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Ethernet Communications Modules, 3rd Edition, Rev. D
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Chapter 7: Maintenance and Troubleshooting
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Diagnosing Network Cable Problems
If you are experiencing communication problems, swapping cables is one of the simplest
diagnostic procedures you can perform. If the network operates correctly with a different cable,
you have isolated and cured the problem. If possible, use a short run of cable to test the network
because problems with longer cable runs can be more difficult to diagnose and are more often
intermittent.
If you are unable to swap cables, verify the proper operation of all other network components.
You probably have a cable problem if you have verified that your:
• ECOM module is working correctly.
• ECOM module configuration is correct.
• RLL program or PC program is correct.
• Hubs are working correctly.
• Windows configuration is correct.
• Network adapter card is the correct type, and it is working correctly.
It is a good maintenance practice to test network cables periodically and maintain a permanent
record of cable characteristics. A number of cable test instruments are available to test
10/100BaseT and 10BaseFL networks. These instruments will check the electrical or optical
characteristics of your cabling, including:
• Continuity - This is a check to make sure the communication pairs are wired correctly,
and that the wires are continuous from end to end. In the case of fiber optic network
this is a test to be sure light is transmitted from one end of the cable to the other.
• Attenuation - This refers to the amount of signal loss over the cable segment
at the signal frequency of interest. The 10/100BaseT specification allows
for a maximum signal loss of 11.5 decibels (dB) for the entire link at the
signal frequency used by 10Mbps Ethernet. The 10BaseFL specification
calls for the optical loss in link segment to be no greater than 12.5 dB.
• Crosstalk - Crosstalk occurs when a signal in one pair of wires is electromagnetically
coupled to an adjacent pair. This is critical for10BaseT networks which are susceptible
to noise interference. 10BaseFL networks are virtually immune to noise interference.
NOTE: Any significant difference between the cable characteristics of the transmitter and receiver can cause
communication errors.
Ethernet devices continually monitor the receive data path for activity as a means of verifying
their link is working correctly. When the network is idle, each network device (including the
ECOM module) sends a periodic link test signal to verify that the network is working. If the
link test signal or other network activity is not received periodically, the LINK/LINKGD LED
on the ECOM module is turned off.