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From: gordon@sneaky.lonestar.org (Gordon Burditt)
Newsgroups: comp.dcom.modems
Subject: Re: Hardware Flow Control
Message-ID: <60652@sneaky.lonestar.org>
Date: 21 Jun 91 20:12:25 GMT
References: <42910619180524.0004025717NB2EM@mcimail.com>
Organization: Gordon Burditt
Lines: 44

You are describing one method of hardware flow control that provides
flow control in both directions.  There are other methods, incompatible,
of course.

>o The CTS is asserted when the modem buffer is full (256 bytes?).

Depending on your definition of "asserted", I thought CTS was
asserted to indicate the terminal/computer should send, and de-asserted
to indicate it should not.  The CTS should be asserted when the modem 
buffer is ALMOST full.  80-90% is a nice number.  The size of the modem
buffer is up to the modem manufacturer.  Some use at least 2k.  The modem 
should give some advance notice to stop sending.

It is impossible to design a stop-on-a-dime flow control scheme using
serial transmission.  For one thing, a serial port can't stop transmitting 
in the middle of a character, so at least one character is likely to arrive 
after CTS drops.  In many cases, the system may not be able to "take back" 
another character already fed to a UART and buffered by it.  Don't be
at all surprised at 2-3 characters arriving after dropping CTS.

>o If a PC asserts RTS, it causes the *remote* modem to stop sending data 
>although a few bytes still get through.

There is no communication channel for a local modem to tell a remote modem
to quit sending in many modem protocols, including non-MNP 2400, 1200,
and 300 baud (we're not talking about control-S/control-Q).  The local
modem quits sending.  The local modem's buffers fill up.  If possible,
the local modem tells the remote modem to quit sending.  This is possible
in PEP, MNP4 and above, and probably any other protocol with error control.
The remote modem's buffers fill up.  The remote modem tells the computer
to quit sending, assuming it's using flow control.

Flow control need not even involve a remote modem.  Ask a modem for a
several complete sets of parameter settings, and it's likely to dump out 
enough data that a system can't take it at a full 19.2kbps.

>o There is no way to signal the local modem to stop sending data.

There is no way to have absolute stop-in-an-instant flow control.
However, assuming proper setup, RTS will stop the local modem from
sending.

					Gordon L. Burditt
					sneaky.lonestar.org!gordon