rs232.txt

在VB中实现通讯的例子

      1.3 Register Summary (Continued)

      REG 3 : Line Control Register (LCR)

      RS-232 line parameters are selected by writing to this register.

         bit 7    : DLAB = 0
         bit 6    : BREAK on(1), off(0).
         bits 5-3 : Parity None(000),ODD(001),EVEN(011),MARK(101),SPACE(111)
         bit 2    : One stop bit(0), two stop bits(1).
         bits 1-0 : Data bits = 5 (00), 6(01), 7(10), 8(11).

      Parity  Meaning
       Odd    The parity bit is 1 if the sum of the data bits is odd.
       Even   The parity bit is 1 id the sum of the data bits is even.
       None   There is no parity bit.
       Mark   The parity bit is always set to 0.
       Space  The parity bit is always set to 1.

      When the Divisor Latch Access Bit (DLAB) is 1, registers 0 and 1
      become the LS and MS bytes of the Baud Rate Divisor registers.

      The baud rate is computed as (115200 / BaudRateDivisor). Thus,
      common baud rates correspond to divisors as follows:

         Baud   Divisor      Baud  Divisor      Baud  Divisor
          300    0180        4800   0018       38400   0003
         1200    0060        9600   000C       57600   0002
         2400    0030       19200   0006      115200   0001

      NOTES:
      (1) Must write BF hex to LCR before EFR [16650 ONLY] can be
      accessed (see 16650 data sheet).
      (2) Must set DLAB = 1 (80 hex) before 64 byte FIFO bit can be
      accessed (see 16750 data sheet).























     RS-232 Serial Communications                               Page 8

      1.3 Register Summary (Continued)

      REG 4 : Modem Control Register (MCR)

      RTS, DTR, loopback testing, and General Purpose Outputs #1 and #2 are
      controlled by the Modem Control register as follows:

         bit 7 : Clock select. X1 (if 0), X4 (if 1). [16750 ONLY]
         bit 6 : IR enable [16650 ONLY]
         bit 5 : Interrupt type select [16650 ONLY] or
                 Flow control enable [16750 ONLY].
         bit 4 : Enable local loopback.
         bit 3 : Enable GP02. Necessary for UART interrupts.
         bit 2 : Enable GP01.
         bit 1 : Set / clear RTS.
         bit 0 : Set / clear DTR.

      REG 5 : Line Status Register (LSR)

      Reading the Line Status register provides status information as
      follows (1 for TRUE, 0 for FALSE) :

         bit 7 : FIFO data error [16650 & 16750 ONLY].
         bit 6 : Transmitter Empty.
         bit 5 : Transmitter Buffer Empty (TBE).
         bit 4 : BREAK detect.
         bit 3 : Framing error.
         bit 2 : Parity error.
         bit 1 : Overrun error.
         bit 0 : Data Ready.




























     RS-232 Serial Communications                               Page 9

      1.3 Register Summary (Continued)

      REG 6 : Modem Status Register (MSR)

      Reading the Modem Status register provides the following status
      information (1 for TRUE, 0 for FALSE) :

         bit 7 : DCD status.
         bit 6 : RI status.
         bit 5 : DSR status.
         bit 4 : CTS status.
         bit 3 : Delta DCD status.
         bit 2 : Delta RI status.
         bit 1 : Delta DSR status.
         bit 0 : Delta CTS status.

      The delta bits (bits 0 through 3) are set whenever one of the status
      bits (bits 4 through 7) changes (from 0 to 1 or from 1 to 0) since
      the last time that the Modem Status register was read. Reading the
      Modem Status register clears the delta bits.

      REG 7 : Scratch Register

      There is no function associated with register 7.  It does not exist
      in early versions of the 8250.

































     RS-232 Serial Communications                               Page 10

      2.0 Modems

      A modem is used to extend the distance over which you may
      communicate. Without a modem, your RS-232 cable is limited to a
      maximum of approximately 50 feet.  But with a modem, you can
      communicate literally around the world.

      2.1 Modem Standards

      Two modems can communicate over a telephone line only if they are
      both using the same signaling frequencies and modulation, which are
      determined by the the modem standards used.  Modem standards can be
      divided into three sets: (1) speed, (2) data compression used, and
      (3) error control.

      The Bell standards (103 & 212A) are those of AT&T.  The CCITT (The
      International Consultative Committee for Telephone and Telegraph)
      standards are designated as 'V. '.

      Speed

           Bell 103  :   300 baud
           Bell 212A :  1200 baud
           V.21      :   300 baud
           V.22bis   :  1200 & 2400 baud
           V.32      :  4800 & 9600 baud
           V.32bis   :  4800, 7200, 9600, 12000, and 14400 baud
           V.34      :  V.32bis plus 16800, 19200, 21600, 24000, 26400,
                        and 28800 baud.
           V.34bis   :  V.34 plus 31200 and 33600 baud.
           USR X2    :  US Robotics 56KB standard (33.6 KB uploads).
           K56flex   :  Rockwells 56KB standard (33.6 KB uploads).
           V.90      :  The new 56K standard.

      Data Compression

           MNP 5     :  Microcom Networking Protocol (proprietary).
           V.42bis   :  International data compression standard.

      Error Control

           MNP 2,3,4 :  Three level error correction (public domain).
           V.42      :  International error correction standard.















