Panasonic KX-FLB802CX Service Manual

21KX-FLB802CX/KX-FLB812CX/KX-FLB802CXS/KX-FLB812CXS6.3.3. MODEM CIRCUIT OPERATIONThe modem (IC607) has all the hardware satisfying the CCITT standards mentioned previously.When the SOC (IC600) is brought to a low level, the modem (IC607) is chip-selected and the resistors inside IC are selected by theselect signals from SOC (IC600) A[0]-A[5]. Commands are written through the data bus, and all processing is controlled by the SOC(IC600) according to CCITT procedures. Here, the INT signal dispatched from IRQ (pin 1 of IC607) to the SOC (IC600) implementspost processing.This modem (IC607) has an automatic application equalizer. With training signal 1 or 2 at the time of G3 reception, it can automati-cally establish the optimum equalizer. The modem (IC607) operates using the 28.224MHz clock (X604).Facsimile Transmission/DTMF Line SendThe digital image data on the data bus is modulated in the modem (IC607), and sent from pin 25, 26 via modem IC607, Analog gatearray IC209(24→22), amplifier IC204(6→7) and the NCU section to the telephone line.Refer to CHECK SHEET(P.165).Facsimile ReceptionThe analog image data which is received from the telephone line passes through the NCU section and enters pin 29 of the modem(IC607). The signals that enter pin 29 of the modem (IC607) are demodulated in the board to digital image signals, then placed onthe 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 sen-sitivity.It can be set in the service mode.Refer to CHECK SHEET(P.165).Busy/Dial Tone DetectionThe path is the same as FAX receiving. When it is detected, the carrier detect bit of the resistor in the modem (IC607) becomes 1,and this status is monitored by SOC (IC600).Call Tone TransmissionThis is the call signal which is generated the ASIC (IC604) and sent to the speaker.