Switch operating functions
- Alternate action
- Secretarial shift
Alternate action switches change between open and closed on each alternate cycle.
In most cases, they use a “heart cam” design, a roughly heart-shaped cam track parallel with with plunger’s motion. A follower pin moves around this track, and when it reaches the top of the track, it holds the plunger in its locked position. This gives a visual indication of the state of the switch. The cams on the heart-shaped track appear to exist to ensure that the follower pin traverses the track in the correct direction. The existence of the cams means that the follower pin must be permitted to move perpendicularly to the track; this is often achieved by placing a spring behind the pin (a leaf spring in the case of Alps KFL, and a coil spring in SMK J-M0404 and RAFI switches), but in some cases (as with Datanetics DC-50 and DC-60 series, and Alps SKCL) a flexible plastic arm or steel wire is used instead.
Cherry however preferred rotary mechanisms. They offered a non-latching arrangement for their E series microswitches, as seen in Cherry Precision Switches catalogue C-663 (dated 1965). Here, their E33-00G and E34-00G “push-push” assemblies use a rotary intermediate part within an external assembly. The diagram shows that after actuating the switch, the plunger returns to its home position.
Cherry would later use a similar design with M6 switches. US patent 3770923 filed in January 1972 depicts an alternate action system with a rotary latching system. This design would be used again in MX series switches.
The only other manufacturer known to use a rotary system in a keyboard switch is Omron; their design (as shown in US patent 4495391) is very similar to that of Cherry’s, but the latch wheel has a rotational symmetry of 180° instead of 120°.
Secretarial, or secretary shift is a shift lock mechanism in electronic keyboards that mimics the way that shift lock works on a manual typewriter. In typewriters, the shift keys physically shift the type basket, and pressing shift lock also locks the type basket into position. Releasing the type basket involves pressing shift.
Lock keys in electronic keyboards typically handle lock keys using either an indicator lamp or by way of an alternate action key. To reduce training costs when moving staff from manual typewriters to electric typewriters or terminals, some products were fitted with a mechanism that mimicked the mechanical shift lock of manual typewriters. Here, the shift lock keyswitch would be mechanically locked when pressed, and then released by either shift key.
Secretarial shift was offered by a number of electronic keyboard manufacturers. The following are listed in approximate date order.
No details on the Micro Switch SW Series implementation have been discovered; its existence is known from Product Brochure SW (373). The date of introduction is not known, but as SW Series was introduced around 1968, it is possible that Micro Switch had this design ready at this point. Secretarial shift (as secretary shift) was also offered with SD Series, introduced in 1975 or 1976, and likewise no details on the mechanism are known.
Clare-Pendar’s design is covered by US patent 3626120, filed in September 1970. A sliding plate held by a coil spring is pulled into a notch in the keystem of the shift lock key. A vertical bar is placed above each shift key, allowing the keycap itself to push on the bar and slide the plate aside, releasing the shift lock key.
Cherry supported secretarial shift with M6 series, and described their mechanism in US patent 3678255, filed June 1971 (although the switch shown in the patent is their reed switch type that has never been seen). Where Clare-Pendar used a coil spring, Cherry used a flat spring that rests against a keyswitch. The sliding plate of Clare-Pendar’s design is replaced with a rotating plate attached onto the shift key. This is one of the very few designs that has been observed; it can be seen in a B80-64AB keyboard of unknown purpose.
US patent 4071719—filed in July 1976—covers mechanical shift lock for one of the older series of Licon/Cortron ferrite core switches. ITW filed a later patent, in February 1980—US patent 4295012—covering the successor ferrite core switch series.
Marquardt’s “butterfly” switch series supported secretarial shift, and this was used in Olympia typewriters. No patent or documentation has been found for their mechanism.
Double-action, two-stage or bi-level switches have two separate actuation levels. Pressing the switch normally will cause it to register its normal output. Pressing it further will cause it to register its secondary output. To avoid accidentally triggering the secondary output, the force of the second stage is very high. Double-action switches are most commonly use for auto-repeat: instead of holding a key for a set amount of time to trigger auto-repeat, the key is pressed harder. The advantage of this is that the operator has direct control of when auto-repeat begins (with no need to wait through the initial delay). The disadvantage is that auto-repeat cannot be applied to every key, as it would mean having complex and expensive switches on every key and require double the size of the switch matrix. Double-action switches are normally found in electronic typewriters under backspace, enter and space.