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APPENDIX C: ADVANCED CABLING ISSUES (SKEW)
46
You can measure pair lengths with a LAN cabling tester (TDR) or view pairs by stripping
back a small piece of cable and viewing how tightly the pairs are twisted. The most tightly
twisted pair is the slowest (longest) and the loosest pair the fastest (shortest). You can also
observe relative delays using the test card.
Some cables have a ‘3+1’ construction where three pairs closely match. The fourth pair
should be used for data and the other pairs sorted as in the above table. Other cables have a
‘2+2’ construction where there are two sets of dissimilar pairs. Put red and green on one set
(tightest) and blue & data on the other.
A suitable pair-swapping scheme is easily made using custom wired patch cables at each end
of the Interconnect cable.
Extender with Skew compensation
In more extreme cases, you should consider using an alternative extender with an in-built
Skew Compensation feature such as the ACU2000 series. This allows you to set independent
delay values of up to 42nS on each color signal. Contact Technical Support for details
Patch Cable
To obtain optimal video performance with your existing setup, it is desirable to have the
green signal as the reference (slowest). This is because the human eye is extremely sensitive
to additional green distortion (introduced by delays).
If your system requires a large delay on the green signal, try using a crossover patch cable at
each end of the link (instead of straight patch cables to connect to wall outlets). In many
cases, the crossover should transfer the delay requirement to the blue signal rather than green.
The human eye is much less sensitive to blue distortion and so video quality appears to be
improved. The main cable should be wired according to EIA-568B as shown on page 17.
Using a crossover cable (at each end) will put the green video on the orange colored pair and
the blue video on the green colored pair.
EIA-568A wiring can also be used to the same effect. Contact Technical
Support for details.
The alternative ServSwitch™ Brand CATx KVM Extender range
(ACU2000 series) has been specifically designed for the extension of high
screen resolutions (1600x1200) over long CATx cables (300m).
ServSwitch™ Brand CATx KVM Extenders have fully integrated skew
compensation and independent control of low frequency (LF) and high
frequency (HF) cable equalization.