By connecting pin 2 (Rx- Receive Data) to pin 3 (Tx- Transmit data), we are able to verify that the computer is able to send and receive information through a serial connection. This path handles the actual information being transmitted across the serial connection. This handshake, for the most part, just verifies that the modem, or other DTR / DSR enabled device, is turned on and active.īy connecting pins 1, 4, and 6, we are taking the DTR signal and sending it to the DSR pin and the CD pin allowing us to verify that the computer is able to communicate with a modem.Īdditionally, you can connect pin 9 (RI - Ring Indicator) to this path to verify that the computer is able to detect when a call is coming in from the modem. The computer will then use pin 4 (DTR - Data Terminal Ready) and pin 6 (DSR – Data Set Ready) to create a handshake between the computer and modem. The modem will use pin 1 (CD – Carrier Detect) to tell the computer that there is a carrier tone detected or it has made a connection with a second modem.

It is important to understand the purpose of each connection for troubleshooting serial connections. The above tables and wiring diagram outlines the required connections to create a loop back plug.

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