User Guide

104 Chapter 4 - Operational overview EN
Hook switch control
Another signal coupled optically is the hook switch control. This is a
signal from the formatter that causes an optoisolator on the LIU to go
active (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, there is no DC current 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 recognized as dialing
digits by the central office of the telephone company.
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).
They both must be present to have a valid ring detected by the
product.
The LIU works with the firmware to determine if an incoming signal is
an answerable ring. Circuits on the LIU detect a ring voltage above a
certain threshold and a corresponding signal is passed to the
formatter indicating the ring is above the threshold. The firmware then
measures the time the signal is high and low and qualifies the
cadence to decide if 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. Since it 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
seen 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 once the voltage reaches around 50mA.
The voltage can vary and 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
going 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