January 2001 © TOSHIBA TEC
5 - 15
GD-1060 CIRCUIT DESCRIPTION
5.4.3 Line current detect circuit
NAD/SAD/TWD models
Fig. 5-4-7
The line current detect circuit consists of the photo-coupler (PC3), I/O port GA-2 (IC6) on the Main PWA,
and 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 photo-coupler
repeats turning on and off. This allows the OREVA1 and OREVAB1 to be pulse signals and input to the
I/O port GA-2 to detect the dial tone.
When the handset is lifted from the external telephone, current also flows through the line and the photo-
coupler turns on and off to detect the off-hook status.
CN10
Lb
R67
C7
R86
Line
OREVA1
OREVB1
19
1
2
3
4
6
5
7
8
4
20
19
20
17
18
NCU PWA
Main PWA
IC6
I/O port GA-2
SG
CN9
CN3
PC3
PC3
05-04-04-U
Signal Name
I/O
Active
Description
Destination
OREVA1
I
-
Line Current Detect Signal
I/O port GA-2
OREVB1
I
-
Line Current Reverse Current Detect Signal
I/O port GA-2