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Video display (DDC) information
The Display Data Channel (or DDC) scheme was introduced to allow analog and
digital video displays to provide details about themselves and their capabilities
to the computer’s graphic adapter circuitry. In most applications this is a useful
and positive feature. However, in a highly secure environment this presents two
potential problems:
• Most video displays provide manufacturer, model and serial number
information as part of their DDC data. This unique information could
possibly be used as a marker by anyone attempting to compromise security
within one or more of the connected computers/networks.
• The operation of the DDC scheme could theoretically provide a means to
transfer a small 128 byte packet of data to the computers at each power on
cycle of the ServSwitch.
If your organisation wishes to protect against such scenarios then it is
recommended that the DDC lines are disconnected in the cable between the
ServSwitch and the monitor. Alternatively, Black Box would be happy to discuss
configuring the ServSwitch with a DDC policy to suit your organisation.
ServSwitch Secure DDC policy
The ServSwitch Secure maintains individual DDC memories for each connected
computer port. During manufacture, these DDC memories are each loaded with
a set of default DDC data.
When the ServSwitch is powered on, its response will be determined by the
condition of the DDC signalling pins of the video monitor connector:
• If the DDC pins are connected as standard: the ServSwitch Secure
reads the DDC data from the attached video monitor and loads a copy into
each port memory, which can then be made available to the connected
computers.
•
If no video monitor is connected or the monitor’s DDC signalling pins
are disconnected: The ServSwitch Secure will maintain the default data
held in the DDC memories and make them available to the computers.
•
If the video monitor’s DDC signalling pins have been connected to
ground: The ServSwitch Secure will load a set of default data to the DDC
memories and no DDC data will be made available to the computers. This
provides a means of clearing DDC information about previously attached
monitors.
Note: Most analog video cards will output a video signal without DDC
information. In such installations it may be acceptable to disconnect the DDC
connections from the ServSwitch Secure so that no DDC information is made
available to the computers. However, most DVI graphics cards will not output a
video signal unless they can read the DDC information.
To determine how DDC data is used
Note: The information given here is provided purely as an overview. It is beyond
the scope of this document to provide detailed instructions on how to modify
video display cables, which should only be attempted by a qualified engineer.
If the transfer of DDC information is unsuitable for your installation, you can
take steps to bypass or disable its use. DDC data is sent from the video display
on the following pins of their connectors:
•
Analog VGA (15-pin D-type) connector: pins 12 and 15
•
Digital DVI connector: pins 6 and 7
As mentioned earlier, the ServSwitch Secure unit responds in the different ways,
depending upon how the DDC data lines within the video display cable have
been wired:
DDC pin conditions ServSwitch Secure unit response
Connected DDC data is harvested from the connected video
display during unit power on and written to all
computer port memories.
Not connected Unit retains the DDC data that is already held in the
port memories and continues to present them to the
attached computers. No new DDC data can be sought
from the currently connected video display.
Grounded Unit wipes all DDC data held in memory and presents
no information to the attached computers.
In situations where no DDC information is being supplied, it may be necessary to
use a special driver on the connected computers to inform their graphic adapters
on the appropriate signals to send.
Alternatively, a ‘surrogate’ video display of the appropriate type could be
temporarily connected to the ServSwitch Secure unit in order to harvest the
necessary DDC information. The surrogate video display could then be replaced
by the real one, which has its DDC pin disconnected (not grounded).