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OPEN (Communications) Statement Details
 
Syntax
  OPEN "COMn: optlist1 optlist2" [FOR mode] AS [#]filenum [LEN=reclen]
 
COMn: is the name of the device to be opened. The n argument is the
number of a legal communications device, such as COM1: or COM2:. The
first list of options, optlist1, has the following form:
 
  [speed][,[parity] [,[data][,[stop]]]]
 
The following list describes the possible options:
 
  Option   Description
  speed    The "baud" rate (baud means "bits per second") of the
           device to be opened. Valid speeds are 75, 110, 150, 300,
           600, 1200, 1800, 2400 and 9600.
  parity   The parity of the device to be opened. Valid entries for
           parity are: N (none), E (even), O (odd), S (space), or
           M (mark).
  data     The number of data bits per byte. Valid entries are 5,
           6, 7, or 8.
  stop     The number of stop bits. Valid entries are 1, 1.5, or 2.
 
Options from this list must be entered in the order shown; moreover,
if any options from optlist2 are chosen, comma placeholders must
still be used even if none of the options from optlist1 are
chosen. For example:
 
  OPEN "COM1: ,,,,CD1500" FOR INPUT AS #1
 
If you set the data bits per byte to eight, you must specify no parity
(N). Because QuickBASIC uses complete bytes (eight bits) for numbers,
you must specify eight data bits when transmitting or receiving
numeric data.
 
The choices for optlist2 are described in the following list. The
argument m is given in milliseconds; the default value for m is 1000.
 
  Option   Description
  ASC      Opens the device in ASCII mode. In ASCII mode, tabs are
           expanded to spaces, carriage returns are forced at the
           end-of-line, and CTRL+Z is treated as end-of-file. When
           the channel is closed, CTRL+Z is sent over the RS-232 line.
  BIN      Opens the device in binary mode. This option supersedes
           the LF option. BIN is selected by default unless ASC is
           specified.
           In the BIN mode, tabs are not expanded to spaces, a
           carriage return is not forced at the end-of-line, and
           CTRL+Z is not treated as end-of-file. When the channel is
           closed, CTRL+Z will not be sent over the RS-232 line.
  CD[m]    Controls the timeout on the Data Carrier Detect line (DCD).
           If DCD is low for more than m milliseconds, a device
           timeout occurs.
  CS[m]    Controls the timeout on the Clear To Send line (CTS). If
           CTS is low (there is no signal) for more than m
           milliseconds, a device timeout occurs.
  DS[m]    Controls the timeout on the Data Set Ready line (DSR). If
           DSR is low for more than m milliseconds, a device timeout
           occurs.
  LF       Allows communication files to be printed on a serial line
           printer. When LF is specified, a line-feed character (0AH)
           is automatically sent after each carriage-return character
           (0DH). This includes the carriage return sent as a result
           of the width setting. Note that INPUT and LINE INPUT, when
           used to read from a COM file that was opened with the LF
           option, stop when they see a carriage return, ignoring the
           line feed.
  OP[m]    Controls how long the statement waits for the open to be
           successful. The parameter m is a value in the range 0 to
           65,535 representing the number of milliseconds to wait for
           the communications lines to become active. If OP is
           specified without a value, the statement waits for ten
           seconds. If OP is omitted, OPEN COM waits for ten times
           the maximum value of the CD or DS timeout values.
  RB[n]    Sets the size of the receive buffer to n bytes. If n is
           omitted, or the option is omitted, the current value is
           used. The current value can be set by the /C option on the
           QuickBASIC or BC command line. The default is 512 bytes.
           The maximum size is 32,767 bytes.
  RS       Suppresses detection of Request To Send (RTS).
  TB[n]    Sets the size of the transmit buffer to n bytes. If n is
           omitted, or the option is omitted, the current value is
           used. The default size is 512 bytes.
 
The options from the list above can be entered in any order, but they
must be separated from one another by commas. For CS[m], DS[m], and
CD[m], if there is no signal within m milliseconds, a timeout occurs.
The value for m may range from 0 to 65,535, with 1000 as the default
value. The CD default is 0.) If m is equal to 0 for any of these
options the option is ignored. The CTS line is checked whenever there
is data in the transmit buffer if the CS option is specified. The
DSR and DCD lines are continuously checked for timeouts if the
corresponding options (DS, CD) are specified.
 
The mode argument is one of the following string expressions:
 
  Mode     Description
  OUTPUT   Specifies sequential output mode
  INPUT    Specifies sequential input mode
  RANDOM   Specifies random-access mode
 
If the mode expression is omitted, it is assumed to be random-access
input/output. The filenum is the number used to open the file. The
OPEN COM statement must be executed before a device can be used for
communication using an RS-232 interface.
 
If the device is opened in RANDOM mode, the LEN option specifies the
length of an associated random-access buffer. The default value for
length is 128. You can use any of the random-access I/O statements,
such as GET and PUT, to treat the device as if it were a random-
access file.
 
The OPEN COM statement performs the following steps in opening a
communications device:
 
  1. The communications buffers are allocated and interrupts are
     enabled.
  2. The Data Terminal Ready line (DTR) is set high.
  3. If either of the OP or DS options is nonzero, the statement waits
     up to the indicated time for the Data Set Ready line (DSR) to be
     high. If a timeout occurs, the process goes to step 6.
  4. The Request To Send line (RTS) is set high if the RS option is
     not specified.
  5. If either of the OP or CD options is nonzero, OPEN COM waits up
     to the indicated time for the Data Carrier Detect line (DCD) to
     be high. If a timeout occurs, the process goes to step 6.
     Otherwise, OPEN COM has succeeded.
  6. The open has failed due to a timeout. The process deallocates
     the buffers, disables interrupts, and clears all of the control
     lines.
 
  Note: Use a relatively large value for the OP option compared to the
        CS, DS, or CD options. If two programs are attempting to establish
        a communications link, they both need to attempt an OPEN during at
        least half of the time they are executing.
 
Any syntax errors in the OPEN COM statement produce an error message that
reads "Bad file name."