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Micro Switch SW Series

Contents

Overview

SW and SN Series are Micro Switch’s original series of Hall effect pushbutton switches. Solid State Keyboards use SW Series switches, which are mounted into special frames within the keyboard, and cannot be used separately. By comparison, SN Series switches are fully-self contained and can be snapped into a panel or attached directly to a PCB, depending on the model.

SW Series keyboards are comparatively uncommon. By 1976, Micro Switch had introduced a replacement series of keyboards—SD Series—and these are far more commonly encountered. SW Series keyboards continued to be produced until at least 1995, and the final update to the switch drawings was in 1999.

Via his son Scott, Everett Vorthmann from Micro Switch has indicated that “SW” denotes wired solid-state keyboards, i.e. fully-assembled keyboard units with reed switches.

Design

SW Series switches are all open at the base. The return spring protrudes out from the bottom, and the Hall sensor IC (“lead frame package”) is a slide-in fit. These switches clip into mounting frames placed onto the keyboard matrix PCB. These frames have locating nubs for each return spring, loops to secure each switch into position, and holes to allow the terminals of the Hall sensors to pass through to the PCB. Micro Switch give the reason for the mounting frame arrangement as being a way to prevent keystroke impact from reaching the solder joints on the PCB, although this seems to be contradicted by the fact that the frames are riveted to the PCB.

The design rationale for having open-base switches (instead of fully-enclosed switches) is not known, but one explanation would be serviceability. Since the switches are contactless, they would typically only need servicing to replace a fouled housing or broken plunger. In such cases, there would be no need to supply or replace an expensive Hall sensor: all that is needed to replace a broken switch is to swap out the housing assembly, without the need for any desoldering. Replacement switches bought singly do come with a fresh Hall sensor, as well as an instruction leaflet (except in later years) and a pair of pry tools, suggesting that service technicians would not buy boxed replacement parts as they would soon have a mound of unused sensors, tools and paper.

The return spring is not part of the bill of materials for the switch: the switch models cover only the physical design and choice of Hall sensor. When sold separately, the spring is included, and these parts have separate numbers indicating that they are replacement modules and—for models with a spring weight other than the default of 3 ounces—the weight of the included spring.

Switches are marked with an arrow on the top, which points towards the back of the switch. Confusingly, the corresponding arrow in SD Series switches points towards the front.

The diagram below depicts the general construction of an SW Series keyboard, starting from the printed circuit board (PCB). The PCB is often dual-layer, and consequently it is not uncommon to see some form of insulating material between the PCB and the mounting frames. In one keyboard this seems be to pieces of PCB substrate, while in others it is more like self-adhesive paper.

Metal mounting frames are placed above the PCB and secured with rivets. Each row of keys has a separate mounting frame, and these frames hold the switches in place. Replacement switches on eBay can be found both with the Hall sensor fitted and left separate. It is more likely that the switches were placed into keyboards with the sensor already fitted, but once the IC is soldered in place, the switch module can be extracted without removing the Hall sensor. This allows for easy switch repair.

Encoding and logic

SW Series keyboards were designed to use two-of-N encoding. This arrangement avoids the need for (at the time) expensive matrix scanning circuitry. Each of the sensor’s dual isolated outputs provides one position of that key’s two-of-N code, and the fully-encoded output value of the key is available the instant the key is struck (made possible also by the bounce-free output from Hall sensors). The outputs of Hall sensors are provided by transistors, so inactive switches cannot pass current between outputs, and thus no diodes are needed to prevent wrong-direction current flow through the two-of-N wiring. The two-of-N coding is done entirely with PCB tracks and a small amount of logic, eliminating the need for a diode matrix. The physical encoding took multiple forms depending on the customer requirements and PCB design, as detailed below. The encoder can be DTL or TTL-based (medium-scale integration), using an assortment of separate chips, or MOS-based (large-scale integration), for a reduced part count and significantly greater flexibility in terms of output, such as permitting the shifted output from a key to be unrelated to the unshifted output.

