Current Shareware 1994 January/SHAR194.ISO/modem/mdm400.zip/SFXURT.TXT

Text File | 1993-08-18 | 17KB | 345 lines

                          Intel Corportation
                Personal Computer Enhancements Division

May 5, 1992

SatisFaxtion 200 & 400 Smart UARTs Overview

    The purpose of this paper is to explain the performance and
reliability improvements obtained in multitasking environments like
Windows using the serial port facades inside the SatisFaxtion 200 &
400 rather than an external modem connected to a PC serial port, or
an internal modem that has a conventional UART configured as a PC
serial port.

What is data overrun anyway?

    Before getting too deep into how the Smart UART works, let's
review some of the problems encountered using a high speed modem on
a PC serial port.  Many of today's high speed modems implement
error control protocols such as V.42 and MNP that transparently
remove all data errors caused by impairments on the telephone line.
The PC serial port it is connected to only sees the corrected
error-free data.

     A communication application (e.g., PROCOMM, etc.) typically
uses a routine called by serial port hardware interrupts to collect
received data in a buffer for when it gets busy doing other things
which take a long time -- for example writing received data to
disk.  As this buffer gets close to filling, flow control is used
to tell the modem to stop sending data to the PC serial port.  Flow
control prevents the PC communication application character buffer
from spilling over and losing good data because it is known in
advance that the buffer is getting full.  The local modem being
held with flow control stalls the remote modem off by sending
special V.42 or MNP messages that tell it to wait for a while.

     But now we are using 386 PCs with virtualized, preemptive
multitasking environments like Windows that can distract the PC's
processor for extended periods with no prior warning to the
communication application.  If the modem is not being held off with
flow control and the PC's processor gets distracted for more than 1
character time (or up to 16 characters for a 16550A FIFO UART),
data gets lost at the PC's serial port UART.  Although flow control
does a good job of preventing the communication application's
buffer from overflowing and losing data, it cannot provide the same
service for the UART's tiny buffer.  If characters are lost like
this during a file transfer (Zmodem, etc.), that damaged block must
be sent again as if bad data was coming out of a modem that had no
error control.  If it happens while scrolling your Email, the lost
characters will be missing from the text your are reading (e.g.,
$1000 may appear as $100).

    Confused?  Consider this silly analogy.  Let's say that you
have a 10 gallon bucket of water that you want to get filled to
wash the car, but some creep stole your garden hose, and the faucet
is located right over an extremely fragile plant with shallow
roots.  Cleverly, you run into the house and get 2 coffee cups.  As
one cup is filling from the faucet, you are emptying the other into
the bucket that is a foot or two away.  All you have to do to
prevent spilling water is to get that freshly emptied cup
underneath the filling cup before it spills over -- no sweat!  At
about the 2 gallon mark, a bee lands on your nose, and you're
suddenly not so concerned about how quickly you get that empty cup
back under the faucet.  Eventually, the bee decides you're a pretty
raunchy smelling flower, and buzzes off.  You then look over to
just in time to see your shallow rooted plant floating down the
gutter drain!!

     So then, think of the receiving modem as the faucet, the PC's
UART as the two cups, and you are the PC's processor -- get it?

What the Smart UART does for you

    Both the SatisFaxtion 200 & 400 fax modems have a large gate
array device which in addition to other functions, provides special
circuitry that appears to the PC processor as a standard 16450
compatible serial port.  In reality, it's just a facade that looks
just like a serial port.  Instead of shifting the character bits
serially in from and out to a modem on the far end of an RS-232 cable,
the characters stay intact as bytes.  These characters are transferred
directly between the PC processor and the SatisFaxtion 200 & 400
80C186-16 on-board processor through the silicon.  The special circuitry
inside the device gives the 80C186-16 complete visibility and control
of the 16450 facade internal workings.  The 80C186-16 knows what
the PC processor is doing on the other side of this facade, and
more importantly, has direct control over what the PC processor
sees.

    On SatisFaxtion 200 & 400, since the on-board 80C186-16 knows
whether or not the PC processor has taken the last character that
it deposited into the 16450 interface, it will NEVER try
to put another character in until the previous one is read -- no
data is spilled.  Think of it as a really smart faucet that shuts
off the water if you don't replace the cup in time.  This is true
no matter how fast the data is being moved or how busy the PC
processor is.  The on-board 80C186-16 will wait indefinitely until
the PC processor has taken the last character before trying to give
it a new one.

    This protection against data loss enables you to set your
communication application baud rate (DTE rate) as high as you like
with no fear of losing data.

UART Rate simulation

    The serial port UARTs found on PCs (8250, 16450, etc.) transfer
characters to/from the PC's processor at a rate proportional to the
baud rate set by the communication application software.  For
example, with a setting of 9600 bits per second, a character is
exchanged between the PC processor and the UART at the rate of (10
bits/char) / (9600 bits/sec) = .00104 sec/char.

