Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!zaphod.mps.ohio-state.edu!mips!apple!vsi1!daver!dlb!dave
From: dave@dlb.uucp (Dave Buck)
Newsgroups: comp.dcom.modems
Subject: Re: Modems & CTS
Summary: Let's start over, but answer still the same
Message-ID: <1989Dec22.193224.11219@dlb.uucp>
Date: 22 Dec 89 19:32:24 GMT
References: <25213@cup.portal.com> <1989Dec20.185317.19518@dlb.uucp> <42123@lll-winken.LLNL.GOV>
Reply-To: dave@dlb.UUCP (Dave Buck)
Distribution: usa
Organization: D.L.Buck & Associates, Inc.; San Jose, Calif.
Lines: 41

In <25213@cup.portal.com>, Lance_C_Norskog@cup.portal.com asked:
> I have a question about modem standards and industry practices.
> How is CTS (hardware flow control) implemented?  The old Bell 300/1200
> modems implemented this via separate tones.  Has a separate channel
> for CTS/DTS signaling made it into the subsequent modem standards?

And I replied, rather long-windedly, in <1989Dec20.185317.19518@dlb.uucp>:
> Um, 'fraid you are lost.  CTS is a signal from the modem (DCE) to the
> terminal or computer or whatever (DTE), raised in response to the RTS
> signal.  It can be interpreted as follows: ... (my drivel deleted) ...

I did jump on his reference to the Bell 300/1200 baud modems, with:
> Just to give a bit of the air of authority to my remarks, let me paraphrase
> from the Bell System Technical Reference manuals for modems, in particular the
> Data Set 212A (old 1200 baud) Interface Specification, though the specs are
> representative.

Then, casey@gauss.llnl.gov (Casey Leedom) said in <42123@lll-winken.LLNL.GOV>:
> I believe that Lance was referring to the popular, but non-standard,
> use of RTS/CTS for FULL DUPLEX flow control.  In this scheme, the DTE
> (terminal or computer) raises RTS when it is able to receive data and the
> DCE (modem) raises CTS when it is able to receive data.  This scheme has
> become so popular it has become a de facto industry standard.
Quite so.  A trailblazer option, for example.  Sorry, I was thinking of only
half the issue when I wrote my response.

While I was creating my response, I had a typo, pointed out by Peter M. Weiss
(PMW1@PSUADMIN):
>for example, with the printer acting as a DTE, it could drop CTS when it
					   ^^^ should be DCE, and of course
there's all kinds of pinouts, I'm not going to give a treatise on that!

Back to Lance's original question:
>I'm interested particularly in using CTS/DTS for signaling to the remote
>end of a modem session, with no relevance to flow control.  Can this be done?
I stick with the answer, "no".  CTS is not an end-to-end signal, but a local
convention with the modem, and is sometimes used for flow-control, but not for
signalling to the remote end.
-- 
Dave Buck	{amdahl,sun,megatest,plx,ardent,ubvax}!dlb!dave
D. L. Buck and Associates, Inc.; San Jose, California 95119; (408)972-2825