     RS-232 Serial Communications                               Page 11

      2.2 Flow Control

      With modems using data compression, the modem to modem connection
      will run at various speeds depending on the quality of the line.
      The computer to modem connection will be at a fixed baud rate.
      Therefore, a protocol (flow control) is necessary to synchronize the
      data flow between a modem and the computer to which it is connected.
      Refer to your modem manual for information on flow control protocols
      supported.

      Two flow control protocols are used by most all modems which require
      flow control.  Software flow control is called 'XON/XOFF' (other
      software flow control character pairs are defined but operate the
      same as XON/XOFF) and hardware flow control is called 'RTS/CTS'. Most
      modems which require flow control enable hardware flow control by
      default.

      In XON/XOFF (software) flow control, the computer suspends
      transmitting data if it receives a XOFF character (13 hex) from the
      modem, and continues transmitting when it receives a XON character
      (11 hex).  Similiarly, the computer can signal the modem not to send
      any more data by transmitting a XOFF to it, and can tell the modem to
      continue transmission be sending a XON.

      In RTS/CTS (hardware) flow control, the RTS line is used by the
      computer to signal the modem , while the CTS line is used by the
      modem to signal the computer.  The RTS line is set OFF by the
      computer to tell the modem to suspend transmission, and set to ON to
      tell the modem to continue transmission.  The CTS line is set to OFF
      by the modem to tell the computer to stop transmitting, and set to ON
      to tell the computer to continue transmitting.

      Given the choice, always choose hardware flow control over software
      flow control so that all data transmission is transparent.  If
      hardware flow control is not the default (which it almost always is),
      you should modify your modem initialization string to turn hardware
      flow control on.





















     RS-232 Serial Communications                               Page 12

      2.3 Modem Initialization

      If an application uses a modem (as opposed to using a null modem
      cable), then it should always send an initialization string to the
      modem. Communication programs such as PROCOMM and TELIX always send
      such a string automatically as soon as they start up.

      The particular initialization string depends on the make of your
      modem.  For most modems, the following string (followed by a carriage
      return) should work:

           AT E1 S7=60 S11=60 V1 X1 Q0 S0=0

      Recall that the modem must be in command mode in order to send AT
      commands. To force command mode, send the character string "+-+"
      (without the quotes), preceeded by 1 second of silence, and followed
      by one second of silence.

      Refer to your Modem User's Guide for a full discussion of these
      commands.  A brief description is as follows:

           AT     Modem attention command.
           E1     Modem will echo what you send to it.
           S7=60  Wait 60 seconds for carrier and/or dial tone.
           S11=60 Use 60 milliseconds for tone dialing duration & spacing.
           V1     Display result code as words (not numbers).
           X1     Use the extended result message (CONNECT XXXX) set.
           Q0     Modem displays result codes.
           S0=0   Do not answer RING.

      If the application will answer incoming calls, set the S0 register to
      the ring on which to automatically answer.

      Most modems can be set to the 'factory default' by transmitting

           AT&F






















     RS-232 Serial Communications                               Page 13

      3.0 Other Serial Devices

      To be sure, the modem is the most common serial device. But there are
      many other serial devices such as digitizing tablets, scanners,
      digital cameras, numerical control machines, card readers, panel
      displays, etc.

      Some serial devices (such as modems) use hardware (RTS/CTS) flow
      control, but DTR/DSR flow control and software (XON/XOFF) flow
      control are also common.

      If you are writing a program to communicate with a serial device,
      keep in mind the following:

      (1) Always set DTR and RTS. Many serial devices "play dead" if DTR
          is not set.

      (2) You may need to add a small time delay (0.25 sec) between
          transmitted characters. This can be reduced or eliminated once
          everything is working.

      (3) Make sure that your receive buffer is sufficently large. You want
          to avoid buffer overflow.

      4.0 Shareware Libraries

      MarshallSoft Computing develops serial communications libraries for
      DOS (real and protected mode) and Windows (Windows 3.1, 95/98, and
      NT) marketed as shareware.

      There are specific libraries for C/C++, Delphi, Pascal (DOS only),
      Visual Basic (Windows & DOS versions), and Power Basic.

      WSC4C  WIN C/C++        Windows Standard Comm Library for C/C++
      WSC4D  WIN Delphi       Windows Standard Comm Library for Delphi
      WSC4VB WIN Visual Basic Windows Standard Comm Library for VB
      WSC4PB WIN Power Basic  Windows Standard Comm Library for PBCC

      PCL4W  WIN C/C++        Personal Comm Library for Windows
      PCLVBW WIN Visual Basic Personal Comm Library for C/C++

      PCL4C  DOS C/C++        Personal Comm Library for C/C++
      PCL4P  DOS Pascal       Personal Comm Library for Pascal
      PCL4VB DOS Visual Basic Personal Comm Library for Visual Basic
      PCL4PB DOS Power Basic  Personal Comm Library for Power Basic

      Our libraries have also been sucessfully used from Excel, Access,
      FoxPro, PowerBuilder, Clarion, dBase, Fortran, and Cobol.

      All shareware can be found on both the MarshallSoft BBS
      (256-880-9748) upon request and on the Internet at:

            ftp://ftp.marshallsoft.com/marshallsoft
            http://www.marshallsoft.com/download.htm

      Better yet, visit our web site and take a look around.


     RS-232 Serial Communications                               Page 14