external serial modem

[William Lindley]

You might also check to be sure the phone line is plugged into the LINE 
jack and the phone into PHONE jack on the modem.  Also check the polarity 
of the phone line.  If you don't have one, get a phone-line tester for $5 
at Radio Shack; if it lights RED instead of GREEN, switch the wires inside 
the phone jack.

Crazy as that sounds, some modems won't work with the line plugged into 
the wrong jack, or with a mis-wired phone line.

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http://www.wlindley.com