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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).
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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.
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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.
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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
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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.
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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.
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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.
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The FLYBALL modules are very similar to the so-called
Circuit Blocks made by Philips
(Mullard, Valvo) in the 1960. Like the FLYBALL modules, they were
available in bright colours.
They are also very similar to the
Bausteinsystem (Building Block System)
of Swiss crypto-manufacturer Gretag.
Although the FLYBALL modules greatly reduced the size and weight of the
machines, they were eventually superseded by the Transistor-Transistor-Logic
(TTL) integrated circuits (ICs) 4 that became available in 1967.
Nevertheless, the FLYBALL modules 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 provided. By 1980, the FLYBALL modules
had become extinct and some of the existing KW-7
boards were redesigned with modern ICs.
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NASA = National Aeronautics & Space Administration.
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FLYBALL is sometimes written as FLY BALL.
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MOBIDIC was a defense Mobile Digital Computer development program
carried out at Sylvania.
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IC = Integrated Circuit.
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Devices with FLYBALL modules
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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 on 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.
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Although the actual FLYBALL modules used by NSA in products like the
KW-7 were never patented, there are some patents
of similar products that were filed in the same era. The image below
shows a patent drawing of August 1960 for a very similar manufacturing technique.
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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.
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Any links shown in red are currently unavailable.
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© Crypto Museum. Created: Wednesday 11 May 2016. Last changed: Thursday, 16 October 2025 - 20:45 CET.
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![Early cardwood module, showing a single flip-flop circuit. Photograph Copyright Jeff Wisnia [4].](img/cordwood.jpg)
![Red FLYBALL module cast in epoxy, on display at the NCM. Photograph by Doug Eyre [5].](img/sylvania.jpg)
![Example of a 1960 patent for a cordwood electronic circuit [3]](img/flyball_small.png)