    Unlike a real UART, the SatisFaxtion 200 & 400 could choose to
ignore whatever baud rate the user had selected, transferring data
between the 80C186-16 the PC's processor at the equivalent of
almost 300K bits/sec.  However, by default it intentionally runs
slower than that for the following reasons:

Compatibility

        During the development of the SatisFaxtion 200, At least one
application (Prodigy) expects data transfers with the UART to occur
at a certain rate, and develops timing problems if the actual
transfer rate is significantly different from the expected rate.

Multitasking

    When running a file transfer in the background under Windows on
a relatively slow machine (e.g., 16 MHz SX), an unbridled character
interrupt rate may tax the user's machine so heavily that it
completely consumes all of that machine's processor, essentially
shutting down all other windows for brief periods.  This is
particularly true of DOS communication applications running in a
window.  The user may choose to select a lower rate such that they
can be working on something else in another window without periods
of time that make the application seem sluggish or in a hung state.

Human factors

    Error correcting protocols like V.42 and MNP free up data to
the host PC in blocks rather than one character at a time.  With
SatisFaxtion 200 & 400 running open throttle, text flashes out to
the screen in chunks rather than flowing out in streams like the
lower settings on real UARTs.  This can be a real annoyance in
terms of visual stress, and can also make it really hard to pause
the screen where you want.

    The actual rate that is simulated by the SatisFaxtion 400 is
configurable using the AT"B=n command, where the decimal value n
causes the following:

n=0 (default) The SatisFaxtion 400 runs the Smart UART 1.05
    times faster than a real UART for a given setting.

n=1 The SatisFaxtion 400 runs the Smart UART 2.1 times faster
    than a real UART for a given setting.

n=2 The SatisFaxtion 400 runs the Smart UART 4.2 times faster
    than a real UART for a given setting.

n=3 (best) The SatisFaxtion 400 runs the Smart UART at full
    80C186-16 processor bandwidth, and also minimizes the
    burden per character for the 80C186-16.  This is nearly the
    equivalent of a 300K bit/sec UART.  Also note that if the
    PC communications software sets the baud rate at 115,200
    bits/sec, the Smart UART also run at full processor
    bandwidth, ignoring any AT"B setting.

   The default speed configuration runs at about the same speed as
an actual UART to provide the smoothest and most compatible
environment which will work well for all PC's.  The other settings
are used to obtain higher performance on faster PCs which are not
running timing sensitive communication applications.  The AT"B=3
setting provides the best performance not only because of the high
transfer rate involved, but also removes overhead to the 80C186-16
caused by internal rate simulation routines.  Note that the
SatisFaxtion 200 is fixed at the 4:1 setting with no AT"B command
provision, since there are fewer compatibility issues with a 2400
bit/sec modem.

What are some uses of the AT"B command settings?

    Modems with V.42bis data compression can utilize a PC serial
port rate at least 4 times that of the raw modem speed to prevent
the uncompressed data from becoming bottlenecked at the PC's serial
port.  Thus to run a V.32bis (14,400 bits/sec) modem such as the
SatisFaxtion 400, a serial port rate of at least 57,600 bits/sec is
desirable.  At times, streams of data are encountered which are
highly compressible and would require a serial port rate much
faster than 4 times the modulation rate to prevent bottlenecks.
With an AT"B=3 setting, the SatisFaxtion 400 can outdistance most
regular modems when the PC communication software doesn't support
faster bit rates.

    An example of this is Windows 3.0, where the serial port
drivers do not allow UART settings above 19.2K bits/sec.  Microsoft
chose this limit because operation above 19.2K even on the fastest
machine of that era was believed to be a risky venture with a
16450.

    The new Windows version 3.1 comm drivers allow operation at up
to 57.6 K bits/sec for Windows comm apps, and have also added
16550A support to lessen overrun problems.  Unfortunately, Windows
comm apps developed prior to Windows version 3.1 don't know how to
ask Windows for the higher rates, so users would ordinarily have to
wait for a Win3.1 update to their comm app.  By using AT"B settings
1-3, the customer does not have to wait for an update to obtain
outstanding performance with a V.32bis + V.42bis connection.

    Before leaving the topic of Windows, there are some nasty
problems that need to be discussed pertaining to Windows 3.0 and
Windows 3.1 communications drivers running in enhanced mode on
slower machines.  These problems even affect operation with the
SatisFaxtion 400, and have nothing to do with loss of characters or
data overrun.  At faster character rates, the 32 bit protected mode
(RING0) drivers inside the Windows WIN386.EXE kernel become
unstable and crash.  The result is a GENERAL PROTECTION FAULT which
sometimes hangs the machine, but most often causes the PC to
cold-boot as if the power switch had been cycled.  Hopefully by
working with Microsoft we'll be able to get them to address this in
their next driver release, but until then AT"B=0 or AT"B=1 are the
recommended settings for operation under Windows enhanced mode.
Note that standard mode operation of Windows does not exhibit this
problem.


