RTS (on) Carrier (on) Modem CD (on) Computer pin 4 pin 8 CTS (on) CTS (off) pin 5 pin 5 CD (off) Modem Carrier (off) RTS (off) pin 8 pin 4 Fig. 3-3. Changing terminal to transmit state. Terminal RTS (off) Carrier (off) Modem CD (off) Computer pin 4 pin 8 CTS (off) CTS (on) pin 5 pin 5 CD (on) Modem Carrier (on) RTS (on) pin 8 pin 4 Fig. 3-4. Placing computer in response state. pin numbers and names are those of the pins on the RS-232-C in- terface. Note in Fig. 3-2 the carrier (a continuous signal capable of being modulated to carry information) from the computer to the terminal is on. The modem at the terminal end detects this on pin 8 (carrier detect). Because the terminal wishes to send nothing, its request to send (RTS) pin 4 is off. Therefore pin 5, the terminal's clear to send (CTS), which is driven by the modem, is also off. PROTOCOLS AND CODES 37 Because the computer is seeing no carrier from the terminal, its carrier detect (pin 8) is off. Because it is sending a string of idle characters, its RTS is on and, so is its clear to send (CTS). In half-duplex systems as presented in Fig. 3-3, the terminal must wait for the computer to indicate that it is ready to accept data before the terminal can send any. When the computer is ready to listen, it turns off its RTS and the computer's carrier signal goes off. The modem at the terminal end sees that the carrier is off and turns the CD off. If the terminal has something to send, it turns on its RTS, which tells the modem to start sending the carrier signal. After a timed wait, the terminal modem assumes the computer's modem sees its signal and tells the terminal that it is permitted to transmit by turning on the terminal's CTS. In Fig. 3-4, when the terminal has nothing more to transmit, it turns off its RTS. The terminal's modem in turn dops its carrier signal. The modem on the computer side senses this and turns its carrier detect (CD) off. When the computer has something to say, it turns on its RTS, which tells its modem to start transmitting the carrier sign