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From: ron@topaz.rutgers.edu (Ron Natalie)
Newsgroups: comp.emacs
Subject: Re: 9600 baud problems (was Re: when using termcap, get it right!)
Message-ID: <13403@topaz.rutgers.edu>
Date: Mon, 20-Jul-87 17:30:04 EDT
Article-I.D.: topaz.13403
Posted: Mon Jul 20 17:30:04 1987
Date-Received: Wed, 22-Jul-87 00:45:24 EDT
References: <1373@bobkat.UUCP> <8707061522.AA07395@ucbvax.Berkeley.EDU> <14358@teknowledge-vaxc.ARPA>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 43

A quick summary of RS-232C

The world is divided into DTE and DCE.  DTE is data terminal equipment,
sometimes referred to as the business machine.  This includes what we
refer to as terminals as well as the computers.  DTE's SHOULD have male
DB-25's on them.  DCE is the communications network end.  Modems fall into
this category.  DCE should correspondingly have the female DB-25 connector.

Here are the popular signals.  Note the letters DCE or DTE indicate whether
the signal is sourced from the communications equipment or terminal equipment
respectively:

DSR (Data Set Ready)-DCE- I am up and running and ready to place/receive
    calls. Should remain on whenever the DCE is on and happy.
DTR (Data Terminal Ready) -DTE- I am ready to receive calls.  While many
    people use this to indicate that they are ready and assert it all the time
    when they are up (which is legal), a terminal is allowed to wait until
    the RI line comes up before raising DTR.  Once raised, it should never
    be lowered unless you wish to sever the connection.
RI (Ring Indicator) -DCE- The phone is ringing, there is a call pending...
CD (Carrier Detect), also refered to as RLSD (Recieve Line Signal Detect?)
 -DCE-
    I have detected a valid signal from the remote modem.
RTS (Request to Send) -DTE- Generally ignored in the full-duplex world,
    but this is asserted when the terminal wants to send data to the DCE.
    In the half-duplex or multi-drop environment, the DCE must prepare
    itself to send the data.
CTS (Clear to Send) -DCE- Answer to RTS.  Terminal may send it's data
    now.
and just for completeness
TD (Transmit Data) -DTE- Data from terminal TO DCE
RD (Recieve Data) -DCE- Data to DTE from DCE

Note, that RS-232C has NO FLOW CONTROL.  Let me say this again.  THERE IS NO
FLOW CONTROL.  RS-232C describes the interconnection of a terminal with a
modem.  Some signals like TRANSMIT and RECEIVE DATA are passed through to the
other side.  Other signals may be affected by actions on the other side, but
they are indications from the modem to the terminal specifically.  RTS and
CTS are properties of the transmission of DATA to the MODEM, these signals
go no farther than the modem.  The same for DTR, DSR, CD, and RI.  If you
are using RS-232, flow control needs to be done in the data stream.

-Ron