BASIC SYSTEMS

ter) quickly indicates line or equipment trouble. Using neutral signaling, this may only indicate that a steady space is being transmitted. This condition is known as running open. The teletype appears to be running because it is decoding the constant spaces that cause the type hammer to continually strike the type box, but there is no printing or movement across the page. BASIC SYSTEMS When teletypewriters are wire-connected (looped), the exchange of information between them is direct. When they are not physically joined, exchange of information is more complex. Dc mark and space intervals cannot be sent through the air. The gap between the machines must be bridged using transmitters and receivers. The transmitter carrier wave is used to carry the mark and space information. A keyer is used to change the dc pulses from the tty into mark and space modulation for the transmitter carrier wave. The receiver and a convertor are needed to change the rf signal back to dc pulses. RADIO ACTUATED TELETYPE (RATT) SYSTEMS The Navy’s two basic RATT systems are the tone modulated system known as audio frequency tone shift (AFTS), and the carrier frequency shift system known as radio frequency carrier shift (RFCS). The RFCS sys- tem is commonly called frequency shift keying (fsk). For fsk systems, the transmitter provides a source of rf excitation. Figure 2-15 illustrates a basic fsk Figure 2-15.—Basic carrier frequency shift (RFCS) system. 2-17