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Datanetics batch-fabricated array

Contents

Overview

Datanetics’ first keyboard type was the batch-fabricated “array” system (or elastic diaphragm array). This is one of the earliest if not the earliest full-travel membrane keyboard; Datanetics themselves used the term “elastic diaphragm” instead of “membrane”. The (likely retrospective) term “batch-fabricated” indicates that all the circuitry and switching was manufactured collectively, rather than using discrete switches. The use of membrane technology was continued with the DC-50 switches, although that design used flat metal strips glued to the membrane pieces instead of screen-printed conductive material.

Such keyboards are extremely rare, and as such, very little is known about production variants.

Patents

Patent Title Filed Published Aspect
US 3594684 Electrical interconnection system for multilayer circuitry 1969-05-12 1971-07-20 Membrane-to-PCB connection
US 3663779 Sealed electrical package 1969-05-12 1972-05-16 Sealed membrane assembly
US 3712840 Method of making packages 1969-09-08 1973-01-23 Sealed membrane assembly
US 3773997 Key assembly diaphragm switch actuator with overtravel and feel mechanisms 1971-12-13 1973-11-20 Full travel actuator

Design

In co-inventor Meryl Miller’s notes, these keyboards were described as follows:

1968 – Datanetics Corp. - Developed full travel keyboards utilizing membrane switch technology. First Customer: RCA ( Van Nuys,CA) - some military ( TIPI ) and some commercial

Switch arrangement consisted of multiple contacts for each key positon that closed sequentially. Initial designs used 3 polyester membranes, upper, middle and lower and were positioned on top of a printed circuit board having gold plated contacts.
The Upper membrane had gold plated contacts on the lower or under side, The Middle and Lower Membranes had gold plated contacts on both the upper and lower surfaces. Dielectric spacers with die-cut holes and made of Kapton , separated the 3 membrane layers. Two thin layers of silicone rubber, one on top and one on the bottom completed the switch sandwich. Sealing was accomplished by means of RTV silicone around the perimeter. An expandable silicone rubber bladder was used to accommodate changes in pressure, due to atmospheric changes.

Later designs, used for commercial applications, used 1 less membrane, 2 instead of 3. Otherwise, the switch stack-up was similar. Keytops for prototypes and initial production units were sourced from IBM at $.30 each, regardless of quantity. Later production units used our own molded keytops, but required engraving and filling of legends.

Membrane layers were produced from 5 inch wide strips of either 3mil or 5mil polyester and coating it with a cobalt-nickel alloy, using a proprietary process developed by Dr. Wilhelm of Thin Films Co. in Los Angeles. Switch patterns unique to each membrane surface were silkscreened with an resist. Unwanted conductive material was then removed using an etch process. Subsequently, the resist was removed leaving the patterns of conductive material. Next, the remaining cobalt-nickel circuitry was gold plated using an immersion process.

A mechanical assembly consisting of plungers, guides, return springs, keytops and sheet metal was placed on top of the membrane switch sandwich. ASCII coded output was accomplished by means of the unique circuit routing on the membranes and by the use of discrete TTL transistor logic circuitry. The design required that the upper contact always closed first. To insure this, the upper membrane was made of 3 mil material and the spacer layer under it utilized larger holes than the second spacer layer. No adhesive was used on the spacers.

The method of interconnecting the circuits on the 3 active membrane surfaces to the rigid printed circuit board and to the external I/O wiring was patented. Inventor: Meryl Miller.

1969 – Datanetics Corp. - Produced first large quantity commercial full travel membrane switch keyboard for Conrac Corporation for use with Burroughs CRT computer terminals.

Encoding

None of the patents document the encoding methods used. Meryl’s notes above form the only description of the two-stage sequential encoder. In Users’ choice is name of keyboard game in Electronics, Vol. 42 No. 23, November 10 1969, Datanetics diaphragm keyboards are described as using diode matrix encoding, which was the industry standard technique at that time. Datanetics part 01-02-638 listed on eBay, and manufactured in 1970, appears to use pure TTL encoding, with neither a diode matrix nor multi-step encoding (considering the number of ICs used).

Documentation