TROUBLE-SHOOTING: 7

Voltmeter testing
The use of a voltmeter in cable testing is restricted, but necessary at times to find unusual faults.

• Voltage tests
  These tests can test for the normal or abnormal presence of signal activity on the cable. Ethernet 802.3 uses the following voltage levels,
  • high = -.2 to -.5
  • low = -1.6 to -1.9
  • carrier sense = -.9 to -1.2
  • collision = -1.5 to -1.7

  A streaming node (a workstation continuously sending) is easily identified by the voltage present on the cable.

  Cards can be tested by placing a carrier sense voltage on the bus. This should cease all network traffic. Any variation of the measured voltage indicates that one or more cards are ignoring the carrier signal.

• Noise tests
  Some of the newer hand held fault tools measure noise levels on the cable. This is the mean voltage read when there is no network activity. It is best to measure this on a system which is down and all workstations are turned off.
  Sources of noise are
  • computer power supplies and motherboards
  • power surges and mains supplies
  • large inductive loads, air conditioning, lift motors

  For ethernet, noise measured should be lower than 0.04v per cable segment.

© Copyright Brian Brown, 1988-2000. All rights reserved.