Keytek was a division of Elco Corporation, responsible for computer keyboards; Elco itself was in turn a Gulf+Western Manufacturing Company. They specialised in inductive keyboards, branded “Inductric”.
Inductric was a line of inductive computer keyboards. It first appeared as Mechanical Enterprises Inductric, but by 1982, it was a Keytek product, advertised in various magazines. An article in Electronic Products magazine in September 1982 indicates that Keytek was supplying prototypes of the then-new DIN-compliant version of the Inductric keyboard. Key travel in the DIN-compliant redesign was reduced from around 0.150″ (3.8 mm) to 0.130″ (3.3 mm). Product manager Guy Russel reported to the magazine that the new keyboards would be offered “with two types of tactile feedback: a standard linear pressure and a tactile differentiation” with the tactile feedback intended as a way to compensate for the reduced travel. Thus far, no tactile examples are known to have been discovered.
A 1984 advertisement for model 88KT-15 indicates that it is moving ferrite inductive notes the following:
Depressing a key moves a ferrite core into the field of a balanced differential transformer etched on the PC board. As inductive coupling changes, the transformer becomes unbalanced so a signal is received by the pulse circuitry.
An earlier, 1983 advertisement for models 84KT-14 (unenclosed) and style 84KT-14E (enclosed) describe the keyboard as having contactless keyswitch modules that “require no physical connection to the PCB”.
Although the one known IMS International keyboard from circa 1983 bears IMS International branding and a chip marked “IMS”, the switches are identical to those of Keytek Inductric.
The relationship if any between Keytek and ADI ferrite core keyboards is not known. The general design is the same, but there are noticeable differences, including some ADI keyboards doubling-up on winding tracks in one axis. It is possible that the technology was sold off to ADI at some stage.
Keytek also placed an advertisment “Keyboard Fits IBM Micro” in Computerworld in October 1983. This keyboard, said to be compatible with the IBM Personal Computer, was listed as microprocessor-based, with a price tag of $250. A cable included with the product allowed it to be connected either to an IBM PC or to systems expecting a serial TTL ASCII keyboard.
The following diagram was published, courtesy of Keytek in the article Keyboards downsize to meet DIN requirements, Electronic Products magazine, September 30 1982:
The diagram shows a single key with two pairs of loops, one primary–secondary pair controlled by the ferrite rod (“active”) and the other loop pair passive. Although this is how the switch was patented, earlier Inductric PCBs have (broadly) one passive loop pair per row (or column), while later keyboards (including those from ADI) seem to share each passive loop set between two active loop sets. Inventor Victor Bernin described these pairs of loop pairs as “differential transformers”: one of the four loops is wired in reverse, causing the two loop pairs to cancel out. As shown in the patent, an inactive key is set up as follows:
The polarity of the induced current is opposite in one loop pair with respect to the other, resulting in a net current of zero. When a key is pressed, the ferrite core is lowered into the first pair of loops, causing a much higher level of induction, and removing the balance between the pair of loops; the diagram below is also based on one of the patent diagrams:
It is clear from the information provided that these loop pairs couple without the need of a transformer core. What is less clear is how the early Inductric keyboards, with very few passive loop pairs, achieved balance. Later keyboards appear to share each passive loop set with two active loop sets, both Keytek and ADI.
The reason for this arrangement is not explained as well as it could be. US patent 4300127 “Solid state noncontacting keyboard employing a differential transformer element” notes:
The advantage of a differential transformer over a simple transformer switch configuration is due to the fact that a high degree of balance can be obtained in a printed coil type differential transformer, which translates to a switch with a high on/off ratio or dynamic range which is a desirable switch characteristic. This fact, plus the fact that printed coils are very inexpensive, results in a switch design that is relatively high in performance and low in cost.
- Unidentified IMS 5000 IS keyboard, IMS-branded PCB, circa 1983
- Falco TS-2624 keyboard, circa 1983
- Leading Edge Model M keyboard, DIN-compliant linear, circa 1983
All material was scanned by Bitsavers unless otherwise noted.