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From: ron@topaz.rutgers.edu (Ron Natalie)
Newsgroups: comp.dcom.modems
Subject: Re: RS-232C
Message-ID: <14000@topaz.rutgers.edu>
Date: Sun, 16-Aug-87 16:46:01 EDT
Article-I.D.: topaz.14000
Posted: Sun Aug 16 16:46:01 1987
Date-Received: Sun, 16-Aug-87 22:59:31 EDT
References: <192@caeco.UUCP> <2849@phri.UUCP> <1102@gilsys.UUCP> <2853@phri.UUCP>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 26

>	RS-232C was published back in the days when half-duplex connections
> were much more common.  The RTS/CTS lines are intended to be used to
> negotiate line turn-around in a half-duplex connection.  When the the DTE
> (Data Terminal Equipment, i.e. computer or terminal) wants to send
> something it asserts RTS (Request To Send).  When the DCE (Data
> Communications Equipment, i.e. modem) is ready to accept this transmission,
> it asserts CTS (Clear To Send).

This is correct, it isn't just for half-duplex transmissions though.

Your definitions of DTR/DSR/RI/CD are a bit off.
DSR indicates the modem is happy and ready for work.
DTR means a willingness on the host to answer the call
RI means there is a call coming in
CD means that there is good signal

What happens is that the modem is powered up, it asserts DSR.
The phone RINGS, RI is asserted.
The modem then checks for DTR and only answers the phone when it
    is asserted.
The modem then answers the phone.
While carrier is detected CD is asserted.

Generally, the host should watch CD to see if the connection went
away.  DSR going away means for some reason the modem itself gave
up (like it was turned off).