Panasonic KX-FB423CN Manual page 338

7.3.3. RTC BACKUP CIRCUIT
1. Function
This unit has a lithium battery (BAT501) which works for Real Time Clock IC (RTC: inside IC501).
The RTC continues to work, backed up by a lithium battery even when the power switch is OFF.
The user parameters for autodial numbers, the system setup data and others are in the FLASH ROM (IC523).
2. RTC Inside (IC501) Backup Circuit Operation
When the power switch is turned ON, power is supplied through Q506 to the RTC (inside IC501). At this time, the voltage at
pin 14 of the IC501 is +2.5V. When the power switch is turned OFF, the BAT501 supplies power to RTC through D503 and
Q506. The voltage at pin 14 of IC501 is about +2.2V. When the power switch is OFF and the voltage of +3.3V decreases, pin
14 of RTC (IC501) becomes roughly the same voltage as the battery voltage. RTC goes into the backup mode, in which the
power consumption is lower.
7.3.4. ANALOG SECTION
7.3.4.1. DESCRIPTION OF BLOCK DIAGRAM IN ANALOG SECTION
1. Function
The analog section serves as an interface between the telephone line. The digital board (IC505) for transmission and reception
of FAX signals, switches between the digital board (IC505) and other sections by means of a multiplexer in the NCU section.
The control signals to the individual analog sections are output mainly from ASIC IC501, and the status information for the
various sections is also held in ASIC IC501.
Simple explanations for the various sections are given below.
2. Circuit Operation
[NCU Section]
This is composed of a bell detection circuit, pulse dial generation circuit, amplifier circuit for line transmission and reception,
sidetone circuit, Extension phone line off-hook detection circuit, Remote FAX activation circuit.
Refer to NCU SECTION (P.163).
[Modem (IC505)]
This is used for FAX signal tone modulation, DTMF signal transmission. The DTMF signal is placed into the TX system.
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KX-FB423CN