7.7.2. 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 processingis 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 ().
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, thenplaced 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
180
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