DOCUMENT:Q163333     [winnt]
TITLE   :Autosynch Compatible COM Applications May Fail w/ FIFO Enabled
PRODUCT :Microsoft Windows NT
PROD/VER:4.00
OPER/SYS:WINDOWS
KEYWORDS:kbenv kbbug4.00 kbfix4.00

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The information in this article applies to:

 - Microsoft Windows NT Workstation version 4.0
 - Microsoft Windows NT Server version 4.0
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SYMPTOMS
========

The released version of the Windows NT 4.0 serial port driver (Serial.sys)
does not delimit data in a manner compatible with the Hayes AutoSynch
standard when FIFO is enabled on the COM port. For example, IBM PCOMM
implements the Hayes AutoSynch standard and requires that data to be
delimited be compliant with the AutoSynch standard to operate reliably.

CAUSE
=====

The Hayes AutoSynch standard requires that data be delimited with an escape
character followed by a line status error with data. The last part of an
AutoSynch frame is indicated by line status error data with the parity
error bit enabled followed by 02. In this case, FIFO was enabled, buffering
data on the COM port, and Serial.sys would return only part of the data
delimited correctly.

For example the following data

   FF BF FB BD

should return from Serial.sys as

   7D 01 E1 FF 7D 01 E1 BF 7D 01 E1 FB 7D 01 E1 BD 7D 01 E5 02

where 7D is an escape character, 01 indicates a line status error, and E1
is the line status error data (E5 is line status error data with the parity
error bit enabled). The released version of the Windows NT 4.0 Serial.sys
driver returns the following unusable stream:

   FF 7D 01 E1 BF FB 7D 01 E1 BD 02

RESOLUTION
==========

The file Serial.sys was modified to correctly insert an escape character
followed by a line status error with data when returning buffered data from
FIFO.

The updated file has been posted to the following Internet location:

   ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40/
   hotfixes-postSP2/serial-fix

STATUS
======

Microsoft has confirmed this to be a problem in Windows NT version 4.0.
A supported fix is now available, but has not been fully regression-tested
and should be applied only to systems experiencing this specific problem.
Unless you are severely impacted by this specific problem, Microsoft
recommends that you wait for the next Service Pack that contains this fix.
Contact Microsoft Technical Support for more information.

KBCategory: kbenv  kbbug4.00 kbfix4.00
KBSubcategory: NTSrvWkst nthw
Additional reference words: 4.00 prodnt

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