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206 TimeHub 5500 User’s Guide
097-55501-01 Revision M – January 2009
Chapter 5 Maintaining the TimeHub 5500
Clearing Output Port Alarms
Clearing Output Port Alarms
The TimeHub Output cards contain a sensing circuit that detects output signal
failures. Every output port has a sensing circuit. The sensing circuit detects and
reports on internal component failures associated with the port output circuitry.
When a port alarm is detected on a given output card, there are several responses:
For output cards with FPGA version A, the Fail LED on the output card will
illuminate. For output cards with FPGA version B or later, the Alarm LED on the
output card illuminates. (FPGA version can be seen at the end of output card
version information in RTRV-NETYPE-ALL response).
An OPPTFLXX (Output Port Fail) event is generated, where XX identifies the
specific port number.
If both output port cards in a redundant pair have detected port alarms on the
same port, the OPTALMEXT (“Alarms on same ports - check external
conditions”) event will be generated. See a detailed description of this event in
the
TL1 Reference Guide
.
If both output cards in a redundant pair have detected port alarms but not on the
same ports, the OPPRFL event is generated. See a detailed description in the
TL1 Reference Guide
.
False Reporting of Output Port Alarms
When you connect an output signal from TimeHub to a single-ended device (such as
an oscilloscope or other type of test equipment), it is possible that false port alarms
may be reported. This can occur when the connection from the TimeHub differential
outputs (Tip and Ring) to the single-ended device (which has signal and ground) is
made improperly. Often this connection is made by connecting Tip or Ring to ground
(shell of connector on test equipment) and the other signal (the one that was not
connected to ground) to the signal input on the test equipment. Connecting in this
way creates a path for ground-related noise coming from the test equipment ground
(e.g., 60 Hz from utility, ground-loop effects, etc.) to be injected back into the alarm-
sensing circuit in TimeHub (via the Tip or Ring that was connected to equipment
ground). The result can be that the sensing circuit sees its detection thresholds
exceeded (due to the induced noise) and reports a false port alarm.
This situation can also temporarily occur when connecting or disconnecting
differentially with test equipment if the connection plug has the possibility of
momentarily allowing either of the differential signals to contact the equipment
ground. In this case, you can avoid the possible false port alarm by enabling the
output only after you complete the physical connection to the test equipment.