Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!ginosko!usc!csun!srhqla!tcm
From: tcm@srhqla.SR.COM (Tim Meighan)
Newsgroups: comp.dcom.modems
Subject: Re: Modem control lines
Keywords: Pass-through?
Message-ID: <1180@srhqla.SR.COM>
Date: 13 Oct 89 22:07:38 GMT
Reply-To: tcm@srhqla.SR.COM (Tim Meighan)
Distribution: usa
Organization: Silent Radio, Los Angeles
Lines: 35

In article <413@belltec.UUCP> lance@belltec.UUCP (Lance Norskog) writes:

>I have a peculiar question.  Do old-style analog modems pass through the 
>CTS/DTS signaling along with the data, or do they ignore it?  If CTS/DTS
>is part of the spec'd behavior of modems, how is it handled with high-speed
>digitally buffered and processed modems?

Well, it's not so peculiar, although you are mixing acronyms.  RTS and CTS
are lines usually used for flow control, while DTR (not DTS) is generally
used for equipment control (like resetting a modem to make sure it hangs
up the phone line).  Also, you don't seem to have a good grasp of what
flow control on serial ports is for -- these are NOT signals intended
to be propagated through, like data, to appear at the remote modem's
serial port.

RTS and CTS are used to make sure that the flow of data between the modem
and the LOCAL data producing equipment (computer, terminal, etc) is properly
maintained without data loss.

An example: your local modem connects to a remote modem at 300 baud, but 
the speed of the RS-232 link between your modem and your computer is at
1200 baud.  Since your computer is sending chars into your modem faster
than it can get them over the phone line, your modem has to be able to
occasionally tell your computer to stop talking so it can get caught up.
That's flow control, and that's what RTS and CTS are used for.  

All modems that support hardware flow control, old or new, work this same
basic way.  The method that modems use to exchange data on the analog side
(the phone line) doesn't change the function of RTS/CTS on the serial
port side.

Tim Meighan
SilentRadio

"Guaranteed more reliable, with fewer working parts!"