Encoding grid

Keyboards with an encoding grid permit every key’s output to be defined independently using circuit pathways. A double-sided PCB has row tracks on one side and column tracks on the other side, with each row connected via a dedicated track to one of the two outputs of a switch. Every switch has each of its two outputs wired to two rows in the grid, separately from every other switch with no matrix. The grid columns form the N bits of the switch output, and this code is then converted to ASCII or EBCDIC (or other format) by the encoding logic. This overall implementation is shown in the diagram below, which shows a DTL/TTL logic implementation:

The diagram above is simplified, but shows the general functional layout of the circuit. The VCC and GND traces are omitted for clarity. The strobe line signals when a key is struck; it is suppressed when two or more keys are pressed simultaneously to prevent the host equipment receiving invalid data. In reality, Micro Switch keyboards have a single double-sided edge connector, but here the two sides are shown adjacent to each other for clarity. With DTL or TTL encoding using binary encoders, at least 30 output lines are required (instead of the 12 shown in the diagram for simplicity), due to the number of valid two-of-N codes that are illegal inputs to the encoders (any code where both 1s are directed to the same encoder). A mere 12 lines is possible however with MOS encoding.

Provisions were often made, such as with models 53SW1-8 (in the Beehive Mini Bee), 70SW12-1 and 75SW12-2 for non-encoded keys. This took the form of columns in encoding grid that were routed directly to the edge connector and not via the encoding logic. Such keys were termed “function keys”, numbered from F1 onwards on the circuit diagrams. These function keys could be used to drive external logic circuitry directly, without needing to go through the keyboard’s data bus and ASCII or EBCDIC encoding. Function keys were also used for modifier keys, meaning that Control and Shift not only influenced the encoding process, but could be detected independently on the edge connector.

Model 64SW1-4 is a good example of a full encoding grid. Only the necessary connections are drilled and connected; the rest are left unmodified. Example circuit diagrams for various two-of-N models can be found in the Circuits section below.

The encoding grid can be fed into either DTL or TTL logic or into an LSI chip. DTL and TTL models define the final output codes directly within the encoding grid; with ASCII, the uppercase and control character output can be provided by bit manipulation logic circuitry. MOS-based keyboards had the option of placing the final output codes directly in ROM, for increased flexibility.

The ability or requirement to define the output of every switch via a dedicated pair of drilled and linked pads is both an advantage and a disadvantage: it is comparatively labour-intensive, but it allows a single PCB to supply any number of customers with no custom chips necessary.

DTL or TTL encoding

The conversion from two-of-N codes to ASCII or EBCDIC can be performed using decimal to binary encoders. The high and low nybbles of the output code are each derived from two 7 or 8 line to 4 line encoders; each encoder handles half the input values (1–8, 9–F) for each of the high and low nybble. For 7-bit ASCII only three encoders are needed in total, as the high nybble will only be in the range 0–7 (since ASCII only ranged from 0x00–0x7F) but models such as that used in the Beehive Mini Bee have all four encoders and thus full 8-bit output capability (see the Circuits below). Separate circuitry on the PCB is responsible for modifier key processing, as each key is only capable of a single output value by itself; the structure of ASCII makes modifier key circuitry a simple process.

Even with priority encoders, it is impossible to detect clashes (two or more keys struck at once) using the encoder logic because it is possible for all four encoders to be fed a single bit, a legal input for the encoders but an illegal two-of-N code overall. Instead, an electrical monitor detector (EMD) circuit detects an excessive number of active two-of-N lines using parallel resistance measurement, just as you would use in a self-encoding keyboard.

The logic-based approach can be seen in 64SW1-4. 63SW5-4 uses a MOS chip to process the encoding.

