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Path: sparky!uunet!usc!wupost!csus.edu!netcom.com!harp
From: h...@netcom.com (Gregory O. Harp)
Subject: 57.6Kbps under Linux -- some results...
Message-ID: <!nwn!6h.harp@netcom.com>
Date: Sat, 12 Sep 92 13:26:38 GMT
Organization: Netcom - Online Communication Services  (408 241-9760 guest) 
Lines: 69

Well, since I opened my big mouth and said that 57.6Kbps probably
wasn't possible, I ended up doing some experimenting with exactly
that.

I'll summarize for those who don't want to read the whole thing.  I
can _almost_ do 57.6Kbps with my 50Mhz 486DX.  I only have 16450
UARTs, so those of you with 16550A UARTs and very fast machines should
be able to do it.

[WARNING!  Kernel-hacking ahead!]

First, I had to hack the kernel serial code to handle that speed.
Linux currently only supports 38.4Kbps as "shipped."  BTW, I'm working
with version 0.97pl4 of the code, if you want to follow along.  It's a
matter of modifying the table on lines 328-330 of
kernel/chr_drv/serial.c.  This is the baud rate divisor table.

Anyway pick an entry like the value 384, which is 300bps, and change
it to 2, which is the divisor for 57600bps.  You might want to replace
a less-common baud rate like 50 baud (the divisor to change is 2304),
but most terminal software won't support that.  BTW, if you really
want to try it, the divisor for 115Kbaud is 1, but good luck! ;)

If you want to figure it out for yourself, the formula is:

		divisor = 1843200 / (baud * 16)

Once you rebuild the kernel (Remember to back up!  Oops, am I too
late? ;) ) and reboot, you should be able to tell any software that
you want to use 300 baud (or whichever entry you modified) and you
will actually be set up for 57.6Kbaud.

Now, I tried hooking up to my Amiga 3000 at this rate.  I had some
success, but the file I downloaded was corrupted somewhat and I only
pulled about 4K/sec out of the transfer.

My next experiement was to null-modem my two serial ports together
and do the following:

	cat </dev/ttys0 >foo2 & 
	cat foo >/dev/ttys1

I transferred a 1430343 byte file in 4 minutes and 9 seconds, meaning
I pulled 5744 bytes per second!  57.6Kbaud!  

Now, once again, the file was a little corrupted, but tar was able to
identify it as a compressed archive and it at least made it through a
hundred K or so before crapping out.

I'd be really interested in seeing what a 486DX50 with 16550A UARTs on
it can do.  If someone out there has one (or they want to swap I/O
cards with me ;) ) send me mail at h...@netcom.com and we'll talk.

BTW, the reason I keep saying 16550A and not just 16550 is because a
glance at the code tells me that Linux is only enabling the FIFO if
the UART is the 16550A (refer to lines 265-272 of serial.c).  Linus,
can you tell us why you don't use the FIFO on the 16550?  I've
personally never used the FIFO because my projects were for 16450
UARTs, but I'm not aware of any problems.

Oh well...  Gotta go.  Glad the weather's nice here in Dallas.  Gonna
be a nice afternoon for some horseriding!

Later folks...
  Greg
-- 
-----------------Greg-Harp----------------h...@netcom.com------------------
  Love me, love my ferrets.               "Break out of the mold before
  Or at least love my ferrets. ;)          the mold sets in" -- B52's

Path: sparky!uunet!cs.utexas.edu!sun-barr!olivea!mintaka.lcs.mit.edu!
bloom-picayune.mit.edu!athena.mit.edu!tytso
From: ty...@athena.mit.edu (Theodore Y. Ts'o)
Newsgroups: comp.os.linux
Subject: Re: 57.6Kbps under Linux -- some results...
Message-ID: <TYTSO.92Sep12125450@tsx-11.mit.edu>
Date: 12 Sep 92 16:54:59 GMT
References: <!nwn!6h.harp@netcom.com>
Sender: n...@athena.mit.edu (News system)
Organization: Massachusetts Institute of Technology
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In-Reply-To: harp@netcom.com's message of Sat, 12 Sep 92 13:26:38 GMT
Nntp-Posting-Host: tsx-11.mit.edu

In article <!nwn!6h.h...@netcom.com> h...@netcom.com (Gregory O. Harp) writes:

   First, I had to hack the kernel serial code to handle that speed.
   Linux currently only supports 38.4Kbps as "shipped."  

I've been working on a heavily revised version of the serial driver,
that supports more boards (including the AST FourPort boards) as well
some other neat features.  One of the neat features is a way 
to control what speed "38400" baud means when you specify it
to the kernel, using setserial.  You can set it to mean 38400, 57600,
115200, or "custom divsor".  With custom divisor, you basically get to
specify "div" in the equation 

			baud = 1843200 / (div * 16)

I'll be shipping my new serial driver to Linus soon, so look for it in a
Linux release near you.  Some of the other fixes which should be popular
is that I've managed to use over 100k less memory in the tty drivers,
which means 100k more memory for user programs.  I figure that should be
nice for people who have 2 or 4 meg machines.  :-)

   BTW, the reason I keep saying 16550A and not just 16550 is because a
   glance at the code tells me that Linux is only enabling the FIFO if
   the UART is the 16550A (refer to lines 265-272 of serial.c).  Linus,
   can you tell us why you don't use the FIFO on the 16550?  I've
   personally never used the FIFO because my projects were for 16450
   UARTs, but I'm not aware of any problems.

The initial version of the 16550 which were shipped by National
Semiconductor had a bug, so that their FIFO's where *not* reliable.  The
spec sheets from National specific state that if you have a 16550, you
should not attempt to use the FIFO's.  Fortunately, relatively few
16550's were actually shipped before the problem was discovered, and
most of those ended up in PS/2's.  These days, when people talk about
16550's, they generally mean recent versions of the chips that have the
working FIFO's.  These chips will be labelled: "NS16550A", "NS16550AF",
"PC16550C", or "PC16550CF", (apparently PC16550CF is the most recent
chip version; although I've been assured by National Semiconductor that
most of the bugs which were fixed in the PC16550CF's shouldn't give me
any trouble if I only have a NS16550A).

In any case, because the original 16550's had such a serious bug,
National thoughtfully gave us a software way of detecting whether or not
the chip was one of the chips with the FIFO bug, or not.  All of the
more recent chips will ID themselves as a 16550A; it is only the
original buggy chips that will ID themselves as a 16550.

You generally don't need to worry about this.  Although it might be
prudent to check a serial board that you're thinking about to see
whether it has one of the newer chips, it is very unlikely that if you
were buying a serial board that you would actually run into one that had
one of the original 16550 chips.

							- Ted

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