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MIL-HDBK-232A  Isolators.  Isolators may be used at the point of egress or may be
used to establish the RED/BLACK boundary, if so required (see figure 42).  At
the point of egress, optical or photon coupled isolators may be used.  When
used in a shielded facility, a drive module is coupled to a receive module
via a FOC that penetrates the shield through a waveguide-beyond-cutoff.  The
isolators may be housed in RFI cabinets bonded to the shield to further
guarantee the shield integrity.  In order to limit the number of penetrations
of the shield, channels may be multiplexed to drive the fiber optic isolator,
then be demultiplexed after exiting the facility.  Within the facility, a
RED/BLACK boundary may be required for signal lines which must traverse both
areas but do not pass through COMSEC devices.  For lines which are used for
control, have a low-frequency rate of change, and do not have critical rise
times, relays may be used.  Where higher speeds are needed, along with fast
rise times, electronic relays or photon coupled isolators may be used.  The
ideal placement of this isolation capability is within the same area as the
COMSEC equipment.  The installation design must assure that the isolation
mechanism cannot he bypassed.
Special considerations.  Patch and test facilities (PTFs).  All facilities will incorporate
some method to trouble-shoot defective equipment on a circuit.  This may be
as simple as a single jack field in a common equipment rack of a small
facility to multiple patch racks with automated test and monitor capability
in a large facility.


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