he terminal: The "terminal" may be either a data entry device, a printer, or a large computer. DCE refers to the data communication equipment (the modem). The fact that all systems use the same pins does not mean that all devices with RS-232-C interfaces will operate when connected. A connection must have, not only the right pins, but the right infor- PROTOCOLS AND CODES 35 Table 3-1. Commonly Used RS-232-C Signals SIGNAL NAME MNEMONIC Protective A connection to the terminal's metal chas- Ground Transmitted TDATA Outgoing data path from the terminal's point of view. Received RDATA Incoming data path from the terminal's point of view. Request Activated by the terminal to tell the mo- to Send dem to prepare to receive and retransmit data from the terminal. Clear Activated by the modem to tell the termi- to Send nal that it is ready to receive and retrans- mit data from the terminal. Data Set Activated by the modem to tell the termi- Ready nal that the modem is operational. Signal Return path for all other signals on the Ground Received Activated by the modem to tell the termi- Line Signal nal that the modem has made contact with Detector the modem of the far end, and can sense the carrier. Activated by the terminal to tell the mo- Terminal dem that the terminal is operational. Ready mation on those pins. The timing of the clocks (for both DTE and DCE devices), the required control characters, and the proper idle characters are not ensured by RS-232-C. 3.3.1 Handshaking Handshaking is a process whereby the modem and the DTE device prepare a communications channel for data transfer. Figures 3-2 through 3-4 illustrate the activities on certain lines which must take place before a terminal can transmit and then receive data. This ex- ample assumes a normal half-duplex communications channel. The 36 TELECOMMUNICATIONS 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. 3-2. Communications channel in idle state. Terminal 