HP 3015 Service Manual - Page 114

Fax functions and operation (HP LaserJet 3015 and 3030 only) The following sections describe the fax capabilities of the product. PSTN operation The public switched telephone network (PSTN) operates by having a central office (CO) that generates a constant voltage on the TIP and RING wires (48 V, usually). A device goes onhook by connecting impedance (such as 600 ohms for the U.S.) across TIP and RING, allowing a line current to flow. The CO can detect this current and can send impulses like dial tones. The product generates more signaling tones, such as dialing digits, to tell the CO how to connect the call. The product can also detect tones, such as a busy tone from the CO, that tell it how to behave. When the call is finally connected, the CO behaves as if it were a piece of wire connecting the sender and receiver. This is the period during which all of the fax signaling and data transfer occurs. When a call is completed, the circuit opens again and line current flow ceases, removing the CO connection from both the sender and the receiver. All of this is done with a two-wire arrangement. The two wires are called TIP and RING on all phone systems. On most phone systems, the wires appear on pins 3 and 4 of the RJ-11 modular jack (the one on the LIU). These two wires do not have to be polarized because all the equipment works with either TIP or RING on pin 3 and the other wire on pin 4. This means that cables of either polarity can interconnect and will still work. These are the basic functions of PSTN operation that are assumed in the design of the fax subsystem. The product generates and detects the signaling tones, currents, and data signals that are required to transmit and receive faxes on the PSTN. The fax subsystem The formatter, LIU, firmware, and software all contribute to the fax functionality. The designs of the formatter and LIU, along with parameters in the firmware, determine the majority of the regulatory requirements for telephony on the product. The fax subsystem is designed to support V.34 fax transmission, lower speeds (such as V.17 fax), and older fax machines. Formatter in the fax subsystem The formatter contains the modem chipset (DSP and CODEC) that controls the basic fax functions of tone generation and detection, along with channel control for the fax transmissions. The CODEC and its associated circuitry act to control transmit and receive gains, and to compensate for transmission signal coupling into the receive channel. The formatter also contains circuitry that adds termination impedance to help match the telephone network impedance. LIU in the fax subsystem Two versions of LIU are used in the HP LaserJet 3015 and 3030 all-in-one. One is used in the North American, South American, and Asian countries/regions. The other is used primarily in European countries/regions. 96 Chapter 4 Operational overview ENWW