Shared tracks

MOS encoding removes the need for every key to have dedicated wiring: using modular arithmetic, switches that generate the same value for a particular output can have those outputs tied together. Thus, Q and W as adjacent keys would likely have one output on a shared PCB track. This arrangement can be seen in the Decision Data 8010 keypunch keyboard. Instead of a full encoding grid, the two-of-N columns branch out to multiple switches, and this reduces the size of the PCB. The conversion between two-of-N to output codes is handled via MOS.

58SW5-9 appears to combine a full encoding grid with MOS encoding, but a look at the reverse of the PCB shows that the encoding is custom to that model and unable to be modified.

MOS encoding

MOS-based encoding is a very compact approach. By feeding the entire two-of-N code into a single chip, clashes between keys can be detected by simply checking for too many active input lines, and thus the electrical monitor detector circuitry can be removed. The conversion from two-of-N to output codes can be placed into a look-up table in ROM, although not all models operated this way, with the MOS chip simply consolidating the encoding and collision detection logic. Using a ROM look-up table for the output codes provides the ability for modifier keys to generate output values that do not bear a bitwise relationship to the underlying key. For example, Shift+1 could produce “*” instead of “!”, and entirely custom modifier keys could be implemented. ROM look-up allows the use of a shared-track approach to the circuitry, as keys need only generate internal or intermediate two-of-N codes.

MOS-based encoding is able to use all possible two-of-N codes, without the constraints imposed by a bank of binary encoders. Thus, the number of encoding grid lines can be reduced to half or less than required by DTL or TTL encoding.

Options

N-key rollover

The use of two-of-N encoding means that only one regular key can be active at once. If two keys were to be held simultaneously, the encoder would receive incorrect or invalid input: the output from the two keys would collide, with the encoder seeing the bitwise OR of the two scancodes (this would likely be an illegal two-of-N code). Thus, N-key rollover in SW Series keyboards carries a different meaning to how it is otherwise understood. SW Series keyboards with N-key rollover do not allow unlimited keys to be held: they simply avoid clashes from keys being struck in rapid succession.

SW Series keyboards with N-key rollover use sink pulse Hall sensors, whose outputs are only active for around 10–100 µs from the point that the key is struck regardless of how long the key is held. This prevents clashes arising, with a trade-off that key release cannot be detected. Before the second key is pressed, the first has already been detected and its sensor has shut itself off. Modifier keys still use current level sensors, and these must be wired separately.

Two-key rollover

Even two-key rollover would be subject to clashes with current level switches. Micro Switch define two-key rollover as an electrical interlock that disables keyboard output while two or more keys are pressed at once. Presumably (as this is not stated) once the number of active keys has reduced back to one, the remaining active key will be reported. This implies that a continuous sequence of keys each pressed with the preceding key still active will not be understood, but occasional instances of a single key pressed too early would be recoverable and not result in a missed keystroke.

The Beehive Mini-Bee terminal keyboard uses TTL two-of-N encoding combined with an electrical monitor circuit. This circuit appears to detect when any two keys are held simultaneously, suggesting a two-key rollover arrangement. This may indicate how two-key-rollover was implemented, as details remain difficult to obtain.

Product Brochure SW reported that Honeywell’s Systems and Research Division found that use of N-key rollover reduced operator error by up to 30% over 2-key rollover. It may help to understand this claim in context: as late as 1967 (shortly before SW Series was introduced) and possibly later, Invac were still advertising their photoelectric keyboards with a mechanical interlock that made depressing two or more keys simultaneously physically impossible. Purely electronic keyboards (with no mechanical contrivances such as solenoids or motors) were still relatively new in the late 1960s.

Key repeat

Key repeat in SW keyboards was achieved by the keyboard repeating the strobe signal. The strobe signal indicates when a key has been pressed and tells the host equipment that there is a key code present on the data bus ready to be collected. There were at least three repeat options:

Bi-level (double action)

Bi-level switches—found in 1SW200 Series—have a larger sensor IC, with two separate Hall elements, positioned at the top and the bottom. The plunger has two metal pins protruding from the bottom, each connected to a stiff spring. As the plunger is depressed, these pins make contact with the switch mounting frame. Pressing the plunger further requires these extra springs to be compressed at the same time.

