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 COM1-COM4 May Be Accessed from Basic Through MS-DOS Service - Q76742
 
 Microsoft Visual Basic version 1.0 for MS-DOS does not allow access to
 the COM3 or COM4 device through the Basic OPEN "COMx:" statement.
 However, COM3 and COM4 may be accessed by opening the device as a
 sequential file from Basic, which will access the device through
 MS-DOS.
 
 To access COM3 or COM4 in this manner, you must be using MS-DOS
 version 3.3 or later. Note that using Basic sequential file I/O to
 access the serial port involves polling the port for data. This
 polling system is not guaranteed to be completely reliable, especially
 at baud rates above 1200. For higher baud rates and greater
 reliability, it is recommended that you use a third party library for
 interrupt-driven communications.
 
 More Information:
 
 You can perform both input and output to COM3 or COM4 by opening a
 sequential file of that name from Basic. Because opening a device this
 way will go through the MS-DOS open services, which are general file
 and device I/O routines, any modifications needed to the baud rate or
 parity must be made by using the MS-DOS MODE command.
 
 For example, to set the baud and parity to 2400 bps and no parity with
 8 data and 1 stop bits, use the following command at the MS-DOS
 prompt:
 
    mode com3:2400,N,8,1
 
 The MODE command can be accessed from inside a Basic program via the
 SHELL statement. To avoid possible conflicts with shelling to MODE,
 which is a terminate-and-stay-resident (TSR) program, you should run
 MODE at least once before starting a program that will shell to it.
 
 The following program will open COM3 as a sequential file and write to
 the port:
 
 ' To try this example in VBDOS.EXE:
 ' 1. From the File menu, choose New Project.
 ' 2. Copy the code example to the Code window.
 ' 3. Press F5 to run the program.
 
 SHELL "MODE COM3:9600,N,8,1"
 OPEN "COM3" FOR OUTPUT AS #1
 PRINT #1, "HELLO COM3";
 CLOSE #1
 
 The following program will open COM3 as a sequential device, accept
 input from COM3, and allow the user to terminate the program by
 entering a Q:
 
 ' To try this example in VBDOS.EXE:
 ' 1. From the File menu, choose New Project.
 ' 2. Copy the code example to the Code window.
 ' 3. Press F5 to run the program.
 
 SHELL "MODE COM3:9600,N,8,1"
 OPEN "COM3" FOR INPUT AS #1
 DO WHILE INKEY$ <> "q"
         A$ = INPUT$(1, 1)          ' Get 1 character from COM3.
         IF A$ <> "" THEN
                 PRINT A$;
                A$ = ""
         END IF
 LOOP
 CLOSE #1
 
 Note that because you are accessing the serial port as a sequential
 file, all the rules of sequential file I/O apply. Data will always be
 transmitted as ASCII characters, and a carriage return plus linefeed
 pair will be appended to the end of each block of data written by a
 Basic PRINT statement (unless you place a semicolon [;] at the end of
 the PRINT statement). Errors during communication will not be noted in
 Basic, but may result in a Basic file I/O error instead.
 
 Why Basic Does Not Support COM3 or COM4
 ---------------------------------------
 
 Adding support for COM3 and COM4 communications ports would require
 a larger code size in the development environment, compiler, and
 run-time module. Also, COM3 and COM4 support requires MS-DOS
 version 3.3 or later, plus implementation in the computer's ROM BIOS.
 Microsoft Basic products are targeted to run on as wide a variety of
 systems as possible, but COM3 and COM4 have not been widely available
 on the existing hardware base.
 
 A combination of these and other factors have resulted in Microsoft
 not currently supporting COM3 and COM4.
 
 COM3 and COM4 Support from Third-Party Developers
 -------------------------------------------------
 
 In addition to the method mentioned above, it may be possible for a
 compiled Basic program to access COM3 and COM4 ports by calling
 third-party library routines. These libraries are listed in catalogs
 such as the "Programmer's Connection Buyer's Guide", which can be
 obtained by calling Programmer's Connection at (800) 336-1166 in the
 United States; (800) 225-1166 in Canada; or (216) 494-8899 (customer
 service).
 
 For example, you can use this catalog to find the number for Software
 Interphase, (401) 397-2340, to determine if their product QuickComm
 supports COM3 and COM4 called from a compiled Microsoft Visual Basic
 version 1.0 for MS-DOS program.
 
 Also, Crescent Software (800-35-BASIC) offers a product called
 PDQComm to support COM3 and COM4. (For more details on PDQComm, call
 Crescent Software.)