Rockwell Automation 1788-CNCHKR ControlNet NetChecker User Manual - Manual (Page 15)

ControlNet NetChecker 15
Rockwell Automation Publication 1788-UM001B-EN-P - August 2012
Poor Signal Quality
Poor Signal Quality has the following indications.
A blinking Poor Signal Quality status indicator often indicates that signal reflections are present
on the network. If you receive this indication, check the network for impedance mismatches:
inadequate coaxial cable used for all or part of the trunk cable, no termination or wrong
termination, high contact resistance, or intermittent contact in BNC connectors near the taps.
Generally, the faults are reported at the lower threshold settings and disappear or decrease at the
higher settings.
Use the Bargraph mode to verify the minimum signal level. If the signal level is low (2 or less on
the bargraph scale), check the trunk cable length versus the number of nodes and compare it to
the ControlNet specification. Terminate any unused taps with a dummy load (1786-TCAP).
Also ensure that no BNC connector is in contact with any conductive material such as a metal
wireway or DIN rail.
In disorders caused by impedance mismatches, Poor Signal Quality may be reported along with
Illegal Framing (and possibly Slow Edges) for all the threshold settings. This indicates large
impedance mismatches caused by an open trunk cable or a short-circuit. In such cases, the Node
Finder may have difficulty detecting the presence of some nodes on the network because of the
high amount of distortion.
In cases where the received signal has a low level (less than 2 on the Bargraph scale) with
distortion and/or poor edges, the faults may appear or their recurrence may increase as the
threshold is increased. Make sure that the cable length versus number of nodes does not exceed
the ControlNet specification. Check for possible high signal attenuation caused by inadequate
coaxial media used for the trunk cable or segments. High resistance at the contact points of the
BNC connectors on the trunk cable may also add abnormal signal attenuation.
Status Indicator State
TRAFFIC Green
Slow Edges May also be blinking red
Illegal Framing May also be blinking red
Poor Signal Quality Blinking red
Pass/Fail Yellow or red
Buzzer Sounds
IMPORTANT An unused drop cable should be terminated by a dummy load (1786-TCAP). Do not use a
75 ohm termination along with a bullet connector. The impedance here is not 75 ohms.