Rockwell Automation 1788 NetLinx Media Checker Users Manual - Manual (Page 33)

Calculating the Distance to a Short 33
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.
Rs
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