The product does not take control of the line unless it detects a fax tone or the user causes it to connect
manually. This feature allows the user to make voice calls from a phone that is connected to the product
without being cut off if a fax is not being received.
Downstream device detection
The line voltage monitoring module of the silicon DAA can detect the line state as well as the
downstream device. It tells DSP via DIB that an active device (telephone, modem, or answering
machine) is connected to the auxiliary port on the product (the right side of the dual RJ-11 jack). The
DSP uses the signal to ensure that the product does not go off-hook (and disconnects a downstream
call) until it has been authorized to do so (by a manual fax start or the detection of the appropriate
tones).
Hook switch control
In the silicon DAA the CODEC controls the hook switch directly. The CODEC is activated when it
receives commands from the DSP. When the circuit is drawing DC current from the central office it is
considered off-hook. When no DC current flows the state is considered on-hook.
Ring detect
Ring detect is performed by the line voltage monitoring module of the silicon DAA, and is a
combination of voltage levels and cadence (time on and time off). Both must be present to detect a
valid ring. The CODEC works with DSP as well as the firmware to determine if an incoming signal is an
answerable ring.
Line current control
The DC current from the CO needs to have a path to flow from TIP to RING. The DC impedance
emulation line modulator and DC terminations modules in the silicon DAA act as a DC holding circuit,
and work with the firmware to achieve the voltage-current characteristic between TIP and RING. The
impedance (the current-voltage characteristic) changes corresponding to certain special events, such as
pulse dialing or when the product goes on-hook.
Billing- (metering-) tone filters
Switzerland and Germany provide high-frequency AC signals on the phone line in order to bill
customers.
A filter in a special fax cable (for certain countries/regions), can filter these signals. Because these
billing signals are not used in the U.S., these filters are not present in the U.S. fax cable.
To obtain a special fax cable, contact your local telephone service provider.
Fax page storage in flash memory
Fax pages are the electronic images of the document page. They can be created in any of three ways:
scanned to be sent to another fax machine, generated to be sent by the computer, or received from a
fax machine to be printed.
26 Chapter 1 Theory of operation ENWW
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