Samsung sf150t Service Manual page 39

Circuit Description
5-6 LIU PBA
The LIU (Line Interface Unit) interfaces the MODEM and telephone to the telephone line. The FAX and
telephone portions of the LIU are active with machine power on. When machine power is off, only the
telephone circuitry operates, powered by telephone line voltage. The FAX portion of LIU consists of the
interface between MODEM and telephone line; and the circuits for DC loop feeding, DP signal, loop current
and ring detect. The telephone portion is divided into ringer, dialling, and speech circuits. Refer to the
schematic and connection diagram sections of this manual.
5-6-1 FAX section
MODEM/LINE INTERFACE
This is the path for data and control signals.
¥ CML201 relay: switches telephone line between
FAX and telephone circuits.
¥ U201 pin 3 TIT: single-ended input for
transformer T202.
¨ TIT: Transmit Input from Transformer
¥ U201 pin 40 ROT : Output for driving a
transformer T203 with an AC impedance
exceeding 10Kohm.
¨ ROT: Receive Output Transformer
¥ C212: DTMF and CNG detect path to T201
20Kohm winding under idle conditons; and DC
blocking for 20Kohm winding.
¥ AC impedance: the AC impedance of U201 (I-
LIU) is set at 1000ohm by external capacitor
C228 at U201 pin 8 CI (Complex Impedance
input) port. With an external resistor (R244) at
U201 pin 34 ACI port it can be programmed to
600ohm. And U201 pin 35 CS (Current Shunt
control output) port is N-channel open drain
output to control the external high power shunt
transistor for synthesizing AC and DC
impedance.
¥ DC conditions : normal operating mode is from
15mA to 100mA. An operating mode with
reduced performance is from 5mA to 15mA. In
the line hold range from 0mA to 5mA the device
is in a power down mode and the voltage at
U201 pin 37 LI (Line Input) port is reduced to a
maximum of 3.5V. The DC characteristic is
determined by the voltage at U201 pin 37 LI port
and a R215 resistor between U201 pin 37 LI and
pin 39 LS port. It can be calculated by the
following equation : V
RING DETECT
¥ U201 pin 28(MO) is ring melody output port and
this signal drive the Q207 which drive the
FET201 which drive Photo coupler U204 for
5-8
= V + I X R215.
LS LI LINE
artificial ring.
MF DIAL (Same as telephone section)
¥ U201 pin 2 DMS(Dial Mode Selection) port is set
to VDD by R227 It is M/B ratio 33:66, and no
power operation mode conducts only DP.
¥ MF signal appears (tone level of low group :
typical -14dBm) at U201 pin 4 MFO(DTMF
Generator Output). This signal is leveled by
R226, R225 and C239, then to amplier U201 pin 9
MFI(DTMF Amplier Input).
¥ Line dial signals appear at U201 pin 39 LS(Line
Current Sense Input).
DP DIAL (Same as Telephone section)
¥ U201 pin 2(DMS) is set to Vdd(33/67) or
VSS(40/60) by R227 or R228 resistor.
¥ Dial pulses originate at U201 pin 27(DPn), which
toggles Q205, which drives Q201. The resulting
intermittent voltage interrupts the telephone
line.
¥ Pulse M/B ratio is set by U201 pin 2 MDS port.
Vdd = 33/66, and Vss = 40/60.
¥ U201 pin 35 CS port : Modulation of line voltage
and shorting the line during make period of
pulse dialling.
LLC (Line Loss Compensation) / LOOP
CURRENT DETECT
¥ When the LLC pin option. it is activated, the
transmit and receive gains for both I/O are
decreased 6dB at line currents above 20mA
when the U201 pin 31 LLC is connected to
AGND, and from 75mA when this pin is
connected to VDD. The LLC is deactivated
when LLC pin is connected to VSS.
¥ When CML201 or Hook Switch switches to
telephone line, U201 on the LIU board and CPU
(U1) on the Main board begin communication.
U201 sends an <Ack> message containing the
line current information to recognize a parallel
phone.
SF150T