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 104 Chapter 4 - Operational overview EN