HP LaserJet 3052 Service Manual - Page 117

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." 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. 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. 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. 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. 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. 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. 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. 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. 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. ENWW Fax functions and operation (HP LaserJet 3050 all-in-one and HP LaserJet 3055 all-in- 97 one only)