Safety-protection circuitry
In addition to the safety barrier, the fax card protects against over-voltage and over-current events.
Telephone over-voltage events can be either differential mode or common mode. The event can be
transient in nature (a lightning-induced surge or ESD) or continuous (a power line crossed with a phone
line). The fax card protection circuitry provides margin against combinations of over-voltage and over-
current events.
Common mode protection is provided by the selection of high-voltage-barrier critical components
(transformer and relay). The safety barrier of the fax card PCB traces and the clearance between the
fax card and surrounding components also contribute to common mode protection.
A voltage suppressor (a crowbar-type thyristor) provides differential protection. This device becomes
low impedance at approximately 300 V differential, and crowbars to a low voltage. A series thermal
switch works in conjunction with the crowbar for continuous telephone line events, such as crossed
power lines.
All communications cross the isolation barrier magnetically. The breakdown voltage rating of barrier-
critical components is greater than 5 kV.
Data path
TIP and RING are the two-wire paths for all signals from the telephone network. All signaling and data
information comes across them, including fax tones and fax data.
The telephone network uses DC current to determine the hook state of the telephone, so line current
must be present during a call. The silicon DAA provides a DC holding circuit to keep the line current
constant during a fax call.
The silicon DAA converts the analog signal to a digital signal for DSP processing, and also converts the
digital signal to an analog signal for transmitting data through a telephone line.
The magnetically coupled signals that cross the isolation barrier go through either a transformer or a
relay.
The DSP in the fax card communicates with the ASIC in the formatter using the high-speed serial
interface.
Hook state
Another magnetically coupled signal is the control signal that disconnects the downstream telephone
devices (such as a phone or answering machine). A control signal originating on the DSP can change
the relay state, causing the auxiliary jack (downstream jack) to be disconnected from the telephone
circuit.
The product takes control of calls that it recognizes as fax calls. If the product does not directly pick up
the call, it monitors incoming calls for the fax tone or for the user to direct it to receive a fax. This idle
mode is also called eavesdropping. This mode is active when the product is on-hook but current exists
in the downstream phone line because another device is off-hook. During eavesdropping, the receive
circuit is enabled but has a different gain from the current that is generated during normal fax
transmissions.
ENWW
Fax functions and operation (fax models only)
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