OPEN COM Statement Details
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OPEN "COMn: optlist" [FOR mode] AS [#]filenum% [LEN=reclen%]
Usage Notes
■ If there is no OPEN COM statement in your program, you can reduce the
.EXE file size by linking with the stub file NOCOM.OBJ.
■ Because Visual Basic uses complete bytes (8 bits) for numbers, you must
specify 8 data bits when transmitting or receiving numeric data.
■ Additional options require comma placeholders even if all of the options
are defaults. For example:
OPEN "COM1: ,,,,CD1500" FOR INPUT AS #1
■ If there are syntax errors in the OPEN COM statement, Visual Basic
generates the error message, "Bad file name."
■ Specifying the argument reclen% is effective only in random-access mode.
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.
See: ◄GET Statement (File I/O)► ◄PUT Statement (File I/O)►
■ 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
for the timeout period for the Data Set Ready line (DSR) to go high.
If a timeout occurs, the process goes to step 6.
4. If the RS option is not specified, the Request To Send line (RTS) is
set high.
5. If either of the OP or CD options is nonzero, the statement waits for
the timeout period for the Data Carrier Detect line (DCD) to go high.
If a timeout occurs, the process goes to step 6. Otherwise, the RS232
device has been successfully opened.
6. If there is a timeout, the open fails. The process deallocates the
buffers, disables interrupts, clears all of the control lines, and
generates the message, "Device timeout." In addition, for MS-DOS, the
process sets the value of ERDEV$ to COM and sets ERDEV to a value
that indicates the signal line that timed out, according to the
following table:
ERDEV Value Signal Line
═══════════ ═══════════════════════════════════════════════
128 (80H) Clear to Send (CTS) timeout
129 (81H) Data Set Ready (DSR) timeout
130 (82H) Data Carrier Detect (DCD) timeout
Note: Only the baud rates shown are supported. Any other value for
speed is invalid. The PE parity value turns on error checking for
parity errors. If you set data to eight bits per byte, you must
specify no parity (N).
■ In addition to speed, parity, data, and stop, OPEN COM supports the
options described below. Options can be specified in any order, and
must be separated from one another by commas. There are three types
of options: data mode, buffer size, and handshaking:
• Data-mode options (ASC, BIN, and LF):
Option Description Default
══════ ═══════════════════════════════════════════ ════════
ASC or Specifies treatment of tabs, carriage BIN
BIN returns, and Ctrl+Z in the data stream:
ASC BIN
═══ ═══
Tabs expanded to blanks Yes No
CR forced at end of line Yes No
Ctrl+Z means end-of-file Yes No
Ctrl+Z sent when device closed Yes No
LF Effective only with the ASC option. Used Ignored
to print communication files on a serial (as part
line printer. Causes a linefeed character of BIN
(0AH) to be sent automatically after each default)
carriage return (0DH):
■ The carriage return can be sent as a
a result of the width setting for the
device.
■ INPUT and LINE INPUT, when used from a
COM file opened with the LF option,
ignore the linefeed character.
• Buffer-size options for sequential modes (RB and TB):
Option Description
══════ ════════════════════════════════════════════════════════
RB[n] Receive buffer size (in bytes)
If the RB option is not used or n is omitted, the default
is 512 bytes, unless overridden by the /C option in the
BC or VB command line. Maximum receive buffer size is
32,767 bytes.
TB[n] Transmit buffer size (in bytes)
If the TB option is not used or n is omitted, default
is 512 bytes (max is 32,767).
• Handshake and timing options (RS, CD, CS, DS, and OP):
Option Description
══════ ════════════════════════════════════════════════════════
RS Suppresses detection of Request To Send (RTS).
CD[m] Specifies the timeout period on the Data Carrier Detect
line (DCD). If no signal appears on the DCD line (the DCD
line remains low) for more than m milliseconds, a device
timeout occurs. If a CD timeout occurs, ERDEV contains
130 (82H).
Default if CD option not used,
m is omitted, or m = 0 Range, if m specified
────────────────────────────── ──────────────────────
m = 0, which means ignore the 0 - 65,535 milliseconds
state of the DCD line.
CS[m] Specifies the timeout period on the Clear To Send line
(CTS). If no signal appears on the CTS line (the CTS
line remains low) for more than m milliseconds, a device
timeout occurs. If a CS timeout occurs, ERDEV contains
128 (80H).
Default if CS option not used
or m omitted Range, if m specified
────────────────────────────── ──────────────────────
m = 1000 milliseconds 0 - 65,535 milliseconds
(m = 0 means ignore the
state of the CTS line)
DS[m] Specifies the timeout period on the Data Set Ready line
(DSR). If no signal appears on the DSR line (the DSR line
remains low) for more than m milliseconds, a device
timeout occurs. If a DS timeout occurs, ERDEV contains
129 (81H).
Default if DS option not used
or m omitted Range, if m specified
────────────────────────────── ──────────────────────
m = 1,000 milliseconds 0 - 65,535 milliseconds
(m = 0 means ignore the
state of the DSR line)
OP[m] Specifies how long the OPEN statement waits for all
communications lines to become active:
Default if
Default if OP OP used but
not used m omitted Range, if m specified
─────────────────── ────────── ───────────────────────
10 * CD or DS time- 10 seconds 0 - 65,535 milliseconds
out value, whichever
is greater
• Use a relatively large value for the OP option compared to the CS,
DS, or CD options.