Rockwell-automation 1788 NetLinx Media Checker Users Manual Manual de usuario descargar pdf (Pagina 33)

Calculating the Distance to a Short

1788-UM002A-US-P – April 2000

Short across the shield and any other wire: The

MediaChecker reports the short, but cannot measure its

resistance or determine the distance to the short. You can

use an ohmmeter to measure the resistance; then use the

resistance to calculate the distance to the short.

Terminated cable: The MediaChecker indicates a fault

and displays a resistance. To determine the approximate

length of a terminated cable, subtract the terminator’s

resistance (75 Ω for ControlNet; 120 Ω for DeviceNet)

from the measured resistance; then use the remaining

resistance as Rs in the following procedure. Note that the

terminator’s tolerance will add some degree of error to the

calculation.

Important: When testing Ethernet cable, the

MediaChecker can report the distance to all

the shorts described above because

additional test data is available from other

wire pairs.

Use the following formula to calculate the distance to a

short. Refer to Tables 13 and 14 for cable resistances.

Distance to short =

R1 + R2

Rs: Resistance of the short, as measured by the

MediaChecker or with your own meter.

R1: Resistance per unit of length of one of the shorted

conductors.

R2: Resistance per unit of length of the other shorted

conductor.

Following is a sample calculation:

A DeviceNet Thin cable is shorted across H and the

shield. Here are the resistance values:

• Rs (as measured with an ohmmeter): 10 Ω

• R1 (resistance of H): 0.028 Ω/ft

• R2 (shield resistance): 0.0032 Ω/ft

10 Ω

Distance to short =

0.028 Ω/ft + 0.0032 Ω/ft

10 Ω

0.0312 Ω/ft

= 321 ft

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Rockwell-automation 1788 NetLinx Media Checker Users Manual Manual de usuario Pagina 33