NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/dcom/modems-part1/157

Text File | 1996-08-05 | 5.6 KB | 123 lines

Newsgroups: comp.dcom.modems
Path: rowdy.lonestar.org!nemesis!uhclem
From: [e-mail] (Frank Durda IV)
Subject: Re: A real 16550A
X-Newsreader: Tin 1.1 PL5
Organization: The Big Blue Box
Message-ID: <[e-mail]>
References: <4c1jla$[e-mail]>
Date: Sat, 30 Dec 1995 18:00:30 GMT

[0]In article <4buagu$[e-mail]>, [e-mail]` says:
[0]How can you check your 16550 chip to see if it really has a FIFO. I did
[0]a check on a Gateway 486-66 that suppose to have a 16550 and some
[0]software program said it did't have a buffer.  Is there a shareware
[0]program that checks the 16550 out to see if it really has a buffer?

The problem is that there is the real NS16550A, then there are "similar"
??16550[A]s made by other companies.  Only a few vendors bother to actually
license the National megacell (the only way to assure 100% identical
function), and since National has patents on the added FIFO anyway,
other vendors take certain liberties with the design so that they
can avoid patent infringement AND still work under the Windows driver.
These differences may show up when using other drivers, or particular
events occur that were not well tested or considered in the Windows driver.
If you are using another OS, then you may be in trouble.

In working with numerous modem vendors and 16550-clone makers, I found
that the benchmark to success according to the chip vendors was whether
or not MSD said it was a 16550 plus whether one or two Windows-based
applications using the Microsoft drivers from 3.11 WFW would run correctly.
That was the criteria.  Not very impressive.

National has a program called COMTEST (you can find it on their BBS
and supposedly it is on SIMTEL mirrors) that compares your 16550 against
the National chip.  Of course, the program is biased, as National wants
to show as many differences (read: terrible incompatibilities) between
their part and these "imposters".   I used COMTEST on chips from the other
vendors and they did howl when they found out.  But it did prove a point
that their chips were different.  It was then up to the vendor to
convince me that the differences were so minor that they would never cause
any application to malfunction.

I tested parts made by National, TI, StarTech, and CMD plus some 16550s in
the form of megacells embedded in internal modems.  Here are some of
the results:

(These tests were performed in 1994 and may not reflect the current product
 performance of that vendor)

Part number            Errors aka "differences" reported
National PC16550DV         0 *

National NS16550AFN        0

TI (TL16550AFN)            3

CMD 16C550PE            19

StarTech (ST16C550J)        23

Rockwell reference
modem with internal
16550 (RC144DPi/C3000-25)     117 (test modified so that it would not abort)

Sierra modem with internal
16550 (SC11951/SC11351)        91 (test modified so that it would not abort)

Now you can't learn a lot from the raw error count.  For example, most
of the errors in the modems with internal UARTs was caused by the
UARTs not supporting 5 and 6 bit characters modes.   The real
16550/16450/8250 supports these modes and COMTEST checks for this, but who
uses them in a modern modem?  So for a modem you can discount those errors
(over 50), bringing the difference counts down quite a bit lower.

Some of the remaining problems were still disturbing, like being able
to read 16 bytes from the FIFO and the status still said the FIFO was not
empty, interrupt priorities not identical, that sort of stuff.

On the other hand, TI had the fewest differences, but the differences
they had sounded critical to reliable operation:
    Error (3)...Tx FIFO reset : LS = 0
    Error (7)...BI/FE/EIF test: IIR = c1  LSR = 60
    Error (7)...IIR/LSR not cleared: IIR = x1  LSR = 60
(You will have to look at the COMTEST code to see exactly what test is
 being performed which causes these errors.)

So far, I have not found any non-National parts that report zero errors in
this program.  Note also that even National has made five revisions to
the 16550 over the years and the newest ones behave a bit differently than
the classic NS16550AFN that people worship.  But COMTEST turns a blind
eye to the differences within the National product line and reports
no errors on the National parts (except for the original 16550_), so just
be aware of that bias.

FYI, National reorganized their part numbering system a few
years ago, and the NS16650AFN no longer exists.  The new numbers are
all of the form  PC16550DV, with minor differences in the letters
depending on package material and shape.   In some stores, you will
pay $15 for a NS16550AFN made in 1990 and in the next bin are the
new PC16550 parts with minor fixes National has made since the AFN part,
and they cost half as much.  I know we were buying them in volume
for $5 in 1993.  Expect the price on real NS16550AFNs to keep going up
until people figure out that the new part number is the same as the
old part number.

Oh, if you run COMTEST on a 16550 that is in a modem, you need to
first issue a ATE0&W command to the modem so that it will not echo
any of the test characters.  If you forget to do this, you will
get at least one error:
    Error (6)...Timeout interrupt failed: IIR = c1  LSR = 61

For the record, I made National parts the sole-source when external
16550s were specified on projects I was associated with since the other
vendors either didn't bother to try to explain the differences or
could not.

Frank Durda IV <[e-mail]>|(C) 1995 Frank Durda IV, All
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