Bi-level switches still have four terminals on the sensor; they will thus not have redundant outputs. Because these switches are used for auto-repeat, they must be level instead of pulse switches. It is not known whether they are current sourcing or current sinking.

Lock keys

Lock keys are achieved using one of the following methods:

Illumination

SW illuminated switches are centre-lit using a small bi-pin incandescent lamp, the same as those used in Clare and Pendar switches. These have an ivory plunger. The plunger is tubular and rises up around the lamp. Special keycaps are required for illuminated switches.

Illuminated SW Series switches are rarely encounted; they can be seen however in a 101SW1 keyboard (possibly 101SW1-3-H or 101SW1-4-H).

Tactile feedback

Apparent tactile feedback is demonstrated in the switch model 1SW12-BL force curve. This switch is known from the IBM 3277 keyboard. The only characteristic visible in the photos that is different from normal switches is a pair of horizontal ridges across the back of the plunger, that look like they are designed to catch on something. The force curve also has the double step characteristic of Alps tactile switches.

No mention has ever been made of 1SW12-BL switches being tactile or clicky.

Product groupings

Product and part numbers are divided into two main groupings. Components use catalogue listings of the form “SW-” followed by five digits. This grouping includes magnets, shells, plungers, “spacers” (pry tools), PCBs, sensors etc. Switches, keycaps and complete keyboards have catalogue listings where “SW” is prefixed by a number.

Grouping Usage
1SW… Switches
2SW… Keycaps
500SW… Uncertain; includes 500SW90-3, which is a sealed keyboard plunger
Other nSW… Keyboards, where n denotes the number of keys
SW-nnnnn Components

Switches

Packaging

Replacement (-R) switches are supplied together with the spring and a pair of pry tools for removing the failed switch. Typically, the package also included an instruction leaflet for switch replacement; see under Documentation for a copy of the leaflet. Older switches were wrapped in thin brown paper and placed into a cardboard carton, while more recent switches shipped in a clear plastic packet, and often lack the instruction leaflet.

Before the switch can be placed into the keyboard, the return spring needs to be attached to the plunger. This is achieved by placing the spring over the boss on the plunger and rotating it anti-clockwise. It will then lightly grip the plunger and allow the replacement switch to be lowered into position.

The following photos represent neither a real unboxing nor the precise state in which the package contents were supplied. The sequence is a recreation using a previously-unboxed switch, where the box contents were returned to the box in perhaps a less suitable and less expert way than how they were placed originally. However, the instruction leaflet does unfortunately arrive just as creased as depicted.

View full-size image Micro Switch PK-44600 standard shipping carton for smaller components; this was later replaced by clear plastic packets
View full-size image The leaflet is placed around the inside of the box, and the switch is placed wrapped into the centre
View full-size image Box contents: one switch, one return spring (to be fitted separately), two pry tools and the instruction leaflet
View full-size image Unwrapped box contents

Schema

The part number schema for SW Series switches is as follows:

1SWM(-R(F))

“1”
Prefix for keyswitch modules
“SW”
Series name
M
Model number, in a range of at least 1–301
“R”
Denotes a replacement part
F
Operating force in ounces; where omitted, the operating force is the default of 3 oz

Subseries

Switches in SW Series fall into at least two subseries:

Subseries Range Type Numbering
1SW Series 1SW1–1SW99 Non-illuminated Odd sloped, even stepped
1SW100 Series 1SW100–1SW199 Illuminated; non-illuminated accepting illuminated keycaps Unknown
1SW200 Series? 1SW200–1SW299? Includes double action (bi-level) Unknown
1SW300 Series? 1SW300–1SW399? Unknown

1SW1 Series switches appear to exist in pairs, with each odd-numbered model having a straight keystem for sloped keyboards, and each even-numbered model having an angled keystem for stepped keyboards.

