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ELECTRICAL CIRCUITS
4 - 8
4.2.4 Line current detection circuit
NA/TW models
Fig. 4-8
The line current detection circuit consists of the photocoupler (PC3), ASIC (IC23) on the MAIN board
and the other peripheral devices. It detects a dial tone and the hook status of the external telephone
from the current flowing through the line.
When a dial tone is input from the telephone line, current flows through the line and the photocoupler is
repeatedly turned ON/OFF. This allows the REVA1 and REVB1 to be the pulse signals and input to the
ASIC, then the dial tone is detected.
When the handset is lifted from the external telephone, current also flows through the line and the pho-
tocoupler is turned ON/OFF, then the off-hook state is detected.
Signal Name Type Active Description Destination
REVA1 I - Line 1 Current Detection Signal IC23
REVB1 I - Line 1 Reverse Current Detection Signal IC23
CN10
Lb
R87
C7
R86
Line
REVA1
REVB1
19
1
2
3
4
6
5
7
8
4
20
19
20
66
33
NCU board FAX board
SG
CN9
CN262
PC3
PC3
IC23
ASIC
CN5
66
33
CN5
160
161
MAIN board