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Micro Switch SC, CT and ST Series

Contents

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 ST superseded both series, offering quiet tactile operation.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 appear to have used tall plunger modules. This can be seen in the August 1981 advertisement in Computer Design; the SC10075 listing on eBay shows a plunger module very similar in profile to that shown in the advertisement. 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.

An engineering prototype 63SC35-1 with ICs dated from 1979 and 1980 is currently the oldest known example. SC Series is very rare, with a 1984 ITT Courier 1700 keyboard being the only documented production example. 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

Patent Title Filed Published Product
US 4604509 Elastomeric push button return element for providing enhanced tactile feedback 1985-02-01 1986-08-05 ST

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.

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
Series Type Description
SC 35 Not known
40
56
ST 13 This appears to denote capacitive membrane ST keyboards
22 This appears to denote contact membrane ST keyboards
33 Unknown due to poor documentation

Potentially, for SC series, 56 could denote the linear type and 40 the click type. 35 may relate to the older, taller modules.

Catalogue models

All the examples found in catalogues, from the mid-1980s, are stated to be DIN-compliant. As this product range goes back as far as 1980, it remains to be seen whether DIN compliance was introduced afterwards, although the design appears like it would have always complied.

Electronic Engineers Master 1985–86

Catalogue listing Application Sensing Power requirement Termination Output Modes Keytops
58ST13-1 Typewriter array Capacitive +5 V DC, 250 mA Right-angle header USASCII 1 Sculptured
63ST13-2 Interactive display terminal 4 Stepped
83ST13-5 Interactive display terminal (DEC VT 100) +5 V DC, 350 mA Sculptured
87ST13-1 Interactive display terminal (IBM 3278) +8 V DC, 250 mA Telephone-type header Eight-bit address 1 Sculptured/front stamped
103ST13-1 General purpose +5 V DC, 350 mA Right-angle header USASCII 4 Sculptured
83ST13-1 Personal computer (IBM PC) Telephone-type header Eight-bit address —
90ST13-1 General purpose USASCII —
105ST13-1
63ST22-1 Communications Contact — Pigtail with connector Wired-only X-Y matrix — Stepped
16ST22-1 Adding machine format
103ST22-1 General purpose Sculptured

Low Profile Silent-Tactile Keyboards Comparison Guide (1986)

Catalogue listing Typical applications Key array features Keyboard features
12ST22-1 Numeric entry 3 by 4 block array X-Y wired-only matrix, pigtail termination
16ST22-1 4 by 4 block array
58ST13-1 Typewriter Alphanumeric, status LED for shift lock USASCII encoded, serial or parallel data outputs
63ST22-1 Interactive display terminals Alphanumeric, communication-orientated X-Y wired-only matrix, pigtail termination
63ST13-1 Alphanumeric, status LED for shift lock, communication-orientated USASCII encoded, serial or parallel data outputs
83ST13-5 Editing terminals DEC VT-100 compatible key array, 8 user-defined LEDs USASCII encoded, serial or parallel data outputs, multiple interface options
87ST13-1 IBM 3278 compatible key array IBM key encoded, operate/release codes, serial output, auto-repeat
105ST13-4 DEC VT-200 compatible key array, 3 user-defined LEDs USASCII encoded, serial output, multiple interface options
122ST13-2 IBM 3179/3180 compatible key array IBM 3179/3180 plug compatible electrical interface, audio feedback
83ST13-1 Personal computer IBM PC compatible key array, status LEDs for Caps Lock and Num Lock keys IBM PC plug compatible electrical interface
83ST13-20 USASCII encoded, multiple interface options (including IBM plug compatible)
84ST13-1 Enhanced IBM PC key array, status LEDs for Caps Lock and Num Lock keys
84ST13-2 IBM PC-AT compatible key array IBM PC-AT plug compatible electrical interface, optional IBM PC plug compatible
98ST13-3 Enhanced IBM PC key array, separate cursor keys, numeric keypad USASCII encoded, multiple interface options, including IBM plug compatible
98ST13-4 Enhanced IBM PC-AT key array, separate cursor keys, numeric keypad IBM PC-AT plug compatible electrical interface, optional IBM PC plug compatible
122ST13-12 IBM 3270 PC compatible key array IBM 3270 PC plug compatible electrical interface
122ST13-14 IBM PC plug compatible electrical interface, optional PC-AT plug compatible
90ST13-1 General purpose Alphanumeric array, numeric keypad, 20 special function keys Serial or parallel outputs, USASCII encoded, multiple interface options
103ST13-1 Alphanumeric array, cursor keys, numeric keypad, 14 relegendable function keys
105ST13-1 Alphanumeric array, cursor keys, numeric keypad, 20 special function keys

Examples

Keyboard MICRO part Date Plunger colour Notes Reference
Engineering prototype 63SC31-1 ca. 1980 ? KBref
Bare module 103SC56-1 8310 Black Linear feel Deskthority
Stearns Computer Systems keyboard 94SC56-1 8406 Black Seven keys have LED switch modules; linear feel Deskthority
ITT Courier 1700 92SC40-23 8420 Black Click type Deskthority
IBM 73x3832 Quiet Touch — 8716 Blue Internal part numbers are all SC series Deskthority
Unknown module 122ST13S-25E 8728 Blue Deskthority
Telex 211757-001 122ST33S-46E-J 8826 Blue No internal details documented, so we cannot understand type 33 or see whether this is capacitive or conductive Deskthority
Bull 115ST T. I. 60164180-001 115ST13-8E-1-J 8827 Black Flickr
Memorex Telex 1KBDC88T01CA 88ST13-24E-J 8929 Black Geekhack
GRiD GRiDCase 1520 72ST22-1-J 8935 Black Deskthority
GRiD GRiDCase 1520 72ST22-1-J 8940 Black Deskthority
Bull 115ST T. I. 60164180-001 115ST13-8E-1-J 9136 Black Deskthority

Parts

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

Part Description Source
SC10075 SC actuator assembly, uncertain keycap mount, stepped plunger eBay

The following additional actuator parts were reported by one “cfraser” at Deskthority who (like so many others) vanished in the middle of a forum conversation; potentially these parts may also have been sold as replacements:

Part Description Source
SC-10090 Regular switch module 103SC56-1 schematics (Deskthority topic)
SC-11002 LED switch module
SC-10109 LED switch module?

Documentation

All documentation below was scanned by Bitsavers unless otherwise noted.