Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!netcom!gandrews
From: gandrews@netcom.COM (Greg Andrews)
Newsgroups: comp.sys.amiga.datacomm
Subject: Re: XON/XOFF flow control (was re: 19200 baud amiga)
Summary: Something still seems wrong
Message-ID: <26918@netcom.COM>
Date: 5 Mar 91 08:09:47 GMT
References: <1047@faatcrl.UUCP> <26673@netcom.COM> <1051@faatcrl.UUCP>
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
Lines: 37

In article <1051@faatcrl.UUCP> jprad@faatcrl.UUCP (Jack Radigan) writes:
>gandrews@netcom.COM (Greg Andrews) writes:
>
>>Then something's wrong with the buffering used by the communications
>>programs, or with the threshold where the receiver cries "stop!".
>>XON/XOFF flow control itself shouldn't cause dropped characters,
>>the comm programs aren't handling it right.
>
>  The Amiga serial.device does the XON/XOFF handshake in this case, not the
>comm program.  Like I said, at slow baud rates XON/XOFF is fine.  At higher
>rates a few characters get lost.  It needs to do some scaling to alter the
>trigger count down proportionally to the baud rate in use.
>

The need to 'scale down' the XOFF threshold implies that there's 
something that's timing dependent going on.  Since you would get
fewer characters in the port, you don't overflow the leftover
buffer.

This still sounds wrong to me.  The XOFF is sent to the transmitting
computer at the faster speed also.  It should be recognized in time
to halt transmission only a couple of characters later.  If the
receiving computer didn't react quickly (sending the XOFF), or the
transmitting computer didn't react quickly (recognizing the XOFF and
pausing transmission), then you could get an overflowed buffer only
at high speeds.

Another possibility is that the thresholds for RTS/CTS handshaking
and XON/XOFF flow control are the same, and don't leave much room
for overruns.  Since XON/XOFF is a 'slower' method of flow control
(meaning more characters can come in before the flow stops), the
threshold for XON/XOFF should be lower than that for RTS/CTS.

-- 
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| Greg Andrews      |   gandrews@netcom.COM |
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