1SW100 Series switches are not fitted with lamps: unlike 201SN Series, there are not pairs of part numbers depending on whether a lamp is fitted.

No useful data remains on 1SW300-R and 1SW301-R.

The allocation of part numbers to the secretarial shift types is not known.

Plunger colour

SW Series was advertised in Information Display, Volume 5 Number 6 from November/December 1968. In this advertisement, the plungers were all shown to be grey. Leaflet PK 8503 2 (see Documentation, below) describes two plunger types: one grey (with a “hat shaped boss” for the spring) and one black (with a “plunger spring seat”). This document was drawn up in November 1971, but black plungers go back at least as far as 1970 (see under discovered keyboard examples). At this stage, all plungers of the same age appear to have been the same colour, either grey or black depending on age.

At some point in the mid-1970s, switches became colour-coded, primarily according to the sensor type. The oldest SW Series switch on eBay with a colour-coded plunger is from 1977, while the most recent one with a black plunger (of a type that would change to colour) is from 1974. Inspection of the SW and SN charts has allowed a reconstruction of most of the colour code.

Note that illuminated switches appear to all have ivory sliders, regardless of age and sensor type (just as they do in SN Series). This can be seen in a 101SW1 keyboard (possibly 101SW1-3-H or 101SW1-4-H).

White plungers in non-illuminated switches can be seen in an unidentified custom keyboard; frustratingly there is no photo that clearly shows the Micro Switch labels that would give the chassis model and manufacture date.

Plunger colour Era Colour usage
Grey ca. 1968–1970 Possibly all switches
White Uncertain Uncertain; in the TI Silent 700 Model 733 keyboard it denoted sink pulse
Black Early to mid-1970s Possibly all switches
Mid-1970s onwards Alternate action and dummy switches; bi-level; additional types
Red Mid-1970s onwards Sink level
Pale red
Blue Mid-1970s onwards Sink pulse
Green Mid-1970s onwards Source level

Model interpretation

Precise details on the various switch types are rare, in particular the output type of each model. However, some of these details can be derived from existing information.

Keyboard 54SW11-14 uses 1SW17 switches for most keys and 1SW11 for the modifier keys, which indicates that 1SW17 (sloped) and 1SW18 (stepped) must be sink pulse, with 1SW11 being a level type (sink level or source level). According to the SW and SN charts, 101SN1B1 and 1SW17 both used sensor SD-10023 (replacing SW-11504), and 101SN1B1 is demonstrated to be pulse type by Ed Nisley, as expected from the “B” designation shared with SD Series switch part numbers. This confirms the expectation of 1SW17 and 1SW18 being sink pulse. 1SW Chart 12 indicates that 1SW17 and 1SW37 are both sink pulse (by way of using the same sensor), and implies that both types have blue plungers (much of the data was previously deleted as parts went obsolete). Switch 1SW18 is confirmed to be blue, and the sensor inside is marked “B” as expected.

Keyboard 59SW9-1 has primarily blue switches (which will be sink pulse) with red switches for the modifier and repeat keys, and a few unexplained black switches. The red switches will be a level type. Switches 1SW51 and 1SW52 are both red types, and each have sensors marked “A”. This again corresponds with SD Series designation and would make these sink level.

This leaves green plungers, and source level switches, with the suggestion that these two correspond. 1SW41 is documented as being green, and as having sensor SW-11897. A number of types also have this sensor, including 201SN1C1, 201SN1C2, 201SN4C1 and 201SN4C1. “C” in SD Series denotes source level, which appears to confirm that indeed the green switches are source level as anticipated.

Although it is not possible to be completely certain about the above details, the various details all appear to fit in a way that makes sense.

