Using the CD Line

Monitoring the Carrier Detect (CD) line of the RS-232 interface is another technique able to detect carrier in answer or originate mode. This assumes that &C1 or the corresponding DIP-switch has been set and the cable is wired properly. You will have a more robust controller if you use result code scanning rather than the RS-232 lines.

If you use CD, you do not know when the modem has given up waiting for the carrier, or why. If the line is busy, you may want to re-try the dial operation. If there is no dial tone, the user should be notified.

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