Re: 14.4 Modem: "CD" indicator light remains on

[dempson@actrix.gen.nz (David Empson)]

Phoenyx <quazar@dcnet2000.com> wrote:

> By default, this light should not be on. It detects the presence
> of a carrier signal. This means the phone signal when the line
> goes off hook.

Not exactly.  "Carrier" from the modem's point of view means that the
modem has established an end-to-end data connection with the other
modem.

If you watch the carrier detect (CD) light on a correctly configured
modem, and listen to the connection noises, you will see that the light
comes on after the modem has finished its whistling and chattering for
the negotiation sequence; it usually happens about the same time as the
modem speaker going quiet.  The light then goes out again when the
connection is lost and the modem hangs up.

> The carrier is also used (sometimes) to hang up the modem by dropping the
> line (considered rude in the old BBS systems).

You're confusing carrier detect (a signal from the modem to the
computer) with DTR (a signal from the computer to the modem).

Carrier detect is used by a bulletin board (for example) to detect that
the caller has hung up abruptly.  It is absolutely essential for the
operation of a system which automatically answers the telephone and
needs to operate unattended.  For an end user, it is only of minor
interest.

DTR ("Data Terminal Ready") can be used (on a PC or a Super Serial Card
for an Apple II) to hang up the modem connection, assuming the modem has
been configured to behave this way for loss of DTR.  This is usually
done by including &D2 in the initialization string.

The Mac, Apple IIgs and Apple IIc only have a single flow control output
signal, and it is usually required for hardware handshaking (RTS), so
DTR hangup is not an option.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand