Another widely used gigabit
interface is the High-Definition
Multimedia Interface (HDMI). It was
designed for and is used primarily for
video data transfer. It is found in
HDTV sets, DVRs, Blu-Ray players,
set top boxes, video games,
camcorders, digital video projectors,
and even some PCs and laptops.
HDMI is designed to carry
uncompressed video and either
compressed or uncompressed audio.
The general configuration is shown in
Figure 2. With today’s high pixel
count, long color words, and fast
refresh rates, speeds must be fast to
keep up with the video action. The
first versions of HDMI could handle
1.65 Gb/s. Later versions bumped
this up to 3. 4 Gb/s. The latest version
2.0 delivers 6 Gb/s — enough to
support the latest 4K ultra HD video.
As for cables and connectors,
cable length is restricted to five
meters to achieve those elevated
data rates. With special cable, lengths
of up to 15 meters are possible. You
can also buy repeaters and extenders
to string that out for as many meters
Connectors are another matter.
The standard has 19 pins. A larger
extended version has 29 pins. Mini
and micro 19-pin connectors are also
available for some devices.
Interface technology for HDMI is
pretty simple. It is strictly simplex;
that is, one way only from the video
source to the destination. However,
multiple channels are used. (Refer
again to Figure 2.)
There are three differential data
channels for video data and control
information. These channels use what
is called time minimized differential
signaling (TMDS) to time-multiplex
the video, audio, and control
information. The interface even
incorporates a standard I2C interface
to handle data for the type of display.
The interface also includes the high
bandwidth digital content protection
encryption software to prevent the
recording of proprietary video.
PCI Express is the serial data
interface that replaced the PCI and
PCI-X parallel interfaces in PCs and
laptops. It is the interface on the
motherboards and backplanes in
computers that interconnect the
processors, co-processors, memory,
and peripheral chips. The interface is
also implemented on connectors for
attachment to expansion cards.
What PCIe does is replace those
32-bit parallel busses with one or
more serial data channels. Each
channel is a pair of differential lines,
so that full duplex/simultaneous
transmit and receive operations are
possible. Each of these channels can
transmit at gigabit speeds depending
on the version. Version 1 channels
run at 2. 5 Gb/s, version 2 at 5 Gb/s,
version 3 at 8 Gb/s, and version 4 at
16 Gb/s. If you need a greater data
rate, you just use four, eight, or 16 of
these channels in parallel. Data bytes
are interleaved between channels.
For example, with four channels (x4)
at 5 Gb/s, you get a total transfer rate
of 20 Gb/s. That translates to two
gigabytes per second, or 2 GB/s
since 8B/10B encoding is used. The
maximum rate with sixteen 16 Gb/s
channels is 256 Gb/s.
At these speeds, range
is short; typically inches on
a PC bus that runs on a
motherboard. Short cables
are available for testing.
Data is transferred in
packets using a standard
protocol which includes a
32-bit CRC for error
detection and correction.
Most connections are just
point-to-point, but some
applications use a fast PCIe
switch IC that lets one
CPU serve multiple
One of the interfaces
you will hear about is Low
Signaling, or LVDS. This is
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THE LATEST IN NETWORKING AND WIRELESS TECHNOLOGIES
■ FIGURE 2. The HDMI source may be a cable or satellite TV set top box, while the
destination is an HDTV set.