Autosynch Compatible COM Applications May Fail w/ FIFO Enabled (163333)

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. This problem was corrected in the latest Microsoft Windows NT 4.0 U.S. Service Pack. For information on obtaining the service pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

S E R V P A C K