rs232.txt

在VB中实现通讯的例子

                        RS-232 Serial Communications


                              USERS MANUAL



                               Version 1.3

                               Aug 14 1998







                            Copyright (C) 1998
                            All rights reserved



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     RS-232 Serial Communications                               Page 1

                            C O N T E N T S



      Chapter                                                   Page


       1.0 The UART...................................................3

           1.1 UART Types.............................................3

               National 8250..........................................3

               National 16450.........................................3

               National 16550.........................................3

               StarTech 16650.........................................3

               Texas Instruments 16750................................3

           1.2 UART Operation.........................................4

           1.3 RS-232 Signals.........................................5

           1.4 UART Registers.........................................6

           1.5 Register Summary.......................................7

       2.0 Modems

           2.1 Modem Standards.......................................11

           2.2 Flow Control..........................................12

           2.3 Modem Initialization..................................13

       3.0 Other Serial Devices......................................14

       4.0 Shareware.................................................14


















     RS-232 Serial Communications                               Page 2

      1.0 The UART

      The heart of serial communications is the "Universal Asynchronous
      Receiver Transmitter", or UART for short. The UART is responsible for
      controlling the computer's RS-232 port.

      1.1 UART Types

      UARTs can be broken down into two classes: buffered and unbuffered.
      The 8250 and 16450 are unbuffer, while the 16550, 16650, and 16750
      are buffered.

      1.1.1 National 8250

      The National 8250 was the original UART used in the IBM PC and
      compatibles, and are only suitable up to 9600 baud on slower DOS
      (before the IBM/AT) machines. The 8250A is somewhat faster than the
      8250, but should also be limited to slower DOS machines.

      1.1.2 National 16450

      The National 16450 was designed to work with the IBM PC/AT (16-bit
      data bus) and faster machines. Faster than the 8250, it still has a
      one-byte buffer. The 16450 chip is suitable for many DOS applications
      and some Windows applications up to about 38400 baud.

      1.1.3 National 16550

      The National 16550 features 16-byte transmit side and receive side
      FIFO buffers. The interrupt trigger level (on the receive side) can
      be set at 1, 4, 8 or 14 bytes. The TX FIFO trigger is fixed at 16
      bytes.

      The FIFOs on the original 16550 UARTs did not work. The 16550A soon
      followed the 16550 and is the "standard" UART for Windows machines.

      The 16550A is recommended as the minimum chip on any new serial board
      purchase.

      1.1.4 StarTech 16650

      The StarTech 16650 features 32-byte FIFOs and on-chip flow control,
      and can be run at up to 460800 baud. It is also pin for pin
      compatible with the 16550 UART.

      1.1.5 TI 16750

      The Texas Instruments 16750 features 64-byte FIFOs and on-chip flow
      control and can be run at up to 921600 baud, but is not pin for pin
      compatible with the 16550 UART.








     RS-232 Serial Communications                               Page 3

      1.2 UART Operation

      The purpose of the UART is:

      (1) To convert bytes from the CPU (Central Processing Unit) into a
      serial format by adding the necessary start, stop, and parity bits to
      each byte before transmission, and to then transmit each bit at the
      correct baud rate.

      The first bit is always the start bit, followed by 5 to 8 data bits,
      (optionally) followed by the parity bit, followed by the stop bit or
      bits.

      (2) To convert the incoming stream (at a specified baud rate) of
      serial bits into bytes by removing the start, stop, and parity bit
      before being made available to the CPU.

      The UART is capable of operating in one of two modes, 'polled' and
      'interrupt driven'.  The serial communications functions in the BIOS
      use the polled method.  In this approach, the CPU is typically in a
      loop asking the UART over and over again if it has a byte ready.  If
      a byte is ready, the polling code returns the byte. But, if the next
      byte comes in before the polling code is executing again, then that
      byte is lost.

      In the interrupt driven approach, when a byte is received by the
      UART, an 'Interrupt Service Routine' (ISR) is executed immediately,
      suspending temporarily whatever is currently executing. The ISR then
      moves the byte from the UART to a buffer so that the application
      program can later read it.

      The 16550 can be programmed so that a receive (RX) interrupt is not
      triggered until 4 (or 8 or 14) bytes have been received, while the
      16650 can be triggered at up to 30 bytes, and the 16750 can be
      triggered at up to 56 bytes. This can significantly reduce the CPU
      processing time, since 14 (or 30 or 56) bytes can be moved at once.

      Transmitted bytes are queued up awaiting transmission.  When a byte
      is moved from the UART transmitter holding register to the UART
      transmitter shift register, an interrupt is generated and the next
      byte is taken from the transmitter buffer by the ISR and written to
      the UART holding register.

      Up to 16 bytes can be written at once to the transmitter FIFO buffer
      while processing one transmitter interrupt if an 16550 UART is used,
      while the 16650 can write up to 32 bytes at once, and the 16750 can
      write up to 64 bytes at once.











     RS-232 Serial Communications                               Page 4

      1.3 RS-232 Signals

      RS-232 is the name of the serial data interface standard used to
      connect computers to modems.

      A summary of the serial port pins and their function follows.  For
      more detailed information, refer to one of the many books dealing
      with RS-232 interfacing.

