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Section 150-232-100
Revision 01
Page 52
3.07 Two additional loopback points are added for each doubler that is used. NDU1 and NDU2 are executed
towards the network. CDU1 and CDU2 are towards the customer. All four loopbacks can be initiated by the 16 bit
in-band commands in SPLB states A1LB, A2LB & A3LB as described in Tables 17, 18, and 19 respectively. NDU1
and NDU2, along with the rest of the loopbacks towards the network, can be issued from the Loopback Menu shown
in Figure 27. See Section C paragraph 3.10 below for Front Panel-initiated loopbacks. The simplest HiGain system
includes two loopback locations: The HLU-232 List 1D and the HRU-412. The most complex HiGain system
application includes the HLU-232 List 1D, HRU-412 & two in-line Doublers. Refer to the PairGain HiGain Intelligent
Repeater Application Note # 910, Part # 325-910-100, for more SPLB details.
3.08 When T1 loopback tests are performed on the HiGain system with metallic loopback connections at either end,
the DS1 code that exists at the metallic loopback interface may be different from the DS1 code being received at the
opposite end when the DS1 user code is set to AUTO. This is caused by the fact that in the AUTO DS1 code mode,
the HLU-232 List 1D & HRU-412 set their own code independent of each other. Each end sets its transmit code to
match its receive code. Thus if one end is receiving AMI and the other B8ZS, their codes are different. For
example, if the HRU-412 has a metallic loopback and the HLU-232 List 1D receive pattern code is first changed from
AMI to B8ZS, and second, the all “0” pattern is sent into the HLU-232 List 1D, the HLU-232 List 1D changes to its
B8ZS mode while the HRU-412 remains in its AMI mode, and thus loops-back the all “0” pattern. This causes the
HRU-412 to indicate a LOS condition, which then causes the HLU-232 List 1D to output the AIS pattern.
3.09 The HiGain system may take longer than normal to respond to in-band loopback commands when its framing
mode is set to UNFR and the in-band commands are sent in either an SF or ESF mode. The frame bits override the
command bits and cause errors in the command sequence. These errors cause the HiGain system to reject some
sequences. This can extend the detection interval.
3.10 Depressing both the MODE and SEL buttons on the front panel for at least three seconds initiates a MANUAL
loopback session. This session allows the user to SEL one of eight HiGain system loopbacks. The message,
“MAN...LPBK”, appears on the front panel display followed by the message “NLO?”. If the SEL button is now
pressed, an “NLOC” loopback is executed and the message changes from “NLO?” to “NLOC”. If the MODE button
is pressed instead of the SEL button, “NLO?” is replaced by “ND1?”. This now allows an “NDU1” loopback to be
executed (if it is a Doubler application) with the SEL button. If this same routine is followed, all eight loopbacks
(NLOC, NDU1, NDU2, NREM, CLOC, CDU2, CDU1 & CREM) are presented and can be initiated in the sequence
listed. This interactive button procedure permits any of the eight HiGain loopbacks to be executed/activated. The
next loopback option can be presented by pressing the MODE button however, the previously executed loopback will
remain active until the SEL button is pressed and a different loopback is activated. If neither button is pressed for a
period of 30 seconds and no loopback is in effect, the manual loopback session terminates and the normal margin
displays reappear. If any loopback is in effect, the 30-second time-out is inhibited. The active loopback and the
manual loopback session continue until the loopback times out in accordance with the user selected LBTO setting.
Only one loopback can exist at any given time. Pressing both buttons again for three seconds, terminates any active
loopback, ends the MANUAL loopback session and returns the display to normal mode. Note that these same
loopbacks can be initiated from the RS-232 maintenance port by choosing the LOOPBACK MODE, option D, from
the MAIN MENU. This displays the Loopback Menu (Figure 27), from which any of the eight loopbacks can be
initiated/terminated.