HP laserjet 3390, 3392 all-in-one Manuale Di Servizio
LIU in the fax subsystem
Two versions of the LIU are used in the HP LaserJet 3390/3392. One is used in the North American,
South American, and Asian countries/regions. The other is used primarily in European countries/
regions.
South American, and Asian countries/regions. The other is used primarily in European countries/
regions.
The following are features that are different in the European version:
●
A different voltage-protection device
●
Current limiting of the dc current
●
Filtering for the countries/regions that use billing or metering tones
●
Control of the impedance of the dc circuit through the SHUNT signal
Because the TIP and RING are not polarized, all LIU circuitry that connects to TIP and RING is
bidirectional. The LIU operates identically when TIP and RING are reversed.
bidirectional. The LIU operates identically when TIP and RING are reversed.
Safety isolation
The most important function of the LIU is the 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 all-in-one reliability in the telecom environment.
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 all-in-one 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 1.5 kV.
rating of barrier-critical components is greater than 1.5 kV.
Safety-protection circuitry
In addition to the safety barrier, the LIU protects against RING and over-current events.
Telephone over-voltages can be either differential mode or common mode. The RING can be transient
in nature (a lightning-induced surge or ESD) or continuous (a power line crossed with a phone line). The
LIU protection circuitry provides margin against combinations of RING events.
in nature (a lightning-induced surge or ESD) or continuous (a power line crossed with a phone line). The
LIU protection circuitry provides margin against combinations of RING events.
Common mode protection is provided by the selection of high-voltage-barrier critical components
(transformer, relay, and optoisolators). The safety barrier of the LIU PCB traces and the clearance
between the LIU and surrounding components also contribute to common mode protection.
(transformer, relay, and optoisolators). The safety barrier of the LIU PCB traces and the clearance
between the LIU and surrounding components also contribute to common mode protection.
A voltage suppressor (a crowbar-type SIDACTOR) 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 RING events, such as crossed power lines.
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 RING events, such as crossed power lines.
When the differential RING is continuous, the resulting over-current causes the thermal switch to
become high impedance, disconnecting RING from the network and protecting the crowbar from thermal
overload. The thermal switch automatically resets to low impedance after the over-current condition is
removed, reconnecting RING to the network.
become high impedance, disconnecting RING from the network and protecting the crowbar from thermal
overload. The thermal switch automatically resets to low impedance after the over-current condition is
removed, reconnecting RING to the network.
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Chapter 4 Theory of operation
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