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From: gordon@FTP.COM (Gordon Lee)
Newsgroups: comp.protocols.tcp-ip.ibmpc
Subject: Re: SLIP failing at high baud rates?
Message-ID: <9104191316.AA14684@ftp.com>
Date: 19 Apr 91 13:16:42 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: gordon@ftp.com
Organization: The Internet
Lines: 40

Attn Russ:  this a bug report for SLIP8250

    From: Tim Tsai
    Subject: Re: SLIP failing at high baud rates?
    
      I've been using SLIP8250 at 19.2K for more than half a year now, and
    I've not had any serious problem with it

There's been a lot of talk about SLIP failing in PC's at high speeds,
then again there have been many people like Tim who claim to have seen
no problem.  In order to clear up confusion and doubt, here's the story:

It's a Hardware Problem.  (I've always wanted to say that for real)

haha, enough fun.  The gory detail:

Both the clarkson SLIP8250 driver and FTP's SLIP 
driver for PC/TCP are built to drive an 8250 UART chip.  
The 8250 UART generates one interrupt for each character sent or
received.  A "transmit holding register empty" interrupt tells the
driver that the chip sent the last byte it was asked to send, and now
it is ready to send another.  Well, remember that the chip may also 
be receiving characters at the same time.  Under heavy load, interrupts 
of both types may be flying back and forth at high speed, and in weaker
board designs, some are dropped.  The trick to a successful driver is 
to put a timeout in the loop used to wait for a transmit completion
interrupt.  This ensures you don't wait forever (PC lockup).

This only happens in certain hardware setups, I use the following setup
to reproduce the condition: Everex EV170A serial board in an AGI 1700C (286).
I setup this PC and another PC with a SLIP link and let them ping each
other non-stop, eventually, the AGI would hang.  The timeout prevents this.

   - GL

PS.  the fix is in PC/TCP 2.05 pl1 for both SLIP and PPP.

== Gordon Lee                 FTP Software Inc
== voice: (617) 246-0900      26 Princess St
== fax:   (617) 245-7943      Wakefield, MA  01880