https://ardent-tool.com/docs/patent/EP0282637B1.pdf

Page 8 physical

The improved pin arrangement of Fig. 15 is minimizes radiated emissions:

a. Every fourth pin on each side of the feature card edge connector 36
(Fig. 8) is connected directly to ground or decoupled to ground at radio
frequency (A.C. ground) as shown in Fig. 20 Connector 37 pins are
therefore similarly connected.

b. The A.C. ground pins on one side of the edge connector are offset by
two pins from those on the other side. This results in a symmetrical
grounding pattern where no signal is more than one pin spaced from an
A.C. ground.

c. The symmetrical A.C. ground arrangement tends to cancel fields
radiated by a signal relative to two grounds. 30

d. Signals that are switched simultaneously on the bus are arranged with
one half of the signals on one side of the card tab 36 and the connector
and one half on the other side in close proximity to one another. This
encourages cancellation of fields. This is done with the address bus as
well as the data bus.

e. Signals with fast rise times of high frequencies are arranged
adjacent to A.C. grounds.

f. The connector is miniaturized to one half of the pin to pin spacing
of known connectors to further reduce the loop area.

g. The audio system signals utilize a separate ground (not shown) to
minimize coupling of digital and audio information. This ground is
connected to the digital signal ground at a single point.

h. All current is returned through the connector grounds only and not
through the rear bracket connector or cable lines.

https://ardent-tool.com/docs/patent/EP0282637B1.pdf

Page 3 physical

In the prior art personal computer systems, a planar board containing
the processor logic is positioned by mating pins and holes to the base
of a sheet metal frame also having side walls. One section of the side
walls includes an inwardly ex- tending sheet metal shell (often referred
to as a "shadow box") welded thereto and having vertical slots to permit
connection of I/O feature cards within the frame to external peripheral
units via cables and I/O connectors. The cards are connected
electrically and mechanically to the board via card/board connectors
inserted into the board and lower contact tabs on the cards forced into
the card/board connectors. Low carbon steel I/O brackets connected to
the cards are adapted to engage the shell and cover the vertical slots,
a screw securing the upper end of the bracket to the shell and a contact
spring forcing the lower end of the bracket against the shell. Similar
brackets are attached to the shell in slot positions not having feature
cards.

This prior art configuration has several disadvantages. The location of
the installed card/bracket assembly in the direction of shell is
established primarily by the engagement of the card tab in its
associated connector in the system board. The dimensional relationship
between the adjacent surfaces of each bracket and slot is established by
tolerance variations in a number of dimensions, such as the distance
between the card tab and holes through which threaded fasteners operate
to fasten the bracket in place, the distance between the holes in a card
bracket through which these fasteners operate and the surface of this
bracket adjacent to the shell, the distance between this surface of the
shell and fasteners which attach the system board in place in the frame,
the distance between the holes for these fasteners operating in the
system board and the holes used to locate the card/board connector in
this assembly, the location of the pins extending from this connector
within the holes in the system board, and the squareness of the shell
with respect to the surfaces of the frame on which the system board is
mounted.

Because of the large number of dimensions involved, relatively large
variations in this distance between the adjacent surfaces of the I/O
bracket and slotted surface of the shell occur in the process of
manufacturing system units. If this distance is too large, the bracket
is warped during installation so that a substantial gap exists between
these surfaces. In other words the contact spring and screw hold the
lower and upper portions of the bracket against the surface of the
shell, but the connection of the card to the board pulls the central
portion of this bracket away from the adjacent surface of the shell.
This warping presents an obvious visual problem when the system unit is
viewed from the rear and establishes slots through which electromagnetic
radiation can escape from the unit.

Grounding electrical contact between the bracket and the shell is
established only at top and bottom. This is a particular disadvantage
because cable connectors are typically fastened to such card brackets in
their central regions, so that the electrical resistance of the brackets
along their length becomes a part of the high-frequency resistance
between such connectors and the frame. This resistance makes it more
difficult to control the electromagnetic radiation of cables attached to
such connectors and acting as antennas.

On the other hand, if there is a mechanical interference between the
adjacent surfaces of bracket and the shell, the frame must be deflected
to allow the installation of the card assembly. This condition can make
such installation difficult or impossible, causing a serious problem in
the manufacturing or field environment and causing stress in the feature
cards.

The engagement of the contact spring with a blank bracket often
establishes a torque on the bracket so that the central region of this
bracket is warped away from the adjacent surface of the shell, also
presenting a visual problem when the unit is viewed from the rear and
establishing slots through which electromagnetic radiation can escape
from the unit.

Such problems make it difficult to maintain EMC integrity below the FCC
Class B limit at clock frequencies greater than 8 MHz. It is therefore a
primary object of the present improvement to an arrangement which
obviates the disadvantage of this prior art configuration. A computer
system according to the prior art portion of claim 1 is disclosed in
Electrical Design 45 News, vol. 32, no. 4, 19.2.87, pages 114-126.