Models

Switch model numbers appear to have started from 1SW1. 1SW1 uses a grey plunger, as shown in advertisements from 1968. The top of the plunger contains a circular hole, as also seen in some older Licon and Cortron switches, something seen in some later models. By around 1970, the plunger was redesigned (as illustrated in installation guide PK 8503 2), and it appears that the replacement models were given new numbers starting from 1SW11. 1SW1 is presently the only known original switch model.

The “R” suffix indicates a replacement switch sold separately in a box along with an instruction leaflet, return spring and two “spacers” (pry tools). The digits that follow the “R” indicate the weight in ounces of the spring supplied in the box and are only seen on “R” models. The exact spring weights are not yet determined, but taken at face value, “1.5” would be a half force switch (used together with a support switch on wide keys); “2” would be 2 oz semi-light, and “8” would be a heavy 8 oz switch. Testing with an R8 switch demonstrates that it is in the region of twice the weight of a normal switch, but proper testing is impossible without a spare mounting frame to hold the switches together.

Clearer details of the switches will be forthcoming, pending completion of collection and analysis of charts. The details below are a combination of the various charts together with the results of analysis and interpretation. The plunger colours listed are a combination of the documented colours (per the charts) and the observed colours.

A source of “↓” indicates that the switch is given in the keyboards table that follows the switches table.

Catalogue listing Action Output Plunger colour Plunger style Source NSN
1SW1 Momentary Grey Sloped Unspecified keyboard
1SW1-R Parts listings
1SW11 Momentary Source level Black, green Sloped 5930-00-524-0338
1SW11-R eBay (date 7409), 1SW Chart 1 5930-00-524-0338
1SW11-R1.5 1SW Chart 1
1SW12-R Stepped 1SW Chart 1, eBay (date 7703) 5930-01-141-2002
1SW12-BL Momentary tactile Black Stepped Deskthority
1SW13-R Alternate Black Sloped 1SW Chart 1, eBay (date 8121) 5930-01-039-3169
1SW14 NSN 5930-01-174-7525
1SW14-R
1SW15-R Momentary Black Sloped 1SW Chart 1
1SW17 Momentary Sink pulse Black, blue Sloped 5930-01-046-3393
1SW17-R 1SW Chart 1 5930-01-046-3393
1SW17-R1.5 1SW Chart 1
1SW17-R2 eBay (date 9541)
1SW17-R8 eBay (date 8444)
1SW18 Stepped NSN 5930-01-337-6279
1SW18-R eBay 5930-01-155-5558
1SW19-R Momentary Blue Sloped 1SW Chart 1
1SW21-R Momentary Black Sloped 1SW Chart 1
1SW23-R Parts listings
1SW31-R Momentary Green Sloped 1SW Chart 1
1SW37-R Momentary Sink pulse Blue Sloped 1SW Chart 12
1SW41-R Momentary Green Sloped 1SW Chart 1
1SW43-R Momentary Red Sloped 1SW Chart 1
1SW45-R Momentary Green Sloped 1SW Chart 1
1SW51 Momentary Sink level Black, red/pale red Sloped NSN 5930-01-046-0767
1SW51-R 1SW Chart 1, eBay (date 8446), eBay (9511) 5930-01-036-5181
1SW51-R1.5 1SW Chart 1
1SW52 Stepped NSN 5930-01-337-6280
1SW52-R eBay (date 8742)
1SW52-R8 1SW Chart 1
1SW53-R Alternate Black Sloped 1SW Chart 1 5930-01-175-8352
1SW54-R Stepped 1SW Chart 1
1SW55-R R&J Components Corp
1SW71 Black Sloped stemless eBay (date 9412)
1SW101-R Momentary illuminated White Sloped 1SW100 Chart 1 5930-01-039-3167
1SW107 1SW100 Chart 3
1SW107-R 1SW100 Chart 3
1SW108-R Momentary, no lamp terminals 1SW100 Chart 3 5930-01-039-3168
1SW111-R Parts listings
1SW111-HR Momentary illuminated Logic scan White Stepped 1SW100 Chart 34
1SW113 Alternate action illuminated Sink level Sloped 1SW100 Chart 6
1SW113-R
1SW117 Momentary, illuminated Sink pulse White Sloped 1SW100 Chart 5
1SW117-R
1SW118
1SW118-R
1SW121 Momentary illuminated Logic scan White Sloped 1SW100 Chart 7 5930-01-354-7030
1SW121-R
1SW151-R Indicator None 1SW100 Chart 4
1SW152-R 5930-01-039-3170
1SW181 Momentary illuminated Sink level White Sloped 1SW100 Chart 1
1SW181-R Momentary 1SW100 Chart 1, eBay
1SW201-R Double action (bi-level) Black Sloped Own collection
1SW204-R Double action (bi-level) Black Stepped Own collection
1SW300-R “Momentary” Black Stepped 1SW Chart 1
1SW301-R “Momentary” Black Sloped 1SW Chart 1

