Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!sdd.hp.com!apollo!holbrook
From: holbrook@apollo.HP.COM (Alan R. Holbrook)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Elementary Modem Control (was Re: CTS/RTS...)
Message-ID: <503f6ae3.20b6d@apollo.HP.COM>
Date: 8 Mar 91 21:01:00 GMT
Sender: root@apollo.HP.COM
Lines: 47


In article <7134@emory.mathcs.emory.edu>, km@mathcs.emory.edu (Ken Mandelberg) writes:
|> Is there a way to get at least partial support for CTS/RTS external to
|> the program? For example, while I think doing flow control on incoming
|> data is inherently a software issue for the DCE (drop RTS if you can't
|> buffer any more data), outgoing flow control is more a hardware issue
|> for the DCE. The UART on the DCE can told not to transmit (and thus not
|> go ready) unless it sees CTS from the DTE.

Whoa up a minute, thar, podner...  If I remember my basic modem control
stuff from years ago when I was working for Racal Milgo, you've got some
very basic misconceptions of how data comm works.  RTS (Request to Send)is a 
control signal from either the DTE or DCE to the modem, saying "I have data 
to transmit."  CTS (Clear to Send) is a signal back from the modem saying
"I'm ready to send the data for you."

What you want to say in the first case...

|> data is inherently a software issue for the DCE (drop RTS if you can't
|> buffer any more data), outgoing flow control is more a hardware issue

... is really "for the DCE (drop DTR if you can't buffer any more data),..."

and in the second case...

|> for the DCE. The UART on the DCE can told not to transmit (and thus not
|> go ready) unless it sees CTS from the DTE.

... is really "unless it sees CTS from the modem attached to the DTE."

Basic modem flow control to transmit data:

    DTE/DCE raise DTR
    wait for DSR
    raise RTS
    wait for CTS
    transmit

Basic modem flow control to receive data:

    watch for DCD
    rcv

There are subtleties based on what dux and what protocol, but those are the basics.

    Alan Holbrook
    DataComm techie to Noah on the ark...