Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!usc!zaphod.mps.ohio-state.edu!think!ames!dlb!dave
From: dave@dlb.uucp (Dave Buck)
Newsgroups: comp.dcom.modems
Subject: Re: Modems & CTS
Message-ID: <1989Dec20.185317.19518@dlb.uucp>
Date: 20 Dec 89 18:53:17 GMT
References: <25213@cup.portal.com>
Reply-To: dave@dlb.UUCP (Dave Buck)
Distribution: usa
Organization: D.L.Buck & Associates, Inc.; San Jose, Calif.
Lines: 53

In article <25213@cup.portal.com> Lance_C_Norskog@cup.portal.com writes:
>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?
>Do smart modems implement it and operate properly?

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:  RTS is raised by the DTE to
indicate that it wishes to send, and that the modem should thus place a
carrier on the line.  The modem raises CTS to indicate that it has done so,
and that enough time has passed for the carrier to have reached the other
end of the line and hit the echo suppressors on the circuit.  The delay
between RTS and CTS is often 'strapped' in the modem at a fixed delay, usually
in the range 0-8 msec for "local" calls, perhaps 100 msec for long distance,
and 250 msec or so for a link that might go over a satellite link.

Since the DTE isn't to begin transmitting until CTS goes high, many DTEs have
a hardware or software implementation that holds off transmitting data when
CTS is low.  Thus, CTS can often be used as a flow-control handshake between
the DTE and the other device.  For direct connect of printers to computers,
for example, with the printer acting as a DTE, it could drop CTS when it
wished to stop the computer from further printing (temporarily).  Local
connection flow control mechanisms vary greatly, but CTS is a common hardware
mechanism.  For modems that do some local buffering of data, such as the
well-known Trailblazer, CTS can be used to stop the DTE from transmitting
more data for the moment.

W.R.T. "CTS/DTS", I suspect you mean RTS/CTS.

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.

	An ON condition of the CTS circuit indicates to the DTE
	that the modem will transmit data on the TxD circuit.
	If CTS is off, the modem will ignore TxD.

>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?

Generally, no.  Some older modems, such as Bell's 202, have a "reverse channel"
which is a separate carrier ... the modem is half-duplex, with most of the data
flow on the primary carrier, going in one direction, and the reverse channel is
a carrier in the opposite direction.  Not commonly found on today's modems.

>Lance Norskog
>thinman@cup.portal.com
-- 
Dave Buck	{amdahl,sun,megatest,plx,ardent,ubvax}!dlb!dave
D. L. Buck and Associates, Inc.; San Jose, California 95119; (408)972-2825