Futaba MD series

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Futaba MD series

Manufacturer	Futaba
Inventor	Hideo Chiyuushiya, Katsuki Shimizu, Masao Miyagishima, Shiyouhei Tanaka, Sadanori Okada, Riyouko Yamaguchi
Family	Futaba linear switch
Switch type	Linear
Sense method	Metal leaf
Rated lifetime	10M (KBM)
Bounce time	5 ms (KBM)
Peak force	3.5±0.8 oz (99±23 g) (KBM)
Pretravel	0.03±0.01″ (0.8±0.25 mm) (KBM)
Total travel	0.12±0.01″ (3.0±0.25 mm) (KBM)
Keycap mount	Custom cruciform, bar
Switch mount	Plate mount
Patents	JPS5944725A (1982)

Futaba MD series (formerly Futaba complicated linear) a provisional name for series in the Futaba linear switch family. The GRI equivalent series is KBM.

Nomenclature

The series name MD-4P is cited by Korean website ePartsHub, with a picture of Sandy's bar mount switch and the description "various switches".<ref name="epartshub" /> With MD-4PCS being a known part number, this would put both bar and cruciform mount in the same series. However, the cruciform mount, illuminated version is cited by a related website as "MD-7", suggesting MD-4P is the bar mount version, MD-4PCS is the cruciform mount version, and that "MD" is the common aspect of the part numbers.<ref name="ecplaza" /> Provisionally this is being used as the series name.

ePartsHub cite the switch as being made by Sejin. The only known PCB photograph of the Futaba Texas Instruments TI-99/4A keyboard bears both the Futaba and Sejin logos, but all cited instances of the product family (Futaba MD-4PCS, the Acorn BBC Microcomputer simplified linear switches, and the complicated linear switches in the Lear Siegler ADM 36) all cite simply Futaba as the source. While Sejin may have been the manufacturer, the series was clearly marketed by Futaba.

The previous "complicated" moniker arose due to the significantly greater complexity over the visually very similar Futaba simplified linear.

Description

Like most other Futaba linear switches, the shell is cylindrical with truncated sides, with a square top above the plate. It sits in a circular hole in the plate. The legs are threaded at the top and screw into place; the base plug of the shell is retained in place by the legs. The legs are forced inside small holes in the contact plates, and in forcing the holes larger, the legs make a good connection with the contact plates.

The switch has been seen in black with a deep orange base plug, in white with an orange base plug, all black, and all white. The black version with the orange base cap has part number MD-4PCS.<ref name="MD-4PCS" />

The distance between the plate and PCB is 12.7 mm.

The switch comprises 16 parts in total, making it one of the most intricate switches ever designed (more so as it is only a linear switch):

• Shell
• Base plug
• Two threaded legs
• Return spring
• Leaf spring
• Contact assembly:
  • Base
  • Lower and upper contact leaves
  • Plastic separator
  • Rubber top
  • Seal
• Slider:
  • Main slider shaft
  • Separate slider side (drops into the main shaft)
  • Rubber ring
  • Black dot

Operation

Two contact plates are placed around a plastic separator, inside a flat base. Above this is a thin flexible rubber sheet, which is sealed onto the sides of the base via an adhesive square. The leaf spring runs parallel to the slider, and it has a central tine with a nub that presses down on the rubber sheet. Via the rubber sheet, the pressure is transferred to the flexible legs of the movable contact, which touch the stationary contact.

The contacts have long exterior legs, with holes in them. The legs of the switch are of a greater diameter than these holes; when first screwed into the switch, they force a larger hole into the contact plates. As the metal is twisted inwards, it makes lateral contact against the sides of the legs.

GRI KBM

This switch was also sold by GRI as the KBM (keyboard switch, mechanical).<ref name="GRI-KBM" /> While GRI keyboards and keypads have been found with the standard "bumblebee" colour scheme, GRI also had bright white switches made with GRI branding.<ref name="HaaTa-GRI" />

KBM switches are described as having 3.5±0.8 oz operating force, which at just short of 100 g would make this the total force; assuming 35 g preload, then the actuation force would be around 67 g. Pretravel is given as 0.03±0.01″ and total travel as 0.12±0.01″; this gives pretravel of 0.8 mm and total travel of 3 mm.

GRI's part numbers are:

KBM-01-01

Momentary

KBM-01-01-AA

Alternate action

Specification

The following table compares the specification provided by GRI with that reported to be found in FS-5104P4.<ref name="epartshub" />

Parameter	Sejin	GRI
Total force	100±25 gf	3.5±0.8 oz (99±23 g)
Total travel	3.1±0.5 mm	0.12±0.01″ (3.0±0.25 mm)
Pretravel	0.8 mm or more	0.03±0.01″ (0.8±0.25 mm)
Overtravel	0.5 mm or more
Rated lifetime	10M	10M
Bounce time (max)	2 ms new, 5 ms at end of life	5 ms

MD-4P series has a mounting pitch of 19 mm.

It's not known whether FS-5104P4 (Futaba-Sejin? Futaba Specification?) covers the alternate action switches.

Variants

Cross mount

The cross mount version has a non-uniform cruciform slider approximately 4.0 mm by 4.7 mm, with 1.2 mm wide arms, taking a 4 mm deep keycap stem. It has been seen in black with an orange base cap, white with an orange base cap, all black<ref name="EBAY-Altair" />, and all white.

The white/black/orange version has part number MD-4PCS. GRI KBM switches are also this variant.

The cross mount variant is keycap-compatible with the Futaba simplified linear switch.

The Futaba linear lock switch is the alternate action switch in this family.

