Technical information

Technician’s Handbook
74
Test/Verify (Loop Start)
Verify far-end (CO) circuits by using a standard telephone set in place of
the Loop Start trunk circuit.
Verify near-end (PBX Loop Start) circuits by connecting to a local PBX
ONS circuit.
Check for appropriate voltages on Tip-Ring in each call state.
During off-hook condition, 18 mA DC minimum should be drawn from the
CO (typical 25-40 mA).
Some Key/PBX systems may be polarity sensitive; reverse Tip-Ring to
verify.
Stop Dial
- -
Receipt of Digits
Audible Tones - Busy,
Reorder, Ringback depen-
dent upon digits dialed and
called party condition
-
Answer
Supervision
Some COs can reverse
current flow
Tip-Ring leads acti-
vated (shorted); Loop
Current flows in line
Disconnect
Supervision
Near- end termination:
PBX/Telephone can break
Loop Current
Near- end termina-
tion: PBX/Telephone
can break Loop Cur-
rent
Table 6: Loop Start Line/Trunk Summary (continued)
Signal
Loop Start Line PBX
Outgoing
Loop Start Line PBX
Incoming
(Sheet 2 of 2)