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From: jr@LF-SERVER-2.BBN.COM (John Robinson)
Newsgroups: comp.emacs
Subject: Re: 9600 baud problems (was Re: when using termcap, get it right!)
Message-ID: <8707061522.AA07395@ucbvax.Berkeley.EDU>
Date: Mon, 6-Jul-87 11:22:59 EDT
Article-I.D.: ucbvax.8707061522.AA07395
Posted: Mon Jul  6 11:22:59 1987
Date-Received: Tue, 7-Jul-87 04:41:13 EDT
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>> One problem that no one has pointed out is that the EIA-232-D
>> definition does not have any hardware flow control defined.

I don't know about 232D (which is less than a year), but 232C does
talk about half-duplex operation, as I remember.  Here it is mandatory
to watch the request-to-send/clear-to-send protocol, so that the two
modems can coordinate their sending and receiving.  The modems, for
their part, have to turn their carriers on and off in response to what
the computers/terminals do.  Hardware flow-control can be based
loosely on this model (Await CTS when you want to send; don't assert
DTR [or else RTS] when you don't want to receive).  Most UART chips
can be wired so that they won't send unless they see CTS, providing
the send direction automatically.

When it works, hardware flow control is nice.  It is faster (the
terminal has to do some work to throw away all that padding - in fact,
the VT101 works so hard that NO amount of padding is enough at 9600
baud) and it is not affected by termianl modes - it just keeps going
even in raw mode.  However, because they associate the control leads
with half-duplex, most modems can't pass their value transparently to
the other end, so hardware flow-control won't work over modems.

I wonder if anyone has ever made an intensely interactive application
like emacs work over a half-duplex line!

/jr
jr@bbn.com or jr@bbnccv.uucp

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