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Functional electronic circuits

In the 1950s, the invention of the transistor made it possible to build an electronic circuit in much less space, using far less energy, than was possible with valves (tubes). This led to a wave of miniaturisation that would eventually lead to the development of the Integrated Circuit (IC).
One manufacturing method that was frequently used from the mid-1950s onwards, was the so-called cordwood structure: conventional parts like resistors and transistors, mounted vertically between two parallel printed circuit boards (PCBs). It became quite common to divide an electronic circuit into functional blocks, each of which were then built as a cordwood module.

The image on the right shows an example of a standard flip-flop circuit that is built as a cord­wood structure. Note that many thousands of these flip-flops are fitted on a few mm2 today.
Early cardwood module, showing a single flip-flop circuit. Photograph Copyright Jeff Wisnia [4].

Cordwood modules came in a wide variety of dimensions and functions and were by no means standardized. In the late 1950s and early 1960s, computer manufacturers, such as IBM, Univac and Burroughs, used cordwood circuits to make their systems smaller, more flexible and more service friendly. In their race to the moon, NASA 1 made heavy use of cordwood circuits in the Apollo program, where size, weight and power consumption were major design constraints.
When the NSA made the transition from valves (tubes) to transistors for the development of their (digital) cipher machines, it was decided to develop a series of standard functional blocks, or modules, from which the entire machine could be constructed. By assigning each module a generic function, such as AND, NOR, XOR, flip-flop, adder, comparator, noise generator, etc., they could be reused in other designs. Furthermore, it greatly simplified and standardized board repair. The project to develop these cordwood modules, was given the NSA name FLYBALL. 2
Development of the FLYBALL modules took place at Sylvania Electronic Systems-East in Needham (Massachusetts, USA), under the super­vision of Maurice I. Crystal, an engineer who had joined the company in 1957 and (co)developed the logic circuits and memory systems for the DoD MOBIDIC 3 and ASD-1 computer projects [2].

Each FLYBALL module was given a different logic function and was identified by its own specific colour, such as the red one shown on the right.
Red FLYBALL module cast in epoxy, on display at the NCM. Photograph by Doug Eyre [5].

The image above shows the various manufacturing stages of a FLYBALL module, cast in epoxy, as it is on display at the NCM in Fort Meade (Maryland, USA). Standard electronic components, like resistors, capacitors, diodes and transistors, are mounted between two double-side printed circuit boards (PCBs), with its contact leads extending at the bottom side. Once the circuit was complete and tested, it was potted in a very hard epoxy-like substance of a specific colour. The reason for this was twofold: it protected the circuit again moist, but also against prying eyes and reverse engineering. Removing the substance would almost certainly result in a damaged circuit.
Each type of module was given its own specific colour to identify its function. Although there is no information about the colours and functions of the FLYBALL modules in the public domain, we hope to be able to present a complete list in due course, which we will share on this page.

Flyball modules were used in NSA encryption devices, such as the KW-7 and the KG-13. The KW-7 consists of 12 plug-in cards that can each hold up to 21 FLYBALL modules, organised as 3 rows of 7 modules each. The image on the right shows one of the KW-7 boards as an example.

Although the FLYBALL modules greatly reduced the size and weight of the machines, they would soon be superceeded by the Transistor-Transistor-Logic (TTL) that became available in 1967. Nevertheless, they remained in production for several years as the KW-7 and KG-13 machines were widely spread and new machines and spare parts had to be supplied. By 1980, the FLYBALL modules had become extinct and some of the KW-7 boards were redesigned with modern ICs.
  1. NASA = National Aeronautics & Space Administration.
  2. FLYBALL is sometimes written as FLY BALL.
  3. MOBIDIC was a defense Mobile Digital Computer development program carried out at Sylvania.

All FLYBALL modules have the same form factor, but each type has its own unique colour and a specific layout of the contact leads. The diagram below shows the red FLYBALL module with part number ONO 017841 as an example. The arrow at the top is used to determine the orientation.

Each module measures 48.5 x 16.7 mm and is 19.6 mm high. The contact leads extend from the bottom of the unit and are arranged as two rows, spaced 10 mm apart, similar to the later dual in line (DIL) ICs. When mounting a FLYBALL module a PCB, the contact leads protrude pre-drilled holes and are then bended and soldered onto the required contact pads at the bottom of the PCB.
E-AJN card Close-up of the flyball modules Solder side Footprint of the grey flyball module at the solder side of the PCB

Although the actual FLYBALL modules were never patented, there are some patents of similar modules that were filed in the era. The image below shows a patent drawing of August 1960 for a very similar manufacturing technique.

Example of a 1960 patent for a cordwood electronic circuit [3]

Flyball Modules
Below is a list of the circuit blocks that have been identified. We currently have no information about the logical function of each block, so any additional information would be most welcome.
ONO number Colour Description
07836 Yellow Unknown, but probably XOR circuit
07837 Bright yellow ?
07838 Orange ?
07840 Bright red ?
07841 Red Unknown, but probably D-type flip-flop (shift register)
07842 Grey ?
07843 Black ?
07844 Brown ?
07845 White ?
07846 Dark blue ?
07847 Blue ?
07848 Light blue ?
07850 Violet ?
07852 Bright orange ?
07853 Green ?
07855 Light green ?
08350 - Relay, Hi-G Inc. 2WPBA-21.1
08575 White ?
08654 Metal ?
08851 Mint ?

FLYBALL modules were used in the following NSA devices:
  1. Jerry Proc, Flyball Modules
    Retrieved May 2016.

  2. E.W. Jervis, MOBIDIC D, Final Report, 1 July 1958 - 1 February 1963
    FR63-3N. Sylvania Electronic Systems-East, February 1963. p. 7-5.

  3. Vito D. Elarde, US Patent 3151278
    Filed 22 August 1960. Issued 29 September 1964.

  4. Jeff Wisnia, Image of early cordwood circuit with single flop-flop
    Retrieved May 2016.

  5. Doug Eyre, Photograph of Sylvania FLYBALL module cast in epoxy
    Photograph taken at the National Cryptologic Museum (NCM).
    Date unknown. Obtained via [1].

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Crypto Museum. Created: Wednesday 11 May 2016. Last changed: Monday, 02 January 2017 - 07:12 CET.
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