The four fiber-optic transceiver connections
are on the front panel of the XMCFPGA-
05F. This is typical for PMC and
XMC cards as well as for many of the
boards they plug into. Rear transition modules
(RTMs) are available for many boards,
permitting connections via the backplane.
Likewise, CompactPCI and VME boards,
along with their relatives, can take I/O connections
via the backplane, but fiber-optic
and coax connections are less common and
non-standard. This will be changing, at
least when it comes to VITA standards.
New connectors and placement standards
are in the works at VITA, leading
to prototype boards like those from DRS
Technologies using new coax connectors
(Fig. 10). The approach has key advantages
when it comes to rugged designs. It
allows for easier board replacement since
front-panel connections aren’t required.
VITA 46.14, a variant of VPX, specifies
the RF connections. It looks like it
will support four and eight coax connections
and is suitable even on a 3U board. It
essentially takes away from the digital I/O
connections, but it’s a reasonable tradeoff.
Similarly, more than half the rugged
backplanes are custom, so having a mix
of connectors for the various slots is common
already. Running connections out of the backplane has significant advantages,
including less noise, higher data rates, and
less crosstalk versus individual connections
via the front panel.
The proposed VITA 46.12 standard
addresses fiber connections on the back of
VPX boards. The standard specifies three
off-the-shelf fiber connectors, including
Mechanical Transfer (MT), Expanded
Beam (EB), and LuxCis-type.
The MT type is a self-aligned pair of
mating shells and pins that maintain the
positional accuracy of fibers in the form
of a ribbon cable. MT connectors are an
established commercial high-connectiondensity
standard offering. Unfortunately,
this approach doesn’t provide specific
alignment of individual fibers, and cleanliness
is a requirement. It will likely be used
in less rugged environments.
The EB connectors use a spherical lens
at the end of each fiber to expand the beam.
The connectors bring the two lenses on
each side of the connector into close proximity,
but without physical contact. These
connectors are less sensitive to alignment
or contamination and can tolerate many
mating cycles without degradation.
The LuxCi s - type connector s ar e
designed to qualify for ARINC 801 aerospace
applications. They fit into other
rugged applications, too. The connectors
consist of metal shell pairs with multiple
signal connections using ceramic ferrules
for fiber alignment. This approach maintains
positional accuracy even in highvibration
environments.
The proposed coax and fiber VPX standards
target level 2 line-replaceable modules
(LRMs) typically found in space and
air vehicles as well as many military platforms.
This allows for quick replacement
of defective components. The task is much
more difficult if front-panel connections
were involved. Having all I/O via the backplane
makes replacement simple and reliable.
It can also simplify the issues associated
with cooling and environmental
isolation, since all connectors are at one
end of the board.