Bar mount

All-black shell with a white slider<ref name="Sandy-bar_mount" />, white<ref name="eBay-PTC-45A" /> (base plug not visible), and black with an orange base and white slider.<ref name="GH-BP1210-D" /> One minor difference between the examples examined, is that the bar mount variant was found to have a transparent plastic cover over the contact assembly, instead of black rubber-like material.

This version may be model MD-4P.<ref name="ecplaza" />

Multiple-switch return key

TeleVideo keyboards (specifically TS-800A, TS-802 and TS-825) have been found with dual switches for the Return key. <ref name="GH-TeleVideo-8xx" /> These switches are white with a cream base plug, and otherwise appear to be identical to the white/orange switches used on the rest of the keyboard. The reason for having two switches side by side instead of a stabiliser is not known, nor why the switches are a different colour.

Illuminated

The illuminated version, with a crucform keystem, is given as MD-7;<ref name="ecplaza" /> it is not clear whether this is a series name or model number.

Keyboards

• Atari CX85 (ca. 1982) (black version)<ref name="Atari-CX85" /> <ref name="Atari-CX85-2" />
• GRI Model 771 Keyboard Subsystem (given as GRI KBM), as listed in Viper magazine, 1979, volume 1, issue 9
• GRI 9016 ASCII Series Keypad<ref name="GRI-KBM" />
• Later Krown Porta-printer keyboards (ivory with black base)
• Newbury 8000 series keyboard (white)<ref name="Newbury-8000" />
• Plantronics Vu Phone 3200
• TeleVideo Model 925 CRT Terminal keyboard (1981) (white version)<ref name="Televideo-925" />
• TeleVideo TS-800A satellite station keyboard (white with orange base, and white/cream for dual enter switch)<ref name="GH-TeleVideo-8xx" />
• TeleVideo TS-802 workstation keyboard (as above)<ref name="GH-TeleVideo-8xx" />
• TeleVideo TS-825 keyboard (unknown product) (as above)<ref name="GH-TeleVideo-8xx" />
• Televideo 910

Other devices

• PTC-45A machine keypad (white bar mount)<ref name="eBay-PTC-45A" />
• Canon Canola BP1210-D (black/orange bar mount)<ref name="GH-BP1210-D" />

Gallery

Cross mount

The following switches (part number MD-4PCS) are NOS from Electronic Surplus.

• 
  Futaba complicated linear switches
• 
  The two legs are threaded and can be unscrewed
• 
  The base plug is retained by the legs
• 
  The legs connect to the contact assembly via the bent edges of the holes
• 
  The contact assembly slides out easily; note that the empty recess above the slider holds the latch track block in the Futaba linear lock
• 
  Contact assembly removed from the switch
• 
  Contact assembly with leaf spring on top
• 
  Leaf spring and contact assembly
• 
  The seal can be eased off safely with a sharp knife
• 
  Contact assembly opened up
• 
  Top contact plate removed, and the rubber sheet removed from the seal
• 
  Separator plate removed, and top contact plate inverted to show the plating
• 
  Bottom contact plate removed
• 
  Base seen from above and from below
• 
  Base of the slider
• 
  Part of the slider is a separate piece that comes out separately
• 
  Slider side piece has rubber ring around connecting peg
• 
  The spring rests against a rubber circle of unknown function
• 
  Detailed views of the slider
• 
  Shell detail, from below and above
• 
  All sixteen components of the switch

No switches were harmed by this disassembly process.

Bar mount

The following switch was damaged during disassembly as it was not obvious that the pins had to be unscrewed.<ref name="Sandy-bar_mount" />

• 
  Top view
• 
  Side view
• 
  Three-part slider
• 
  Slider parts
• 
  Transparent contact assembly
• 
  Contact assembly detail
• 
  Switch partially disasembled

Specifications

• GRI KBM specification

References

<references> <ref name="ecplaza">ECPlaza — Push Button Type Switches</ref> <ref name="MD-4PCS">(Wayback Machine) Electronic Surplus — MD-4PCS - Switch, P/B. NO Keyboard. Package of 20. Archived 2012-01-17. Retrieved 2015-07-26.</ref> <ref name="GRI-KBM">(Wayback Machine) GRI — 9016 ASCII Series Keypad Archived 2000-08-18. Retrieved 2015-07-26.</ref> <ref name="HaaTa-GRI">Flickr — GRI Model KBM 01 01 Retrieved 2015-07-26.</ref> <ref name="EBAY-Altair">eBay — Altair George Risk Keyboard 2 756 013 A w 16 Pin Connector Used with My Merlin (DEAD!)</ref> <ref name="Sandy-bar_mount">Sandy — Futaba_complicated_Linear</ref> <ref name="GH-TeleVideo-8xx">Geekhack — Televideo Terminal Keyboard Posted 2013-10-09. Retrieved 2015-07-26.</ref> <ref name="Atari-CX85">Google+ — Atari CX85 Dated 2010-03-25. Retrieved 2015-07-26.</ref> <ref name="Atari-CX85-2">Japanese keyboard blog — Atari CX85 (Japanese only) Dated 2013-10-20. Retrieved 2015-07-26.</ref> <ref name="Newbury-8000">eBay — Newbury 8000 series vintage computer keyboard. retro computing at its best (DEAD!)</ref> <ref name="Televideo-925">Deskthority — Bunch from a storage unit</ref> <ref name="eBay-PTC-45A">eBay — Machine Keyboard PTC 45A with Missing Key ^{[Volatile reference]} Retrieved 2015-07-26.</ref> <ref name="GH-BP1210-D">Geekhack — What are theses switches? Posted 2013-12-15. Retrieved 2015-07-26.</ref> <ref name="epartshub">ePartsHub — MD-4P</ref> </references>