HP Color LaserJet CM2320 Service Manual - Page 123

Fax functions and operation (fax/memory-card models only) The following sections describe the product fax capabilities. Computer and network security features The product can send and receive fax data over telephone lines that conform to public switch telephone network (PSTN) standards. The secure fax protocols make it impossible for computer viruses to be transferred from the telephone line to a computer or network. The following product features prevent virus transmission: ● No direct connection exists between the fax line and any devices that are connected to the USB or Ethernet ports. ● The internal firmware cannot be modified through the fax connection. ● All fax communications go through the fax subsystem, which does not use Internet data-exchange protocols. PSTN operation The PSTN operates through a central office (CO) that generates a constant voltage on the TIP and RING wires (48 V, usually). A device goes on-hook by connecting impedance (such as 600 ohms for the U.S.) across the TIP and RING so that a line current can 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 like 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 the line-current flow ceases, removing the CO connection from both the sender and the receiver. On most phone systems, the TIP and RING wires appear on pins 3 and 4 of the RJ-11 modular jack (the one on the fax card). 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 basic functions of PSTN operation 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. Receive faxes when you hear fax tones In general, incoming faxes to the product are automatically received. However, if other devices are connected to the same phone line, the product might not be set to answer automatically. If the product is connected to a phone line that receives both fax and phone calls, and you hear fax tones when you answer the extension phone, receive the fax in one of two ways: ● If you are near the product, press Start Fax on the control panel. ● Press 1-2-3 in sequence on the extension phone keypad, listen for fax transmission sounds, and then hang up. ENWW Fax functions and operation (fax/memory-card models only) 111