UARTs with FIFOs (16550A, etc.)

    So you may also ask, what do the FIFOs in a 16550A do for you?
The 16550A receiver FIFO is a buffer which can hold up to 16
characters before losing any data.  The 16550A has a programmable
receiver trigger which can be configured to request service as soon
as a single character arrives, allowing the host PC's processor to
be distracted for up to 16 more character times without overrunning
the FIFO and losing data.  This can work very well for systems that
occasionally have very long latency problems, but are otherwise not
heavily affected by the overhead caused by having to service the
UART once per each character.

    The receiver trigger has 3 other settings which configure the
16550A not to request service from the PC processor until it has
collected up to 4, 8 or 14 characters, resulting in much less
overhead per request (and less latency protection).  This reduction
in overhead can be used either to give more of the PC processor
back to other applications that are running, or can also be used to
handle a higher data rate.  Note that if more than 4 character
times have elapsed since the last character arrival, the 16550A
will generate a request to prevent data from being trapped in a
partially full FIFO.

    Getting back to our analogy, it's the same as having bigger
cups.  Since the cups are larger, you are not constantly busy
swapping cups for a given flow rate.  You can use this extra time
between cup swaps to chat with your neighbor, or if you are
ambitious you could open the faucet more to get the bucket filled
faster.  Just make sure you don't get distracted too long as the
water level approaches your chosen full mark.

    Ideally, the 16550A receiver trigger should be set to the
largest number of characters that won't cause any characters to be
lost at that baud rate because of PC processor latency.  On slower
SX machines that take longer to task switch, a low trigger point
may be required.  The lower trigger point causes more frequent
requests (overhead) of the PC processor as well, so a lower DTE
rate setting may be the only alternative if the PC processor can't
keep up with the flow rate.

    Interestingly enough, in Windows 3.1 Microsoft chose to set the
16550A receiver trigger at the highest possible setting of 14
characters, favoring low overhead for PCs fast enough to always
respond to requests within 3 character times.  Although this
provides 3 character times of latency protection whereas before in
16450 mode there was only one, slower 80386SX equipped may lose
characters at a rate where a receiver trigger setting of 8 may have
provided adequate latency protection and efficiency to operate
error free.

    You may be wondering which is better for running under Windows
-- a 16550A that reduces burden, or the SatisFaxtion 200 & 400
Smart UART interface?  The answer is really both!  At a given baud rate,
the 16550A when set to a 14 character receiver trigger will generate
less overhead for the PC Processor.  On the other hand, no matter what
nasty transient things you may do in Windows that affect latency, the
Smart UART is never going to drop a character on you.

Glossary:


COM Port

    A logical device name used with DOS that references a serial
port in the IBM compatible PC.  COM device names (COM1, COM2, etc.)
are normally assigned in sequence by the order in which they are
discovered by the PC's ROM BIOS when the machine is booted.  These
names are often incorrectly used to indicate the physical IO
address at which a serial port if configured.  See also Serial Port

8250
16450

    These are UART devices developed by National Semiconductor
Inc., which are commonly used in IBM PC compatible computers.  They
each have the ability to buffer only one character for both receive
and transmit.  See also definition of UART below.

16550A

    This is a UART device developed by National Semiconductor Inc.
that contains two 16 byte FIFOs, one for receive and one for
transmit.  The receive FIFO reduces the likelihood of losing
received characters caused the hosting microprocessor's inability
to remove the prior character in time, and also reduces the hosting
microprocessor's overhead by collecting more than one character
before interrupting it.  The transmit FIFO reduces the overhead
caused by interrupting the processor each time a character is sent.
It was originally developed for the IBM PS/2 line of personal
computers, but has recently been growing rapidly into ISA and EISA
bus applications as well since the appearance of high speed modems
and laptop serial file transfer packages.

FIFO

    First In, First Out.  This is a memory or storage area that
preserves the order of data, queuing the data up until the
recipient is able to remove it.
Latency

    The delay from when a piece of data or service is requested
until the time it is actually delivered or removed.
Serial Port

    The combination of a UART and an external connector (usually
DB-9 or DB-25) which is used to operate devices such as modems,
serial printers, and mice.

UART

    Universal Asynchronous Receiver/Transmitter.  The purpose of
this device is to break down entire characters into a single bit
serialized stream that is sampled by the remote end at known
intervals and re-assembled into characters.  By using only a single
wire in each direction to carry the information, substantial
savings are realized in cabling and connector costs.