AD8134

Rev. A | Page 16 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).