Brother IntelliFax-3550 Service Manual page 45

• The primary function of the NCU (which is shared by facsimile and telephone units) is to
switch a line to the facsimile unit or to the telephone, which is carried out by the line relay.
• Since the direct connection of a facsimile equipment to the line is not allowed for protect-
ing the line, it is essential to insert a line transformer between the line and the facsimile
equipment to insulate them from each other in the direct current band.
The above two components, line relay and 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 hook state by detecting the loop current.
• If the facsimile equipment has an automatic answering facility (TAD), the NCU should be
equipped with a calling signal detector which detects a calling signal and tells it to the
CPU in the FAX engine (ASIC).
• The circuit related to a line transformer allows the line transformer to be invariant by se-
lecting 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 are also
required depending upon 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 in order to prevent them from
flowing out onto the communications line.
• The high-impedance transformer circuit detects the remote activation, and F/T switching
sent from the line in ON-HOOK state without any interference to the line.
• The telephone circuit includes an amplifier that amplifies output signals of the handset mi-
crophone. It also includes a transistor that amplifies receive signals from the communica-
tions lines to sound the handset receiver.
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