Safety-protection circuitry In addition to the safety barrier, protection against RING and over- current events is an LIU function. 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 crossed power line with a phone line). 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.
A voltage suppressor (a crowbar type SIDACTOR) provides differential protection. This device becomes low impedance at approximately 300V 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.
Data path The magnetically coupled signals that cross the isolation barrier go either through a transformer or a relay. The TIP and RING signals use a transformer as a means to cross the isolation barrier. TIP and RING are the two-wire paths for all signals from the telephone network. All signaling and data information comes across them, including fax tones and fax data. The telephone network uses DC current to determine hook state of the telephone, so it is necessary to have line current present during a call. On some line interfaces this current flows through the transformer, but on the HP LaserJet 3330mfp LIU, it is blocked by a capacitor from going into the transformer.
Blocking the current with a capacitor prevents the core of the transformer from being saturated by the high DC voltage, which can affect the AC signal quality. Rather than through the transformer, this 102 Chapter 4 - Operational overview EN