HP laserjet 3390, 3392 all-in-one Manuale Di Servizio
Hook switch control
Another optically coupled signal is the hook switch control. This signal from the formatter causes an
optoisolator on the LIU to activate (making an open circuit into a short circuit). When shorted, the circuit
is drawing dc current from the central office and is considered “on-hook.” When open, no dc current
flows and the state is considered to be “off-hook.”
optoisolator on the LIU to activate (making an open circuit into a short circuit). When shorted, the circuit
is drawing dc current from the central office and is considered “on-hook.” When open, no dc current
flows and the state is considered to be “off-hook.”
For pulse dialing, the optoisolator is toggled on and off with a particular timing sequence to generate
pulses that the central offices of the telephone company recognizes as dialing digits.
pulses that the central offices of the telephone company recognizes as dialing digits.
Ring detect
Another optically coupled signal on the LIU is ring detect. Ring detect is a combination of voltage levels
and cadence (time on and time off). Both must be present in order for the all-in-one to detect a valid
ring.
and cadence (time on and time off). Both must be present in order for the all-in-one to detect a valid
ring.
The LIU works with the firmware to determine if an incoming signal is an answerable ring. When circuits
on the LIU detect a ring voltage above a certain threshold, a corresponding signal is passed to the
formatter to indicate that the ring is above the threshold. The firmware then measures the amount of
time that the signal is high and low, and qualifies the cadence to determine whether it is a valid ring.
on the LIU detect a ring voltage above a certain threshold, a corresponding signal is passed to the
formatter to indicate that the ring is above the threshold. The firmware then measures the amount of
time that the signal is high and low, and qualifies the cadence to determine whether it is a valid ring.
Line current control
The dc current from the CO needs to have a path to flow from TIP to RING. Because the current does
not flow in the transformer, a path through some transistors and diodes (the dc hold circuit) acts like a
simulated inductor (thus replacing the function of the transformer). This dc hold circuit is the main
component of the voltage-current characteristic between TIP and RING.
not flow in the transformer, a path through some transistors and diodes (the dc hold circuit) acts like a
simulated inductor (thus replacing the function of the transformer). This dc hold circuit is the main
component of the voltage-current characteristic between TIP and RING.
Two other functions of this block of circuits are the current limit and the SHUNT for Europe. Current limit
basically maintains a constant current from TIP to RING after the voltage reaches about 50 mA. Even
if the voltage varies, the current will remain the same.
basically maintains a constant current from TIP to RING after the voltage reaches about 50 mA. Even
if the voltage varies, the current will remain the same.
The SHUNT function changes the impedance (the current-voltage characteristic) during certain special
events, such as pulse dialing and when the devices goes on-hook.
events, such as pulse dialing and when the devices goes on-hook.
During such an event, the firmware tells the DSP on the formatter to activate SHUNT. A signal goes to
the LIU through an optoisolator (optically coupled), which changes the state of some transistors so that
the board has a linear current characteristic (normally, it is current-limited). Then more current can flow
to the network, as required by these special events.
the LIU through an optoisolator (optically coupled), which changes the state of some transistors so that
the board has a linear current characteristic (normally, it is current-limited). Then more current can flow
to the network, as required by these special events.
Billing- (metering-) tone filters
Switzerland and Germany provide high-frequency ac signals on the phone line in order to bill customers.
The LC (inductor, capacitor) notch filters are provided on the European LIUs to prevent these billing
signals from adversely affecting the data path and the dc hold circuit.
The LC (inductor, capacitor) notch filters are provided on the European LIUs to prevent these billing
signals from adversely affecting the data path and the dc hold circuit.
An additional filter in the cable (for certain countries/regions) can, in some cases, supplement these LC
notch filters. Because these billings signals are not used in the U.S., these filters are not present on the
U.S. LIU.
notch filters. Because these billings signals are not used in the U.S., these filters are not present on the
U.S. LIU.
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Chapter 4 Theory of operation
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