Newsgroups: comp.dcom.modems
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Modem control lines
Message-ID: <1989Oct17.170047.18468@utzoo.uucp>
Organization: U of Toronto Zoology
References: <1180@srhqla.SR.COM> <2532@cs.yale.edu>
Date: Tue, 17 Oct 89 17:00:47 GMT

In article <2532@cs.yale.edu> Owens-Christopher@cs.yale.edu (Christopher Owens) writes:
>... 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.

Actually there are no such lines in the standards, although today's
modems often use RTS and CTS for that purpose anyway...

>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...

It wants to.  However, it can't, in most of the standard modem protocols,
because there is no such signal.  If you can't accept data as fast as the
other end sends it, you lose some of it.  Simple as that.

>So, according to my understanding, RTS and CTS are in some indirect
>sense passed through the telecommunications link...

Nope.  Because RTS and CTS were *not* meant as flow-control signals,
there is no provision for doing this in most modem protocols.  I think
Telebit's PEP will do it, but that's a very different story from the
rest of the modem world.

>... Does a plain
>analog modem (e.g.: Bell 212) do anything of this kind?

Minor clarification:  Bell 212 is not a plain analog modem.  To find a
plain analog modem -- one which just passes the state of its input-data
lines to the other end at all times -- you have to go back to Bell 103
at 300 baud.

>If not, what
>happens when one drops RTS at one end of the connection?

In a standard modem, what happens if you drop RTS is that the modem either
pays no attention or stops sending carrier.  The standard purpose of RTS is
to tell the modem "commence transmitting", and CTS is for it to tell you
"I am transmitting, go ahead and send data".  They are meant for half-duplex
modems in which only one end can be transmitting carrier at a time.  Old
full-duplex modems either require RTS to be on at all times when connected,
or ignore it entirely.
-- 
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry henry@zoo.toronto.edu