OPEN COM Statement Details

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◄Syntax► ◄Details► ◄Example► ◄Contents► ◄Index► ◄Back► ────────────────────────────────────────────────────────────────────────────── Opens and initializes a communications channel for I/O. The OPEN COM statement must be executed before a device can be used for communication using an RS232 interface. Optimization Note ■ If there is not an OPEN COM statement in your program, you can reduce the size of the .EXE file by linking with the stub file NOCOM.OBJ. OPEN "COMn: optlist1 optlist2" [FOR mode] AS [#]filenum% [LEN=reclen%] ■ If there are syntax errors in the OPEN COM statement, BASIC generates the error message, "Bad file name." ■ The argument filenum% is the number used to open the file. ■ The OPEN COM statement has many options, which can be separated into two option lists: - optlist1 contains the most general and often-used parameters; - optlist2 contains options used in special situations. ■ optlist1 specifies the communication line parameters and has the following syntax: [speed] [,[parity] [,[data] [,[stop]]]] ■ Following are the valid values for optlist1 options: Option Description Range (default in bold) ═══════ ══════════════════════════════ ═══════════════════════════ speed Baud rate (bits per second) 75, 110, 150, 300, 600, 1200, 1800, 2400, 9600 parity Method of parity checking N, E, O, S, M, PE (none, even, odd, space, mark, error checking) data Number of data bits per byte 5, 6, 7, or 8 stop Number of stop bits 1, 1.5, or 2(1) ────────────────────────────────────────────────────────────────────── (1) The default value is 1 for baud rates greater than 110. For baud rates less than or equal to 110, the default value is 1.5 when data is 5; otherwise, the value is 2. ■ Options in this list must be entered in the order shown. Use comma placeholders for defaults. For example, OPEN "COM1: ,N,8," FOR INPUT AS #1 ■ 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). Important ■ Because BASIC uses complete bytes (8 bits) for numbers, you must specify 8 data bits when transmitting or receiving numeric data. ■ If any options from optlist2 are chosen, comma placeholders must still be used even if all of the optlist1 options are defaults. For example: OPEN "COM1: ,,,,CD1500" FOR INPUT AS #1 ■ The optlist2 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. ■ The 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. Ignored Used to enable communication files (as part to be printed on a serial line printer. of BIN Causes a LF character (0AH) to be default) automatically sent after each CR (0DH). Note that the carriage return may be sent as a result of the width setting for the device. Note also that INPUT and LINE INPUT, when used from a COM file that was opened with the LF option, ignore the line feed. ■ The 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 QBX 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, the default is 512 bytes. ■ 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 is 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 is omitted Range, if m specified ────────────────────────────── ─────────────────────── m = 1000 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 times the CD or 10 seconds 0 - 65,535 milliseconds DS timeout value, whichever is greater Usage Notes ■ 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. ■ Use a relatively large value for the OP option compared to the CS, DS, or CD options. ■ The argument mode 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 mode is omitted, it is assumed to be random-access input/output. ■ 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 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