General Procedures / Information
6-72 12/04 FaxCentre F110
Line control
EXT_LOOP:
Loop current input signal. This signal is used to indicate that a loop current is drawn by an external
phone (if Off-Hook). External loop current status (EXT_LOOP) also changes if Ringer applied Par-
allel Phone state changes
The loop detection detects the current that flows in the external parallel phone. The detection
should bi-directional and work from 5mA to 100mA without disturbing the external phone.
INT_LOOP:
Internal loop detected. This signal is used to check the voltage in the LIU.
The voltage is measured just after the rectifier bridge. The threshold level can be adjusted by
R817 and R818. The transistor Q805 then drives the optocoupler Z801.
External loop current status (EXT_LOOP) also changes if
PD changes
Ringer applied
Parallel Phone state changes
Line polarity reversal
Flash key dialled
The two signals EXT_LOOP and INT_LOOP are then used by the firmware to decide whether or
not switch the relay.
Table 2:
EXT_LOOP State
0 Loop current, external Phone Off-Hook
1 Default, no loop current
Table 3:
INT_LOOP State
0 Default, Off Hook or line disconnected
(LIU_HOOK=0)
1 Line connected(LIU_HOOK=1) and On Hook
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