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Keyboard switch characteristics

The following is a brief guide to switch terminology with a focus on the types of switches found in keyboards. Most of these types are not found in keyboards, but presumably to increase flexibility and profitability, many keyboard switch series offered additional switch configurations for non-keyboard applications. As such, examples are given of switch configurations commonly encountered in older keyboard switch series.

Please note that this page is not authoritative and may contain errors. Due to inconsistencies within the industry, terminology of switches varies between manufacturers.

Contents

Terminology

The following is a brief explanation of terms relating to switch configuration. These will become clearer once the following section has been reviewed.

Open
A switch is “open” (as an adjective) when it does not conduct electricity, i.e. when the switch is “off”. Just as with an open circuit, there is a gap that prevents electricity from flowing through it. As a verb, “open” means the same as “break”, below.
Closed (adjective)
A closed switch conducts electricity (the gap in the circuit has been closed); that is, a closed switch is “on”.
Make (verb)
To close the switch contacts and make an electrical connection.
Break (verb)
To open the switch contacts and break the electrical connection.
Normally open (NO)
The switch contacts are open when the switch is released, and closed when the switch is pressed. When used to describe a pushbutton switch (such as in a keyboard), it means that the switch conducts electricity when it is pressed. This is the arrangement used for keyboard keys and in other simple pushbuttons.
Normally closed (NC)
The switch contacts are closed when the switch is released, and open when the switch is pressed: pressing the switch temporarily disconnects the electrical circuit. This is the opposite of normally open, and is not used in keyboards.
Pole
See below.
Throw
See below.

Poles

Each separate (and electrically independent) set of switch contacts is called a “pole”. A simple switch, that controls a single electrical circuit, has one pole (and one throw) and is referred to as “single pole single throw”. Each pole has an input terminal (where electric current enters), and one output terminal for each throw.

A switch with two or more poles is effectively a set of separate switches controlled by a single button, rocker, toggle or lever.

Throws

A “throw” is a non-off position for a switch. As noted above, a simple switch is single pole single throw: it controls a single circuit, and its only positions are off and on. As such, it has only a single on position, i.e. one throw.

If a switch can connect the input terminal to either of two output terminals, it is denoted “double throw”. A double throw switch has two possible exits for the current:

The number of throws is relative to each pole, not to the switch as a whole. Therefore, a double pole double throw switch has two poles (two sub-switches) and each pole has two throws each (rather than two throws across the entire unit).

Forms

The “form” of a switch is a shorthand for—depending on usage—the arrangement of the contacts or of the switch as a whole.

Form A
An alternative term for SPST-NO; in some cases form A can refer simply to NO, with SPST-NO in these instances written as “1 A” or “1 form A”
Form B
An alternative term for SPST-NC or sometimes simply NC
Form C
An alternative term for SPDT

Common configurations

In the diagrams below, the released state of each switch is drawn with its circuit diagram symbol. The pressed state is drawn with a modified symbol purely for illustrative purposes, to show switch operation. Current flowing through a switch is shown in red.

As noted previously, most of the configurations given below are not used in keyboards, but were offered in a number of keyboard switch series. The examples should help explain the options listed by manufacturers in their literature.

Both the pole/throw/type (e.g. “SPST-NO”) and count/form (e.g. “1A”) notations are used, with the latter given in Cherry format.

SPST-NO, 1A

Single pole, single throw, normally open (form A, or 1A in Cherry notation) is the typical configuration of a keyboard switch. A single circuit pathway is connected when the switch is pressed.

DPST-NO, 2A

Double pole, single throw, normally open (2A in Cherry notation) is seldom found in keyboards. A number of switch series offer this as an option, but keyboards almost always use SPST.

For keyboards, DPST has the advantage of allowing the switch to continue functioning even if one pair of contacts becomes contaminated or defective. If one contact pair should develop chatter, this should also not pose a problem as the other pair of contacts will continue to make reliable contact.

So far, only one keyboard has been found that appears to have simple redundant switching with DPST switches, is a Cherry G80-0229 with an undocumented variant of Cherry M7.

Other examples with DPST switches do not appear to be wired for simple redundancy. These are the Bendix-made NASA keyboard (with Clare-Pendar S84020 switches) and the Siemens SIMATIC S5 PG675 keyboard (the Cherry-made version with M8 switches — the Siemens-made keyboard has not been examined). The Bendix keyboard has some peculiar arrangement where the terminals on one side are connected together (as you would expect for redundant wiring) but those on the other side are not connected to each other; in fact, the whole keyboard is wired in a very unconventional arrangement with lots of sub-matrices. The Siemens keyboard does seem to be wired for redundant sensing, although some sections seem to use a switch variant with keystroke velocity sensing instead of plain DPST, for no apparent reason (a type of M8 switch without a known part number).

Switch series confirmed by literature or observation to offer DPST include: Cherry M5, M6, M7, M8 and M9; Clare/Pendar high-profile reed; Clare-Pendar S840; Datanetics DC-50; GRI KB and KBLP; MEI T-5.

Switch series where DPST appears to be supported include: FR Electronics RSM and RRE RKS (“1A” in the part numbers printed on the switches, and space for a second magnet and reed capsule), Unitra Dolam M-24-112 (also with space for two magnets and reed capsules).

Switch series where DPST is a possibility (due to switch design) but remains unconfirmed include: SMK J-M0404 (the angled keystem sliders have two actuator ramps), Mitsumi full-size switches (two contact assemblies can be fitted, but this has only been seen as double action to date).

The Cherry MX patent depicts DPST, but this has never been seen.

SPST-NC, 1B

Single pole, single throw, normally open (form B, or 1B in Cherry notation) is an option provided by a number of series, but not for keyboard applications. Here, pressing the switch temporarily breaks the circuit.

Switch series confirmed by literature or observation to offer SPST-NC include: Cherry M7, M8 and M9; GRI KB and KBLP; MEI T-5

DPST-NO/NC, 1A+1B

This is a curious configuration that appears to be fairly rare. It is a double-pole switch with one pair of normally-open contacts and one pair of normally-closed contacts. One would figure that this would be designated “DPST-NO+NC”, but it seems that it is NO/NC based on the RS listing for Otto P9 series. However, GRI designated it DPST-NO-NC.

Pressing the switch temporarily disconnects the active circuit pathway and connects the inactive pathway. Releasing the switch reverts this change.

One possible reason for this design is that it provides a way to implement single pole double throw (sometimes called single-pole changeover) without needing any custom parts. By connecting the two input terminals, the resulting arrangement is equivalent to an SPDT switch.

This type of switch is also unlikely to occur in keyboards.

Switch series that offer DPST-NO/NC include: Cherry M6, M7, M8 and M9; GRI KB and KBLP; MEI T-5.

There are also switch series that included changeover as an actual configuration. This is confirmed for Siemens STB 11 and GRI KB and KBLP; and Sasse series 25 also offered one or more changeover types.

Unusual configurations

Double-throw chaining

UNIVAC keyboards using Micro Switch 1PB87x switches use an unusual technique of chaining double-throw switches. Most switches have their common (input) terminal connected to the previous switch’s normally-closed terminal, meaning that each switch is fed from the previous switch, so long as that switch remains in its idle state:

As soon as a switch is pressed, current flows from common to the normally-open terminal, and all further switches down the chain lose their source of power:

This produces a keyboard that is incapable of arbitrary rollover. However, the keyboards using this technique had a separate output PCB track from each switch (for example, to feed into a diode matrix), and this chaining technique may have been chosen to simplify the PCB routing.

Action

See switch operating functions.

Further reading