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 RJ-11 jack). The DSP
uses the signal to ensure that the product does not go off-hook (and disconnect 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 works with the firmware to achieve the voltage-current characteristic between TIP and RING. The
impedance (the current-voltage characteristic) changes depending on 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 telephone 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.
The product stores all fax pages in flash memory automatically. After these pages are written into flash
memory, they are stored until the pages are sent to another fax machine, printed on the product,
transmitted to a computer, or erased by the user.
These pages are stored in flash memory, which is the nonvolatile memory that can be repeatedly read
from, written to, and erased. The product has 8 MB of flash memory, of which 7.5 MB is available for
74 Chapter 1 Theory of operation ENWW
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