User guide

EN LIU (fax) functions and operation 103
LIU is designed so the current flows through another circuit called a
gyrator or DC hold circuit. This creates the path for the central office
current to flow when a call is in progress.
Hook state
Another magnetically coupled signal is the control signal for
disconnection of the downstream telephone devices (such as a
phone or answering machine). A control signal originating on the
formatter can change the relay state causing the auxiliary jack
(downstream jack) to be disconnected from the circuit.
The HP LaserJet 3330mfp is able to take over a call it recognizes as
a fax. If it doesn’t directly answer the line, it still listens to incoming
calls for the fax tone or for direction from the user to receive a fax.
This idle mode is also called eavesdropping. This mode is active
when the product is on-hook but there is current in the downstream
phone line due to another device being off-hook. During
eavesdropping, the receive circuit is enabled but has a different gain
than during normal fax transmissions.
The HP LaserJet 3330mfp product does not take control of the line
unless it detects a fax tone or unless the operator tells it to manually
connect. This is good because it allows the user to make voice calls
from a phone connected to the HP LaserJet 3330mfp without being
cut off during voice calls.
Downstream current detection
A signal that is optically coupled is the aux phone detect signal. This
signal tells the firmware that there is an active phone (or modem or
answering machine) connected to the auxiliary port of the product
(the right side of the RJ-11 jack).
A circuit that can detect current above a certain threshold generates
the downstream current detection signal. The downstream current
goes through a resistor, generating a voltage. If this voltage is above
the level of the voltage that would be generated by a current at the
threshold level, the signal changes state.
The downstream current detection signal is coupled back to the
formatter through an optoisolator across the safety isolation barrier.
This signal is used by firmware to ensure 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 detection of the appropriate tones).