BROTHER MFC5500ML
Service Manual

� The primary function of the NCU, which is shared by facsimile and telephone units, is to switch the line to the facsimile unit or to the telephone; switching is carried out by the line relay. � Since the direct connection of a facsimile equipment to the line is not allowed in order to protect the line, it is essential to insert a line transformer between the line and the facsimile equipment to insulate them from each other in a direct current band. The above two components, a line relay and a line transformer, are the minimum requirements for the NCU of the facsimile equipment. � If an external telephone is connected to the facsimile equipment, the NCU should have a loop current detector to identify the on-hook/off-hook state by detecting the loop current. � If the facsimile equipment has an automatic answering facility, the NCU should be equipped with a calling signal detector which detects calling signals and notifies the CPU. � The circuit related to a line transformer allows the line transformer to be invariant by selecting the constants of the parts in this circuit so as to conform to the communications regulations or codes of each country. In addition to the above basic components of the NCU, the following components may also be required depending on the additional functions of the facsimile equipment: � The dial pulse generator generates dial pulses within the facsimile equipment. � The surge absorber is a protection circuit which absorbs lightning surges. � The noise filter eliminates noise, including radiation noise, which would otherwise enter the communications line. � The high-impedance transformer circuit detects the remote activation, and F/T switching sent from the line in the on-hook state without any interference on the line. � The MODEM-telephone switching circuit changes the connection of the line transformer to the telephone circuit or to the MODEM. Note that the MODEM always monitors the signals on the line even if the telephone is selected. � The telephone circuit includes two amplifiers; one for amplifying the output signals of the handset microphone and the other for amplifying the receive signals from the communications lines to generate the handset receiver sound.

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