Mirror manufacturer Donnelly Mirrors, Inc. joined the keyboard market with a rather curious approach to encoding keyboards. Little is known about their keyboards, and no confirmed sightings have been made.
In 1972, Donnelly Mirrors filed two patents for their encoding keyboard design:
- US patent 3777082 “Keyboard switch assembly with improved movable contact having cantilever supported central member with radially extending contact fingers”, filed 1972-09-08
- US patent 3803592 “Keyboard and digital circuit therefor”, filed 1972-09-11
The drawings are unusually unclear; the concept in general needs to be depicted in a three-dimensional illustration, but instead a number of indecipherable two-dimensional drawings are shown. In brief, however, this is an encoding design where each switch contains a one-to-many contact arrangement connected to a multiple-bit encoding bus. A single keypress will connect a group of switch contacts, each one setting an appropriate output bit. The movable contacts are etched out of a material such as beryllium copper, and the etched contact layer is affixed to a flexible sheet, such as rubber. The stationary contacts are placed on a substrate below, which could be a printed circuit board. The etched sheet is not flat: the movable contacts take the form of fingers that extend towards the movable contacts. In order to combat switch bounce, and the output signal being read prematurely, the fingers take different angles in order to stagger their connection times, and the strobe connection is made last. This will not itself remove contact bounce, but it helps ensure that the output code does not change after the strobe signal is received.
As with most of not all other encoding keyboard designs, simultaneous registration of two or more keys is impossible. Since the strobe signal is mechanically generated, normal rollover processing will not work. When a second key is operated with a prior key still held, no new output is generated. When the first key is released, the logic circuitry recreates the strobe signal that would have been utilised if the key had been struck in isolation.
Considering the comparatively recent point in time that this design was introduced, the patent covers the use of a ROM to hold the encoding data, as well as the use of a single chip to contain the encoding logic.
The Focus on Keyboards article from Electronic Design in November 1972 briefly mentions Donnelly Mirrors keyboards. An illustration captioned “Donnelly Mirrors evaluates the lifetimes of its flexible-diaphragm keyswitch on this test rig” depicts a full-travel keyboard undergoing testing. All that is said otherwise about these keyboards is that they are classified as “elastomeric, or elastic-diaphragm, switch”. It is not conclusive that the design is the same as that covered by the patents, but it is fairly likely that the magazine article is talking about the same product range.