat” design (depicted: latching)

“Non-flat” design (depicted: tactile with no-LED base)

“Flat” switches are extremely rare, but two dated examples have been found so far, one from 1985 and one from 1987 (out of only four known instances of these switches). Sandy also possesses two latching “flat” switches (found in an NEC 8313-12 keyboard, and depicted by his photograph above), something not seen in the later design. (We are theorising that as the redesign did not allow for a latching mechanism, those few customers still requiring latching switches would continue to receive custom batches of the original design; latching switches at this stage were seldom used outside of Apple, who were not an Omron customer.)

“Non-flat” is familiar design of B3G-S where the front and back of the slider have cut-away recesses, and the opening in the switch shell has slits at the front and back: the front and back of the slider/opening design was modified to be similar to the sides of the slider and opening of Alps SKCL/SKCM switches.

Sadly, neither of us have yet determined more sensible names for these two design characteristics!

Keycaps

The following dimensions come from PRDN 1106, which includes illustrations of the general shape of each series (B34 and B35).

B34 series

B34 keycaps are the older, more rounded style, with a top surface that tapers slightly towards the back. These keycaps were discontinued as of the end of March 1992.

B35 series

B35 keycaps are the newer, angular style.

Changes

The history of B3G-S remains poorly understood. We know that in 1988, Omron issued PRDN 495, listing the part number correspondence between B3G-S and B3G-S*N. This occurred around the time that “flat” appears to have been superseded with “non-flat”.

• B3G-S to B3G-S*N part replacement (PRDN 495 1988-07-01)

The document is summarised thus:

メカキースイッチ形B3G-Sシリーズの一部中止のお知らせ
メカキースイッチ形B3G-Sシリーズの一部を生産中止します。

Translation:

Announcement of discontinuation of some B3G-S Series mechanical keyswitches
Part of the mechanical keyswitch series B3G-S will cease production.

The part replacement table (“生産中止機種と代替機種”) is given thus:

There was no change to the external dimensions or specifications. Subsequent production of the obsolete types was not possible.

So far, no documentation has been found that indicates what modification to the switch necessitated the change in part numbers, and whether this change occurred at the same time that the slider design was updated. A number of changes occurred around this time, including the removal of “JAPAN” from the switch shell, suggesting that the tooling had been shipped abroad. However, this occurred after the redesign from “flat” to “non-flat”, as the latter was seen briefly with “JAPAN” marking.

More dated examples of keyboards might give us better insight into this enigmatic switch series.

“JAPAN” marking

Amber Omron switches are generally seen with the word “JAPAN” deleted from the mould: that part of the mould has been ground away to remove the writing. However, this Focus keyboard (an FK-727, according to the controller marking) ca. late 1986 has Japanese-made amber switches.

Discontinuations

PRDN 1106

PRDN 1777

The suggestion is that B3G-S100N and B3G-S600N were at this point the only B3G-S subseries still in production, and likewise B35 series keycaps contained only rows C, D, E and G.

PRDN 1779

The list of discontinued types is given below. B5GS is the keyboard series identified with B3G-S, such as B5GS-R101-620, which means that the number following “R” denotes the number of keys. The Sony M35 Laptop has two modules inside: B5Y-C073-401 with vase spring switches for the main keyboard area, and B5GS-C010-414 for the ten-key function key block. “BKE” may relate to Sony BKE keyboards. The 114-key Sony SMC-3000G keyboard has PCB model B5GS-R114-534, as well as codes KY-106 and 1-619-995-12 which potentially relate to the “KY” subseries of B5GS.

PRDN 2620

These are switch gangs; see the PRDN for limited illustrations.

Moving click leaf patent

US patent 5015811 “Snap-action pushbutton switch with click sound” describes an alternative variant of B3G-S that uses a sliding click leaf (called a “plate spring”). Citing the typical click leaf design as prior art (redrawn from the Alps SKCM patent), the patent shows a new design with an inverted leaf that is pushed upwards by the slider. The slider bears a ramp that engages with the click leaf, and upon clearing the leaf, the leaf is fired upwards and collides with the top of the switch. The same action occurs when the switch is released, with the slider firing the leaf back downwards. Consequently this guarantees a tactile event and click sound on both press and release.

The patent does not appear to suggest why the leaf spring does not fall down after it has been fired upwards (it is depicted as remaining at the top of the switch), or why it does not deflect off the top of the switch. The spring is not loose when released, as the slider holds it down, but this is not the case when the switch is pressed, so friction alone would have to hold up the spring.

This patent was filed in 1988, which is likely to be after clicky B3G-S switches were introduced. No switches with this design have ever been seen.