Panasonic UF-890 Service Manual page 238

6.2.8 Line Control Board
The following shows a block diagram of the Line Control Board.
Line
Telephone
The Ring Detector consists of a photocoupler, PC2, and its peripheral circuits. The ringing signal is half-
wave rectifier in the Ring Detector, and transferred through the nCTON signal line to the IC60 on the FCB
PC Board. The IC60 observes the signal to distinguish from signals caused by chattering.
The Off-Hook Detector (External Telephone) circuit consists of the photocoupler, PC1, and its peripheral
circuits. When PC1 detects loop current flow, it emits a Low active output signal (nHKOF) to the IC60 which
monitors it for a specified time. If the IC60 detects no change in the Low signal level, it determines that the
External Telephone is Off-Hook.
CML
CML
T2
C5
Ring
Detector
PC2
+24V
CML
PC1
Off-Hook
PLS
Detector
238
PLS
PC2
PC1
LCU PCB
Modem
(IC 91)
(IC 90)
(IC 80)
Port
(IC 60)
Port
(IC 60)
Port
(IC 60)
FCB PCB