150-431-115-01, Revision 01
Maintenance
HLU-431 List 1E
September 22, 1998
23
HAIS Selections
The HAIS option provides two selections for the T1 transmit outputs at both the HLU-431 and HRU for HDSL
loss of sync conditions.
•
1LP
causes the AIS (LOS if ALMP is set to LOS) pattern to be transmitted at both T1 outputs when either
of the two HDSL loops experience an out-of-sync (LOSW) condition or when a margin alarm occurs. 1LP
causes the 12 channels on the surviving loop to be lost as they are replaced by the AIS/LOS pattern. However,
it does notify downstream and upstream equipment of the loss of one HDSL loop or a loop with low margin.
This is the preferred setting for initiating an AIS/LOS state with just one conductor open in either of the
HDSL pairs. Short loops, below approximately 16 dB of loss at 200 kHz, can remain in sync with one
conductor open. Since the loop is still in sync, no LOSW condition occurs. However, the margin on a
one-conductor loop drops from 5 to 10 dB. Thus, if the Margin alarm is set to 5 dB below the normal margin
at turn-up, when one conductor does open, a system alarm occurs and causes the AIS/LOS condition. This
alerts the maintenance personnel of the problem.
•
2LP
requires both HDSL loops to be out of sync (LOSW) before the HAIS signal is transmitted. 2LP
preserves the integrity of the 12 surviving channels when just one loop is lost.
HDSL Line Voltage Operation
A unique feature of the HLU-431 List 1E is that its 200 V output voltage for doubler applications is always bipolar.
This bipolar voltage is required to support the Ground Fault Detection (GFD) circuit, which is also unique to the
HLU-431 List 1E. The 140 V output voltage used in non-doubler unit applications is always negative. This keeps
the HDSL cable pair voltage at or below ground potential, thereby avoiding corrosion problems caused by cable
voltages more positive than ground.
The specific bipolar voltage levels existing between ground and the two loops for a system containing doublers
depends on the following factors:
•
loop length
•
number of doublers
•
type of doublers (which List)
•
whether the HRU is locally powered or line powered
The line voltage power supply is ground referenced, but also ground isolated by 200 kohms. This ground isolation
reduces problems due to induced noise currents and large surge voltages, which are ground referenced. It also
reduces ground fault currents, which improves product safety. Therefore, the product safety depends solely on the
differential voltage across loop 1 and loop 2.
Ground Fault Detect
The HLU-431 List 1E has a GFD circuit that is compliant with paragraph R7-1, Section 7.2.1 of GR-1089-CORE,
Issue 1, Revision 1, December, 1996.
The GFD circuit immediately detects ground faults that occur at any point along any span on any conductor, and
shuts down the HiGain circuit. The HLU-431 then applies power periodically to the first span to detect the ground
fault condition. This power cycling and ground fault protection continues as long as the fault condition exists.
