Data Sheet AD8134
Rev. B | Page 17 of 20
HOW TO APPLY THE OUTPUT PULL-DOWN
FEATURE
The output pull-down feature, when used in conjunction with
series Schottky diodes, offers a convenient means to connect a
number of transmitters together to form a video network. The
OPD pin is a binary input that controls the state of the AD8134
outputs. Its binary input level is referenced to the most positive
power supply (see the Specifications section for the logic levels).
When the OPD input is driven to its low state, the AD8134
output is enabled and operates in its normal fashion. In this
state, the sync-on-common-mode circuitry provides a
midsupply voltage and encoded sync pulses on the output
common-mode voltage. The midsupply voltage is used to
forward bias the series diodes, allowing the AD8134 to transmit
signals over the network. When the OPD input is driven to its
high state the outputs of the AD8134 are forced to a low voltage
irrespective of the levels on the sync inputs. This reverse-biases
the series diodes and presents a high impedance to the network.
This feature allows a three-state output to be realized that maintains
its high impedance state even when the AD8134 is not powered.
This condition can occur in KVM networks where the AD8134s
do not all reside in the same module, and where some modules in
the network are not powered.
It is recommended that the output pull-down feature only be
used in conjunction with series diodes in such a way as to
ensure that the diodes are reverse-biased when the output pull-
down feature is asserted because some loading conditions can
prevent the output voltage from being pulled all the way down.
KVM NETWORKS
In daisy-chained KVM networks, the drivers are distributed along
one cable and a triple receiver is located at one end. Schottky
diodes in series with the driver outputs are biased such that the one
driver that is transmitting video signals has its diodes forward-
biased and the disabled drivers have their diodes reverse-biased.
The output common-mode voltage, set by the sync-on-common-
mode circuitry, supplies the forward-biased voltage. When the
output pull-down feature is asserted, the differential outputs are
pulled to a low voltage, reverse-biasing the diodes.
In star networks, all cables radiate out from a central hub, which
contains a triple receiver. The series diodes are all located at the
receiver in the star network. Only one ray of the star is
transmitting at a given time, and all others are isolated by
reverse-biased diodes. Diode biasing is controlled in the same
way as in the daisy-chained network.
In the daisy-chained and star networks that use diodes for
isolation, return paths are required for the common-mode
currents that flow through the series diodes. A common-mode
tap can be implemented at each receiver by splitting the 100 Ω
termination resistor into two 50 Ω resistors in series. The diode
currents are routed from the tap between the 50 Ω resistors
back to the respective transmitters over one of the wires of the
fourth twisted pair in the UTP cable. Series resistors in the
common-mode path are generally required to set the desired
diode current.
In point-to-point networks, there is one transmitter and one
receiver per cable, and the switching is generally implemented
with a crosspoint switch. In this case, there is no need to use
diodes or the output pull-down feature.
Diode and crosspoint switching are by no means the only type
of switching that can be used with the AD8134. Many other
types of mechanical, electromechanical, and electronic switches
can be used.
VIDEO SYNC-ON-COMMON-MODE
In computer video applications, the horizontal and vertical sync
signals are often separate from the video information
signals. For example, in typical computer monitor applications,
the red, green, and blue (RGB) color signals are transmitted
over separate cables, as are the vertical and horizontal sync
signals. When transmitting these types of video signals over
long distances on UTP cable, it is desirable to reduce the
required number of physical channels. One way to do this is to
encode the vertical and horizontal sync signals as weighted
sums and differences of the output common-mode signals. The
RGB color signals are each transmitted differentially over
separate physical channels. The fact that the differential and
common-mode signals are orthogonal allows the RGB color
and sync signals to be separated at the channel’s receiver.
Cat-5 cable contains four balanced twisted-pair physical
channels that can support both differential and common-mode
signals. Transmitting typical computer monitor video over this
cable can be accomplished by using three of the twisted pairs for
the RGB and sync signals and one wire of the fourth pair as a
return path for the Schottky diode bias currents. Each color is
transmitted differentially, one on each of the three pairs, and the
encoded sync signals are transmitted among the common-
mode signals of each of the three pairs. To minimize EMI from
the sync signals, the common-mode signals on each of the three
pairs produced by the sync encoding scheme induce electric
and magnetic fields that for the most part cancel each other. A
conceptual block diagram of the sync encoding scheme is
presented in Figure 34. Since the AD8134 has the sync encoding
scheme implemented internally, the user simply applies the
horizontal and vertical sync signals to the appropriate inputs.
(See the Specifications tables for the definitions of the high and
low levels of the horizontal and vertical sync pulse voltages).
