XON/XOFF Communications Protocol Not Supported in QuickBasic

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.