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From: Owens-Christopher@cs.yale.edu (Christopher Owens)
Newsgroups: comp.dcom.modems
Subject: Re: Modem control lines
Keywords: Pass-through?
Message-ID: <2532@cs.yale.edu>
Date: 16 Oct 89 16:50:04 GMT
References: <1180@srhqla.SR.COM>
Sender: news@cs.yale.edu
Reply-To: Owens-Christopher@cs.yale.edu (Christopher Owens)
Distribution: usa
Organization: Computer Science, Yale University, New Haven, CT 06520-2158
Lines: 71
In-reply-to: tcm@srhqla.SR.COM (Tim Meighan)

In article <1180@srhqla.SR.COM>, tcm@srhqla (Tim Meighan) writes:
> 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.
> 
[...] 
> 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.

There is something about RTS/CTS and modem links that I have never
fully understood.  My understanding of flow control is as follows.
I'm sorry if I am mixing up RTS and CTS -- I don't have the reference
handy.  By RTS I mean the line from the DTE to the modem telling the
modem that the DTE is ready to accept data.  By CTS I mean the line
from the modem to the DTE telling the DTE that the modem is ready to
accept data.

Let's say I am connected and configured at both ends to use RTS/CTS.
Now if I am at the receiving end of a data transfer, and I find my
input buffer filling, I will drop RTS, which tells my modem to stop
sending characters to me.  Now, presumably, the modem's internal
buffer is going to fill up pretty fast, at which point it is going to
want to send some kind of signal to the other modem to tell it to stop
sending data down the communications line. The exact nature of this
signal would depend upon the modem protocol, of course.  If the other
modem obeys this signal, then pretty soon its buffer is going to fill
up, at which point it wants to signal the computer at its end to stop
sending characters to it, presumably by dropping CTS.  By this time I
have emptied my input buffer, so I re-assert RTS, which tells my modem
to start sending characters to me again.  When my modem's buffer
begins to empty, it sends a signal to the other modem to tell it to
start sending data to it again, at which point its buffer begins to
empty, and it turns on the flow at the far end by re-asserting CTS.

So, according to my understanding, RTS and CTS are in some indirect
sense passed through the telecommunications link in that, if the DTE
at one end shuts off the flow from the modem, the modem at the other
end will eventually shut off the flow from the DTE to which it is
connected.

Now what I don't understand is, what do the modems use for this
mystery signal to communicate with each other?  How sophisticated a
protocol does one need to be using before this happens?  Does a plain
analog modem (e.g.: Bell 212) do anything of this kind?  If not, what
happens when one drops RTS at one end of the connection?  To throw in
an additional monkey wrench, what if one modem is configured to use
rts/cts flow control, and the other is configured to use XON/XOFF.
Assuming a modem protocol like, for example, MNP5, is everything
sorted out correctly such that de-asserting RTS at one end will result
in an XOFF being sent at the other end?

My thanks to anyone who can explain this to me.

/c