150-388-125-01, Revision 01
Appendix C - Functional Operation
HLU-388 List 2E
September 15, 1998
51
CRC fields on the HDSL streams allow the HLU-388 to determine if errors are present on the channel due to
excessive impairments on the HDSl pairs or excessive impulse or crosstalk noise.
The demultiplexer removes data link messages from the HDSL loops and passes them to the microprocessor. This
mechanism allows operations messages and status to be exchanged between the HLU-388 and the HRU.
The reconstructed HDSL data is buffered in a first-in-first-out (FIFO) buffer within the demultiplexer. A
frequency synthesizer, in conjunction with the FIFO, regulates the output bit rate and reconstructs the DSX-1
clock at the exact rate received from the remote end. The HiGain system operates at T1 rates of 1.544 Mbps with
up to ±200 bps of offset.
A DSX-1 interface driver converts the input data to an Alternate Mark Inversion (AMI) or Binary Eight Zero
Substitution (B8ZS) format. The DSX-1 equalizer is programmable to five different lengths, as determined by the
distance between the HLU-388 and the DSX-1 interface. This provides CB-119 specification compliant pulses at
the DSX-1 interface over a range of 0 to 655 feet of ABAM-specification cable. To comply with GR-1089 CORE,
section 4.5.9, the shields of the ABAM cables must be grounded at both ends.
The new T1 transceiver chip allows the unit to process both B8ZS and AMI code inputs, regardless of the DS1
code setting (AMI or B8ZS). Earlier units caused input BPV, if B8ZS patterns were processed while in the AMI
mode. When the newer units are in the AMI mode, they can receive B8ZS but can only transmit AMI. For this
reason HiGain systems containing both the HLU-388 List 2E and older List HRUs will respond differently in each
direction for B8ZS inputs when in AMI mode.
The HLU-388 List 2E contains two separate power converters. The main power supply converts -48 Vdc local
battery to logic power for the HLU-388 circuits. The line power supply converts the -48 Vdc battery to either 130
Vdc (for non-doubler applications) or 200 Vdc for doubler applications), then provides simplex power feed on the
two HDSL line interfaces. The 140 V output voltage used in non-doubler applications is always unipolar. The 200
V output voltage used in doubler unit applications is always bipolar. The line power supply can be turned on or
off by the microprocessor and is automatically shot down in the presence of line short circuits or microprocessor
failure.
HDSL Line Voltage
A unique feature of the HLU-388 List 2E 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-388 List 2E. 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 doubler unit applications depend
on the following factors:
•
loop length
•
number of doubler units
•
type of doubler units (List)
•
whether the HRU is line powered or locally 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 the product’s safety. The safety issue thus depends solely on the
differential voltage across loop 1 and loop 2.
