Shorted Cable; Open Cable; Measuring Distance - IBM 5500 Operational, Installation, And Maintenance Manual

Shorted Cable

Open Cable

Measuring Distance

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Chapter 6: Twinax Client Cabling Requirements and Problem Solving
• Do not use this test to check the quality of work done on the cables by a
contractor.
• If the cable installation is still under contractor warranty, or if a third party
is responsible for the quality of the installation, use extra care so that
quality of the product or workmanship of the cable is not degraded.
• Use this test as a problem determination aid only when instructed to do so
by maintenance procedures, after you have followed all maintenance
procedures, or the problem could not be determined.
• Do not use this test for communication facilities owned or supplied by a
common carrier, such as telephone lines.
You can also use the test to check cables installed on other products for use on
IBM systems.
If there is a cable failure that changes the impedance of the cable, a part of the
signal is reflected back to the signal source.
If the cable failure causes the impedance to be lower than normal, the
reflected signal is out of phase and causes a reduction of the signal.
If the failure causes the impedance of the cable to be greater than normal, the
reflected signal is in phase and causes an increase in the amplitude of the
signal.
The reflected energy is delayed by the time (t) it takes for the transmitted
signal to travel to and from the termination or the fault.
By measuring the time from the start of the B-gate pulse to the start of the
change caused by the reflected signal, you can determine the distance to the
cable fault or cable end.
After you determine the fault, you can determine the distance from the cable
end to the fault by using the following formulas:
For twinax (solid poly) cable:
Df = t (µs) × 324.7 ft/µs
Dm = t (µs) × 99 m/µs
For twinax (Teflon**) cable:
Df = t (µs) × 344.5 ft/µs
Planning, Installation, and Maintenance Guide