User Guide

EN LIU (fax) functions and operation 101
Formatter in the fax subsystem
The formatter contains the modem chipset (DSP and CODEC) that
controls the basic fax functions of tone generation and detection,
along with channel control for the fax transmissions.
The CODEC and its associated circuitry act to control transmit and
receive gains, and compensate for transmission signal coupling into
the receive channel. Also on the formatter is circuitry to add
termination impedance to help match the telephone network
impedance.
LIU in the fax subsystem
Two versions of LIU are used in the HP LaserJet 3330mfp. One is
used in the Americas and for Asia. The other is used primarily in
Europe.
Differences in the European version include:
 a different voltage protection device
 current limiting of the DC current
 filtering for the countries that use billing or metering tones
 control of the impedance of the DC circuit via SHUNT signal
Due to the lack of polarization on TIP and RING, all LIU circuitry that
connects to TIP and RING is bi-directional. The LIU operates
identically when TIP and RING are reversed.
The rest of this section describes the hardware functions of the LIU.
Safety isolation
The most important function of the LIU safety isolation between the
high-voltage, transient prone environment of the telephone network
(TNV - telephone network voltage) and the low-voltage analog and
digital circuitry of the formatter (SELV - secondary extra low voltage).
This safety isolation provides both customer safety and product
reliability in the telecom environment.
Any signals that cross the isolation barrier do so either optically or
magnetically. The breakdown voltage rating of barrier-critical
components is greater than 1.5kV.