Panasonic KX-FHD333BR, Service Manual

Page: 144 / 196

8.7.2. MODEM CIRCUIT OPERATION
The modem (IC505) has all the hardware satisfying the ITU-T standards mentioned previously.
When the ASIC IC501 (61) is brought to a low level, the modem (IC505) is chip-selected and the resistors inside IC are selected
by the select signals from ASIC (IC501) ADR0-ADR4(pin 81~85). Commands are written through the data bus, and all processing
is controlled by the ASIC (IC501) according to ITU-T procedures. Here, the INT signal dispatched from IRQ (pins 108 and 121 of
IC505) to the ASIC (IC501) implements post processing.
This modem (IC505) has an automatic application equalizer. With training signal 1 or 2 at the time of G3 reception, it can
automatically establish the optimum equalizer. The modem (IC505) clock is supplied by X503.
1. Facsimile Transmission/DTMF Line Send
The digital image data on the data bus is modulated in the modem (IC505), and sent from pin 69, Analog SW IC509(1
→
2),
amplifier IC511(2
→
1) and the NCU section to the telephone line.
Refer to CHECK SHEET (P.105).
2. Facsimile Reception
The analog image data which is received from the telephone line passes through the NCU section and enters pin 60 of the
modem (IC505). The signals that enter pin 60 of the modem (IC505) are demodulated in the board to digital image signals, then
placed on the data bus.
In this case, the image signals from the telephone line are transmitted serially. Hence, they are placed on the bus in 8 bit units.
Here, the internal equalizer circuit reduces the image signals to a long-distance receiving level.
This is designed to correct the characteristics of the frequency band centered around 3 kHz and maintain a constant receiving
sensitivity.
It can be set in the service mode.
Refer to CHECK SHEET (P.105).
3. DTMF Transmission (Monitor tone)
The DTMF signal generated in the modem (IC505) is output from pin 69, and the NCU section to the telephone line the same
as facsimile transmission signals.
(DTMF Monitor Tone)
Refer to CHECK SHEET (P.105).
4. Busy/Dial Tone Detection
The path is the same as FAX receiving. When it is detected, the carrier detect bit of the resistor in the modem (IC505) becomes
1, and this status is monitored by ASIC (IC501).
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