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3.5.
General Button Usage – DDC Select
The TX and RX units both have a button labeled DDC SELECT. When the data communications fibers
L1 and L2 are connected, either button will control the state of the DDC operation. Each press of the
button will advance the DDC state from PASS to DYNAMIC to STATIC and back to PASS (a fine-tip
pencil, such as a mechanical pencil, works best). When the units are disconnected, the DDC state of the
TX unit is the master and will be sent to the RX when data communications are restored. In PASS
through mode, the CPU communicates directly with the connected monitor for DDC information. In
DYNAMIC mode the DDC of the monitor is read from the receiver and stored in the transmitter. In
STATIC mode the DDC information in the transmitter is not changed regardless of changes made at the
receiver end.
There is also a default DDC table stored in the device. This table is suitable for use with many standard
PC monitors. If you wish to restore the default DDC table to the product and automatically place the
device into STATIC DDC mode, hold the button down for approximately 7 seconds or until you see the
DDC mode LED change to static mode.
3.5.1. Operation of the Thinklogical DDC Modes
DDC (Data Display Channel) is a VESA standard communications channel between a display adapter
and a video monitor. DDC data is stored in a monitor and describes the monitors characteristics (vendor
name, serial number, frequency range, analog or digital capabilities, etc.). DDC information is read from
the monitor by a video card in order for the video card to gather Extended Display Identification Data
(EDID) and provide video acceptable to the monitor.
Due to the Thinklogical TX emulating a monitor it must provide a DDC table to the video card in order for
the video card to provide images for transport. Some video cards are very un-forgiving of DDC data. An
un-forgiving video card will shut off video output if the DDC is not present at boot up (missing monitor),
hot-plugged or the DDC information does not match expected parameters (digital or analog). Some
video cards are very tolerant and will produce video output under many conditions. Due to the variability
of video card DDC handling, Thinklogical products provide all or a subset of the DDC support modes
detailed below.
3.5.1.1. Dynamic Mode
Dynamic DDC mode is the default and recommended mode of DDC operation. In this mode the RX will
read the DDC data from a monitor under 2 conditions; 1) power up and 2) if the monitor cable is removed
and reinstalled. The RX will only read the DDC information to RAM, it will not be stored on the RX.
Once the RX has validated the DDC data it informs the TX of a change in DDC information. Once
informed of a DDC change the TX reads the DDC information from the RX. If the DDC data read from
the RX is different than the DDC information stored in the TX non-volatile memory, the TX writes the new
DDC information to non-volatile memory on the TX that the video card can read. The TX then informs
the video card that the DDC information has changed so that the video card may read the DDC data and
act appropriately. In this mode the CPU may be booted up with the TX off or disconnected from the RX
since the TX non-volatile DDC memory can be read by the video card without TX power.
MAN-000018
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REVISION A
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