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Q22019: XON/XOFF Communications Protocol Not Supported in QuickBASIC

Article: Q22019
Product(s): See article
Version(s): 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | docerr B_BasicCom | mspl13_basic
Last Modified: 17-JAN-1991

The XON/XOFF handshaking protocol of the ASC option of the OPEN COM
statement is not implemented in any version of QuickBASIC (including
versions 2.00, 2.01, 3.00, 4.00, 4.00b, and 4.50), in Microsoft BASIC
Compiler version 6.00 or 6.00b for MS-DOS, or in Microsoft BASIC
Professional Development System (PDS) version 7.00 or 7.10 for MS-DOS.

Except for XON/XOFF protocol, the other features of the ASC option of
the OPEN COM statement work properly. During communications, the ASC
option expands tabs to spaces, forces carriage returns at the
end-of-line, treats CTRL+Z (ASCII value 26) as the end-of-file flag,
and transmits CTRL+Z when you CLOSE the communications channel.

When you OPEN the COM1 or COM2 communications port and specify the ASC
option, XON and XOFF bytes have no effect. A program will continue to
transmit and pass all received XON and XOFF characters to the program
without halting or resuming transmission. This behavior occurs because
the XON/XOFF handshaking protocol of the ASC option of the OPEN COM
statement is not implemented.

The sentence saying "XON/XOFF protocol is enabled" should be deleted
in the documentation for the ASC option of the OPEN COM statement in
the following manuals:

1. Page 375 of the "Microsoft QuickBASIC Compiler" manual for
   QuickBASIC versions 2.x and 3.00

2. Page 297 of the "Microsoft QuickBASIC 4.0: BASIC Language
   Reference" manual for QuickBASIC versions 4.00 and 4.00b

3. Page 297 of the "Microsoft BASIC Compiler 6.0: BASIC Language
   Reference" manual for versions 6.00 and 6.00b

This documentation error has been corrected in the documentation for
Microsoft QuickBASIC 4.50 and the BASIC Professional Development
System (PDS) for MS-DOS and MS OS/2.

Additional Background Information
---------------------------------

XOFF is also known as CTRL+S, which is a character with an ASCII
value of 19. CTRL+S tells a device or program that knows XON/XOFF
protocol to stop transmission.

XON is also known as CTRL+Q, which is a character with an ASCII value
of 17. CTRL+Q tells a device or program that knows XON/XOFF protocol
to continue transmission.

The CTRL+S (XOFF) and CTRL+Q (XON) bytes pass without special handling
through devices or programs that do not support the XON/XOFF
communications protocol. While it is possible to write a BASIC program
that checks every character for XON or XOFF and then programmatically
starts or stops communications, such a program is unlikely to be as
reliable as the XON/XOFF handshaking that is built into dedicated
commercial communications software or hardware packages.

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