29 DESIGN TALK OKW PLASTIC
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OKW ENCLOSURES, INC.
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Corporation has been
the industry leader in
test point design
For over 70 Years
a driving need for large real-estate boards in a tightly packaged form factor for high-volume
telecom applications. At the other end of the scale, PC104 was conceived as an architecture
without a backplane in that it was just a stack of cards. Innovative, but not very convenient
if you want to find a way of housing it. There also remains the option of designing your own
packaging, typically undertaken by large manufacturers who want to define a brand image
and can afford the vast sums necessary for complex injection molding and special coatings for
EMC shielding, just take a look at test instruments and medical equipment.
The Eurocard form factor
The most enduring standard is still the Eurocard form
factor. Anyone with a new design brief may dismiss this
as only relevant to existing standard bus architectures
such as VME and CompactPCI. However, that is doing
the standard a disservice. The Eurocard form-factor has
been around for a long time (far longer than the recog-
nizable standards that define their usage). Even these
standards contain a strange heritage when you start to look at the detailed measurements.
Basic Eurocard dimensions revolve around a modular approach wherein the smallest board
is some 100 mm ( 3.937”) high and the same in depth. The preferred board size of 100 mm x
160 mm commonly forms the basis of a ‘3U’ module and typically increases in steps of 133.35
mm ( 5. 25”) vertically and 60 mm in depth. The height of any module is based on a vertical
pitch of 44.45 mm (1.75”). This is where you will see the first of the mixed measurements.
The 100mm is a metric size. However, the integer vertical steps are designated as ‘U’. This
44.45 mm supposedly dates back to the British Post Office Type 3000 relay used in telephone
and railway signalling applications with the 44.45 mm being the minimum vertical distance
between the horizontally stacked trays.