Step-By-Step Cable Debugging - IBM 5500 Operational, Installation, And Maintenance Manual

buildings. Lightning protectors are to be installed at both building entry
points. Included are lightning shield wires which are grounded and attached to
earth ground at the entry points of both buildings. Make sure that these ground
wires are correctly installed to maintain a low GPD. Capacitors in the station
protectors will act as a barrier to any DC or low frequency AC voltages.
Where GPD exists within a building, a single station protector can be used on
the twinax cables to reduce the effect of the GPD. Installation of a Broadband
Transformer Unit may help with GPD but use with caution as it may allow
additional noise to enter the system.
Ground Potential Difference Isolation
A Broadband Transformer Unit can be used to isolate a device from the
twinax ground. This will prevent ground potential differences from affecting
the devices, and enable the potential difference to be measured. An OEM
device is available in some areas; however, an IBM part number is not
available.
Testing for GPD
You can determine if a GPD problem exists by varying off all the devices on
the twinax run, and disconnecting the twinax connector at the point in the line
to be tested. For a line going between buildings, this will be at the point the
cable exits the lightning protector.
Measure the voltage (both AC and DC) between the shield of the removed
connector, and the shield of the connector from which it was removed. This
may need to be measured at a number of places along the twinax run.
An alternate method of testing involves the use of a clip-on ammeter. This
may simply be clamped around each cable to be tested. It is not necessary to
remove the connectors, or vary off or power off the connected devices.
Devices connected to twinax runs generally contain zener diodes that will
clamp should the voltage exceed certain limits. If the measured potential
difference exceeds 3 volts DC or AC, you will most likely experience
problems. 1 to 3 volts may cause problems; below 1 volt should not cause
problems. The actual voltage at which failure occurs depends on the
characteristics of the line in question.

Step-by-Step Cable Debugging

To check for open shields without interfering with your normal operation, use
the procedure outlined in "Steps to Check Cable Using a Current Probe" on
page 109.
One device with a distorted display or failure to power up is not assumed to be
a problem with the cable. In this case, the problem is most likely with the
particular device. Typically, if there is a cable problem, one or more devices
on a string will not come on line or will come on line and periodically lose
communications with the host. If no workstations come on line, ensure that
the 5500 system is operational. If bad cabling is found it is the customer's
responsibility to repair the bad cabling or connections.
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