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From: res@colnet.uucp (Rob Stampfli)
Newsgroups: comp.dcom.modems
Subject: Hardware Flow Control - How does it work?
Keywords: hardware flow control
Message-ID: <1991May25.061834.4893@colnet.uucp>
Date: 25 May 91 06:18:34 GMT
Organization: Little to None
Lines: 38

There is an involved discussion going on right now in comp.sys.3b1 about
why some people are having problems making hardware flow control work,
specifically with the Unix-PC.  Generally, there seems to be two
implementations of hardware flow control, as shown in the following excerpt
from "Managing UUCP and Usenet" by O'Reilly & Associates (brackets mine).

My question is, which of the two implementations are modems currently
designed to work with?  Which does Unix provide? If the half-duplex standard
is the way things *really* work, how can the data coming from a modem be
likewise controlled.  Basically, a short tutorial on hardware flow control,
as employed by Unix and current generation modems, would be helpful to me,
and I would predict, to a number of other readers.  I am looking for someone
who knows the facts, and not just speculating.

From "Managing UUCP and Usenet":

  "In the RS-232 standard, [ hardware ] flow control is defined only for
  half-duplex connections -- that is, for connections in which data can be
  transmitted only in one direction at a time.  However, the standard has
  been adapted, de-facto, for full-duplex communications as well.

  "In the half-duplex standard, the DTE [ computer ] asserts RTS when it
  wants to send data.  The DCE [ modem ] replies with CTS when it is ready,
  and the DTE begins sending data.  Unless RTS and CTS are both asserted,
  only the DCE can send data.

  "However, in the full-duplex variations, RTS/CTS is used as a kind of
  throttle.  The signals have the opposite meanings than they do for
  half-duplex communications.

  "When a DTE device is able to accept data, it asserts pin 4, Request to
  Send.  If the DCE is ready to accept data, it asserts pin 5, Clear to
  Send.  If the voltage on RTS or CTS drops at any time, this tells the
  sending system that the receiver is not ready for more data...

Thanks in advance,
-- 
Rob Stampfli, 614-864-9377, res@kd8wk.uucp (osu-cis!kd8wk!res), kd8wk@n8jyv.oh