Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!olivea!uunet!pcserver2!genesis!kdenning
From: kdenning@genesis.Naitc.Com (Karl Denninger)
Newsgroups: comp.unix.sysv386
Subject: Re: Son of FAS?
Summary: Equinox's Xoff times
Message-ID: <1991May1.194927.15814@pcserver2.naitc.com>
Date: 1 May 91 19:49:27 GMT
References: <1991Apr26.013550.20175@netcom.COM> <1991Apr27.231847.8873@pcserver2.naitc.com> <3823@sixhub.UUCP>
Sender: usenet@pcserver2.naitc.com (News Poster for NNTP)
Organization: AC Nielsen Co., Bannockburn IL
Lines: 48
Nntp-Posting-Host: genesis.naitc.com

In article <3823@sixhub.UUCP> davidsen@sixhub.UUCP (bill davidsen) writes:
>In article <1991Apr27.231847.8873@pcserver2.naitc.com> kdenning@pcserver2.naitc.com (Karl Denninger) writes:
>
>| The Equinox drivers and newer Digiboard drivers on the host side use a
>| derivitive of polling.  They use ADAPTIVE polling.  This takes advantage of
>| the fact that you really can't tell the difference between a 50ms delay and
>| no delay in reading characters from a serial port at low baud rates..... and
>| the driver can "learn" the data rate and adjust it's polling rate.  Also,
>| polling to check one bit is VERY fast (ie: is there anything in the buffer
>| that I have to deal with right now).  You can do this from the clock
>| interrupt with nearly no overhead.
>| 
>| Since the board has buffer memory on it, this works very well.  On each
>| "tick" the board can be checked for pending input and output buffer
>| availability.  If there is input pending or output buffer space (and you
>| have output for the card) you can then set a flag for later -- and when you
>| get around to it do the actual input and output in a batch.
>
>  What kind of latency do they get on XOFF? Whats the worst case number
>of characters or ms between the XOFF coming in and the output stopping?
>The obvious answer is 10ms, or about 40 characters, but I hope that's
>wrong, because it's too many.

Equinox guarantees that they can stop the output stream on an Xoff within 10
bit times (ONE character).  All the way to 38,400 baud.  Most of the time
it's at the end of the current character being output when the XOFF is fully
received.

They do this by implementing some of the line discipline on the board, which
allows them to do it >NOW<.

And yes, it really is that fast.  I've never seen a buffer overrun on any
hardware tied to these beasties when the Xon/Xoff flow control is turned on.

Their harware flow control, until recently, wasn't as good.  There were
several characters of slop in there.  Fortunately I have Telebits which need
this feature, and they have more than enough buffer to handle it.  I hear 
they have new drivers which fix that.

If you can't tell I love the Equinox boards... :-)

--
Karl Denninger - AC Nielsen, Bannockburn IL (708) 317-3285
kdenning@nis.naitc.com

"The most dangerous command on any computer is the carriage return."
Disclaimer:  The opinions here are solely mine and may or may not reflect
  	     those of the company.