Re: [TECH] Modem-to-Modem wire connection

From: Clive Wright (
Date: Sun Apr 23 2000 - 20:39:58 CEST

Michele Andreoli wrote:
> Do not worry about that: I know AT command. My only doubt is about
> cable, because usually crossover is needed: transmit pin connected
> to receiving pin, on the other side, and so on.
Unlike RS232 the connection to a modem is a wire loop there
is no separate rx and tx line and a zero volt (ground plane)
connection is not required.
A crossover is not needed but if one is present it will not
make any difference as long as the loop is intact. Telephone
instruments usually receive their power from a (voltage
limited) constant current source at the telephone exchange;
but as modems do not depend on the line for power, the lack
of a DC power source should not present a problem.


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