Keyboards

Schema

The model number schema for SW Series keyboards is as follows:

KSWA-B

K
Number of keys (at least three, as “1” in this position denotes a single switch and “2” denotes a keycap)
“SW”
Series name
A
Unknown
B
Unknown

Subseries

There are at least two known subseries:

12SW Series
Current sinking non-encoded 12-station keypads (can be paired as a 24-station assembly)
16SW Series
Current sinking non-encoded 16-station keypads (can be paired as a 32-station assembly)

Documented

For more details, see Documentation below.

Model Description Output Rollover Lock Keycaps Source
50SW11-50 Keypunch Keyboard System 3 Code NKRO Sloped Product Brochure SW (373)
51SW5-1 Key-to-Tape/Disc Keyboard EBCDIC 2KRO Sloped Product Brochure SW (373)
51SW12-1 Typewriter Keyboard 6-bit address code NKRO Secretary shift lock Sculptured Product Brochure SW (373)
53SW1-2 Teleprinter Keyboard US ASCII 2KRO Sloped Product Brochure SW (373)
61SW12-1 Communications Keyboard Full US ASCII NKRO Electronic with LED Sculptured Product Brochure SW (373)
63SW5-4 Teleprinter Keyboard Full US ASCII 2KRO Alternate action Sloped Product Brochure SW (373)
70SW12-1 Remote Batch Keyboard US ASCII NKRO ? Sculptured Product Brochure SW (373)
75SW12-2 Communications Keyboard US ASCII NKRO Alternate action Sculptured Product Brochure SW (373)

Discovered

The following is not a complete list of all known SW keyboards. It only lists keyboards where at least one switch model is shown, or where it is particularly early or late production. In almost all instances of SW keyboards found online, the switch part numbers are not all shown, or are not shown at all (except for where it seems the switches were not marked in the factory).

Keyboard Micro Switch model Switches Date code Notes Reference
Bare assembly K50326-62SW3 1SW1; possibly others 6933 Sketchfab
Bare assembly 64SW1-4 7012 Earliest example of black plungers Flickr
Bare assembly 12SW1-1 1SW11 7040 Earliest example of 1SW11, earliest visible date 7003 KBref
Hitachi keyboard 60SW5230-173 1SW11, …? 3-74 Manufactured by Micro Switch Yamatake-Honeywell, Tokyo Deskthority
Decision Data 8010 54SW11-14 1SW17 (sink pulse), 1SW11 (source level) 74/34 Flickr
Texas Instruments 914 keyboard 102SW11-1 75/02 A rare example of SW illuminated within a keyboard Deskthority
Engineering prototype K54133-16SW17 7550 Custom keycap mount KBref
Unknown switch panel 8SW3-2 Possibly 1SW11 (unmarked) 7837 The smallest key count to date KAref
Custom keyboard Unreadable 1SW17 (sink pulse), 1SW51, … ca. 1980 Deskthority
Honeywell TDC keyboard assembly 101SW1-4-H Sink pulse, illuminated, … 9546 NRI Industrial Sales

Keycaps

Keycaps for SW, SN and SD switches all come under 2SW. Part Target list 326 separate entries for keycaps, and this list appears to be very far from complete, as presumably every combination of legend and colour has its own part number and, where necessary, its own NATO Stock Number (NSN). There is no single catalogue listing format for keycaps; the catalogue listing numbers are quite varied in their format, and at times very long and complicated (e.g. “2SW1-4C-M-4-2-1598C”). In Part Target’s search result, 2SW1 is associated with around 26 different NSNs.

