Introduction to Data Communications
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The procedure for connecting between 2 PCs using modems and the telephone line is as follows:

DTE(Tx) is originating the call and DTE(Rx) is answering the call. DTE(Rx) is in the auto-answer mode with DTR(Rx) and DSR(Rx) High is ready to answer a call.

1. The communication program controls the handshaking. The DTE (Tx) dials the phone number:

a. PC sends DTR(Tx) - PC is awake!
b. Modem replies with DSR(Tx) - Modem is awake, too!
c. PC sends RTS(Tx) - Ready for some data?
d. Modem replies with CTS(Tx) - Okay send away!
e. PC transmits data on TXD - Initialize dial this telephone number.

2. DTE(Rx) is in the auto-answer mode with DTR(Rx) and DSR(Rx) High, indicating the receive end is ready to answer a call. This has been setup by the communication program similar to dialing the number in the previous step except the modem is told to go to auto-answer mode. The phone rings:

a. Modem sends RI(Rx) - Hey the phone's ringing!
b. Modem picks up phone line
c. Modem sends answer carrier

The modem since it was initialized in the auto-answer mode, picks up the phone line and sends Answer Carrier (2125 Hz). Everytime the phone rings, the RI line goes high. The communication program will usually display the word "ring" on the screen.

Introduction to Data Communications
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