Datanetics batch-fabricated array

Contents

• Overview
• Origin
• Patents
• Design
• Encoding
• Examples
  • Promotional photographs
  • Discovered examples
• Documentation

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 discrete 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.

Origin

As noted under Datanetics history, the membrane keyboards were based on prior work on NCR’s CRAM 2 data storage system. The flexible cards used in the CRAM appear to have been considered a good means for serving as an array of keyboard switches. To minimise contact resistance, gold (or gold alloy) was selected for the contact surfaces. The electroless plating system required an underlying metal onto which to deposit the gold, and cobalt-nickel was used for this purpose, onto which the gold was deposited.

Dick Gerlach devised the encoding system that these keyboards used, where some switches had two sets of contacts, with the top contacts closing first. The exact method of the encoding operation is not presently documented anywere.

Patents

Design

In co-inventor Meryl Miller’s notes, these keyboards were described as follows (emphasis as in original):

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.

Meryl Miller provided a set of photographs of a TIPI keyboard showing the unusually complex membrane assembly:

The diaphragm (circuit) sheets were made from Mylar. The spacer sheets were originally made of “polyester” (presumably also Mylar) with die-cut holes according to Meryl; this was later changed to Kapton as it was available in thinner gauges.

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, manufactured in 1970, appears to use TTL matrix scan encoding.

Examples

Promotional photographs

Meryl Miller provided the following promotional photographs of batch-fabricated keypads and keyboards:

Note that the streaky appearance of these photographs is owed to Hewlett-Packard’s inability to construct a working scanner and is no reflection on the talent of the product photograpers, Bill Butterfield Commercial Photography and William R. Eastabrook Photography.

Discovered examples

Documentation

• Five New Keyboards From Datanetics Will Feature MSI, Computerworld, August 12 1970, from the Internet Archive