Micro Switch SC, CT and ST Series

Contents

• Overview
• Patents
• Technology
• Design
  • Keycap mount
  • Colour
• Specifications
• Models
  • Catalogue models
    • Electronic Engineers Master 1983–84
    • Electronic Engineers Master 1985–86
    • Low Profile Silent-Tactile Keyboards Comparison Guide (1986)
  • Examples
• Parts
  • Actuators
• Documentation

Overview

SC Series, CT Series and ST Series are early membrane keyboard series from Micro Switch. SC Series appears to be the original design, as all parts shared between the series (such as controller ICs and membrane sheets) are allocated SC part numbers.

The series are defined as follows:

SC

Capacitive membrane, where the “S” here may denote “solid state” as with SW, SN and possibly SD

CT

Contact membrane

ST

Silent Tactile: capacitive and contact membrane keyboards, using rubber domes instead of snap action springs

SC and CT are both click-tactile designs, using a snap action stacked-spring assembly. ST superseded both series, offering quiet tactile operation. However, at least two SC Series keyboards have been reported to be linear, and photos of one of these suggests that the linear feel was achieved by splitting the snap-action bar into two fingers.

As originally introduced, SC and CT Series used tall actuator modules. This can be seen in the August 1981 advertisement in Computer Design and in some Symbolics 364000 keyboards, the only keyboard model ever discovered that used the original tall SC actuators. The SC10075 listing on eBay is for replacement tall actuators. It appears that the low-profile version was introduced almost as soon as the series came out; low-profile SC and CT are confirmed as early as March 1981 in Electronic Design magazine, in a photograph that appears to show the full height and low-profile versions side by side. The DIN compliant actuators shortened the plunger barrel and the plunger, and switched to a slot mount instead of a post mount. Keycaps are not interchangeable between the two styles.

An engineering prototype 63SC35-1 with ICs dated from 1979 and 1980 is currently the oldest known example, although it is likely to have been assembled in 1981 (it seems to be DIN-compliant). SC Series is rare, with few documented examples. ST Series is more common, with production spanning at least the range of 1984–1991 based on documented examples and the 1984 advertisement in Computer Design magazine introducing it as a new series. CT keyboards have yet to be encountered.

Patents

US patent 4920343 covers the issue of all-plastic keyboards losing the metal components as a capacitive reference. Quite which keyboard type this patent refers to is unclear, because the SC/CT/ST family is a plate-mounted type. The keyboards covered by the patent no longer seem to have the metal mounting plate. The patent is listed here for now alongside the other capacitive sense patents.

Technology

SC Series capacitive full-travel membrane keyboards appear to date to 1980 or 1981, making them contemporary with Oak Full Travel Membrane. An article “Full travel membrane keyboards seal switching elements within sandwich” from the July 1981 issue of Computer Design documents the principle of operation. There are three layers of capacitor pads: drive, floating and sense. The pairing of sense and float produces a static capacitor that provides detection of the capactive level of an inactive switch, and the pairing of float and drive provides for keystroke detection. These keyboards are fully environmentally sealed, and the article goes on to describe the principles of air pressure management, which seems to take a very similar approach to that of Datanetics batch-fabricated array. The following illustration of the membrane structure is a recreation of the illustration in the Computer Design article:

These keyboards were complemented by CT Series conductive membrane, and KD Series flat touch panels. It is not clear whether KD Series was capactive or conductive, or even both.

SC Series and CT Series provided full travel using actuator assemblies. These plate-mount modules contained a flat spring driven by a coil spring. In the tactile models, this flat spring has over-centre snap-action operation, providing inherent hysteresis, which is noted as a feature of these keyboards. KD Series panels are said to have a snap disc at each position for tactile feedback and to protect the membranes from sharp objects. The earlier US patent 4354074 filed in 1981 depicts an alternate action switch broadly similar to SC Series, but with notable differences, in particular the size and angle of the plate retention arms and the design of the snap action spring. The latter patent shows a snap action spring more like that of Olivetti snap action.

SC and CT series were superseded by ST Series “Silent Tactile” keyboards. These were largely identical, but the metal spring system was replaced with a rubber dome, giving a tactile feel without the associated audible feedback. US patent 4604509, that was filed in 1985, covers the rubber dome design used in ST Series.

Design

Keycap mount

The original full-height switches are Micro Switch mount and share keycaps with SW/SN Series and SD Series. This is not necessarily obvious from a loose switch module, as the cross section of the plunger omits two of the corners:

The DIN-compliant switches were designed to use Honeywell mount. However, at least two mounts are known: Honeywell, and blade slot, where the keycap contains a blade (the opposite of the blade mount found in many 1960s and 1970s switches where the slot is in the keycap). DIN-compliant plungers that take Honeywell mount keycaps are black, and those that take blade mount keycaps are blue.

Colour

Plunger colours are not fully understood due to the scarcity of examples. Full-height plungers have been found in black (in a linear keyboard) and unpigmented white (“clear”). DIN-compliant plungers appear to be black when they accept Honeywell-mount keycaps and blue when they accept blade-mount keycaps. The actuator body is normally black, but it is beige or grey for special positions such as space bar. The GRiDCase 1520 keyboard uses grey actuators for space and, inexplicably, the 1 and PgUp keys. (Space normally uses two actuators, one at each end, with one having a grey or beige body and the other having a normal black body.)

Specifications

Very little is known in the way of specifications for these series. The details below were taken from Micro Switch’s advertisement in Electronic Engineers Master 1983; the SC specifications are from typewriter keyboard model 58SC36-2, while the CT specifications are from the 16CT series of numeric keypads. The former is listed as N-key rollover and the latter as two-key rollover. Although N-key rollover on a numeric keypad would be pointless, it is unlikely that any CT keyboard could achieve more than two-key rollover.

Models

The model numbers fit the patterns defined below:

KSCA-B

KCTA-B

KSTA-B

K

Number of keys

“SC”

SC Series capacitive membrane click feedback keyboards

“CT”

CT Series contact membrane click feedback keyboards

“ST”

ST Series quiet tactile keyboards

A

Keyboard type: see the table below.

B

Unknown; this seems to be a sequential code that indicates the nth keyboard of its type

Catalogue models

All the examples found in catalogues, from the mid-1980s, are stated to be DIN-compliant. No pre-DIN models are currently known from product literature.

Electronic Engineers Master 1983–84

All models are assumed to be click–tactile, as this is how both SC and CT series are described in the advertisement.

Electronic Engineers Master 1985–86

Low Profile Silent-Tactile Keyboards Comparison Guide (1986)

See Micro Switch ST Series on the Deskthority wiki for details.

Examples

Parts

The following are parts that were sold separately, such as for replacement.

Actuators

The 103SC56-1 actuator parts were reported by one “cfraser” at Deskthority who—like so many others—vanished in the middle of a forum conversation. The 103SC56-1 keyboard was not properly documented either.

Documentation

All documentation below was scanned by Bitsavers unless otherwise noted.

• Full travel membrane keyboards seal switching elements within sandwich, Computer Design, July 1981, page 54
• Micro Switch SC, KD, SD and CT Series advertisement, Computer Design, August 1981, pages 39–41
• Micro Switch low-profile SC and CT Series advertisement, Electronic Design No. 6, March 1982 (collected by Marcin Wichary)
• Micro Switch Silent-Tactile advertisement, Computer Design, July 1984, page 58
• Keyboards advertisement, Electronic Engineers Master 1983–84 Volume 1
• Keyboards advertisement, Electronic Engineers Master 1985–86 Volume B