Standard 2SW keycaps are 0.475″ tall.

The catalogue listing numbers were largely not depicted and varied in formatting; thus, they have been normalised according to the Part Target listings for consistency.

Cat. listing Type Size Mount Colour Legend NSN Reference
2SW1-M-4-2-4928 Double-shot 1 unit SW Green PAGE UP 5930-01-213-1690 eBay
2SW1-M-5-2-95 Double-shot 1 unit SW Black N / SO 5930-01-212-5854 eBay
2SW1-M-7-2-282 Double-shot 1 unit SW Black 4 / $ 5930-01-212-2998 eBay
2SW3-9 Blank 1 unit ? ? eBay
2SW603-11 Relegendable 1 unit ? Beige eBay
2SW701-04D-N Illuminated 1 unit SW illuminated Green eBay
2SW79105 Blank 2 unit SW Black Deskthority

Other components

Cat. listing Component
500SW90-3 Sealed keyboard plunger (eBay); these can be seen in a 26SW3-3-S keypad
500SW90-5 Uncertain (NSN 5930-01-368-9087), allegedly a pushbutton switch
SW-10485 Keycap puller; mentioned in the Fairlight CMI System Service Manual (for the CMI IIx)
SW-11485 Keycap puller; depicted in PK 8919 2 for use with SD Series

Maintenance

When fitting a replacement switch module, or returning a module to the keyboard, the return spring needs to be attached or reattached to the plunger. PK 8503 2 instructs repair technicians to “place the spring on the spring boss and compress and rotate it until the last coil of the spring expands around the boss”. For those who instinctively try to rotate the spring clockwise, note that in fact the spring must be rotated anti-clockwise. The leaflet recommends a pencil eraser for the task, but bare fingers are sufficient. Only a small amount of rotation is required for the spring to remain attached to the plunger, although this process can be a bit finicky.

Documentation

Honeywell have kindly provided a collection of SW series switch charts. Only a subset of the charts appear to have survived. The charts themselves were extensively redacted by Honeywell as switches went end-of-live, but they do have some older versions of the charts (as is the case with 1SW Series Chart 1), and with luck, they may find some more older revisions with more switch details.

Product Brochure SW “Solid State Keyboards” and Product Sheet 51SW12-1 “Typewriter Keyboard” are in item SILNMAHTL_31166 at the National Museum of American History Library (part of the Smithsonian Institution), who kindly provided scanned copies of both documents. The dates for these documents are derived from the three-digit codes placed at the bottom of the last page of each. Product Brochure SW—seemingly from March 1973—is mentioned on Electronic Design 17, page 118, from the 16th of August, 1973.

Circuits

The Mini Bee keyboard schematics are from the Mini Bee Computer Terminal Service Manual, scanned by Bitsavers. The keyboard model (53SW1-8) is from the separate Illustrated Parts Breakdown.

Several documents included with a Micro Switch keyboard can be found in a single PDF on the MMcM/parallel-ascii-kbd Wiki on GitHub for a keyboard with PCB model SW-11234. This includes the instruction leaflet PK 8911 1, the specification chart, the PCB layout and circuit diagram, and the physical dimensions of 55SW5-2, even though the keyboard built on SW-11234 is a 56-key keyboard with slightly different keycap widths. The PCB layout gives a good indication of how the MOS two-of-N arrangement is laid out.

See also