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Micro Switch SC, CT and ST Series



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:

Capacitive membrane, where the “S” here may denote “solid state” as with SW, SN and possibly SD
Contact membrane
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 ST superseded both series, offering quiet tactile operation.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.


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


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.


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.


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.)


The model numbers fit the patterns defined below:




Number of keys
SC Series capacitive membrane click feedback keyboards
CT Series contact membrane click feedback keyboards
ST Series quiet tactile keyboards
Keyboard type: see the table below.
Unknown; this seems to be a sequential code that indicates the nth keyboard of its type
Series Type Description
SC 35 Not known
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. No pre-DIN models are currently known from product literature.

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
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


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


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


Part Type Profile Mount Behaviour Angle Shell Plunger Source
SC10075 Regular? Classic Micro Switch Stepped Black White eBay
SC-10090 Regular DIN Honeywell Linear Sloped Black Black 103SC56-1 schematics (Deskthority topic)
SC-11002 LED
SC-10109 LED?

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 was not properly documented either.


All documentation below was scanned by Bitsavers unless otherwise noted.