      Signal Ground Pin 7 (DB25), Pin 5 (DB9)

      The SG line is used as the common signal ground, and must always be
      connected.

      Transmit Data Pin 2 (DB25), Pin 3 (DB9)

      The TX line is used to carry data from the computer to the serial
      device.

      Receive Data Pin 3 (DB25), Pin 2 (DB9)

      The RX line is used to carry data from the serial device to the
      computer.

      Data Terminal Ready Pin 20 (DB25), Pin 4 (DB9)

      The DTR line is used by the computer to signal the serial device that
      it is ready. DTR should be set high when talking to a modem.

      Data Set Ready Pin 6 (DB25), Pin 6 (DB9)

      The DSR line is used by the serial device to signal the computer that
      it is ready.

      Request to Send Pin 4 (DB25), Pin 7 (DB9)

      The RTS line is used to 'turn the line around' in half duplex modems,
      and for hardware flow control in most modems that require flow
      control.  RTS is controlled by the computer and read by the serial
      device (modem).

      Clear to Send Pin 5 (DB25), Pin 8 (DB9)

      The CTS line is used to 'turn the line around' in half duplex modems,
      and for hardware flow control in most modems that require flow
      control.  CTS is controlled by the serial device (modem) and read by
      the computer.

      Data Carrier Detect Pin 8 (DB25), Pin 1 (DB9)

      The DCD line is used by the modem to signal the computer that a data
      carrier signal is present.

      Ring Indicator Pin 22 (DB25), Pin 9 (DB9)

      The RI line is asserted when a 'ring' occurs.


     RS-232 Serial Communications                               Page 5

      1.4 UART Registers

      Data sheets can be obtained from the following sources on the
      Internet.

      16550  www.national.com and www.exar.com
      16650  www.exar.com
      16750  www.ti.com

      These UARTs consists of 8 register ports as follows:

           Offset    R/W   Register
             0       R/W   Receiver (read) / Transmitter (write)
             1       R/W   Interrupt Enable Register (IER)
             2       R     Interrupt Identification Register (IIR)
             2       W     FIFO Control Register (FCR : 16550/650/750)
             2       R/W   Enhanced Features Register (EFR : 16650 Only)
             3       R/W   Line Control Register (LCR)
             4       R/W   Modem Control Register (MCR)
             5       R/W   Line Status Register (LSR)
             6       R/W   Modem Status Register (MSR)
             7       R/W   Scratch register. Not used.

      For the standard PC ports, the UART registers are based at 3F8h
      (COM1), 2F8h (COM2), 3E8h (COM3), and 2E8h (COM4).  COM1 and COM3
      share interrupt request line IRQ4 while COM2 and COM4 share request
      line IRQ3. This means that COM1 and COM3 can't be used concurrently.
      Similarly for COM2 and COM4.

      Four sources of interrupts are possible. (1) receiver error or BREAK,
      (2) receiver data ready, (3) ready to transmit, and (4) RS-232 input.
      Additional sources can be generated by the 16650 (see 16650 spec
      sheet).

      These four sources of interrupts are summarized as follows:

         Source of Interrupt        Action Required to Clear
         Receiver error or BREAK.   Read Line Status register.
         Receiver data.             Read data from data register.
         Transmitter Buffer Empty.  Write to data register or read IID reg.
         RS-232 input.              Read Modem Status register.

      Serial ports are configured as either Data Communications Equipment
      (DCE) or Data Terminal Equipment (DTE). Modems are always configured
      as DCE, while serial printers are (almost) always configured as DTE.
      Most serial (computer) ports are designed to talk to modems and are
      therefore configured as DTE. Serial (computer) ports designed to talk
      to serial printers (rare today) are configured as DCE. Most other
      serial device are configured as DCE.

      A normal RS-232 cable is used to connect two serial ports with
      opposite configuration (DTE & DCE), while a null modem cable is used
      to connect two serial ports of the same configuration (both DTE or
      both DCE).




     RS-232 Serial Communications                               Page 6

      1.5 Register Summary

      REG 0 : Data Register

      Reading from the data register fetches the next input byte, once it
      is ready. Writing to the data register transmits the byte written to
      it over the serial line.

      REG 1 : Interrupt Enable Register (IER)

      The Interrupt Enable register enables each of four types of
      interrupts when the appropriate bit is set to a one.

         bit 3 : Enable interrupt on RS-232 input.
         bit 2 : Enable interrupt on receiver error or break.
         bit 1 : Enable interrupt on transmitter buffer empty (TBE).
         bit 0 : Enable interrupt on received data (RxRDY).

      REG 2 : Interrupt Identification Register (IID)

      Reading the Interrupt Identification (read only) register once an
      interrupt has occurred identifies the interrupt as follows:

         Bit 2  Bit 1  Bit 0  Priority    Interrupt
           0      0      1      none      none
           1      1      0      0 (high)  Serialization or break.
           1      0      0      1         Received data.
           0      1      0      2         Transmitter Buffer Empty.
           0      0      0      3 (low)   RS-232 Input.

      REG 2 : Interrupt Identification Register (IID)

      In the 16550, 16650, and 16750, REG 2 (write only) is also the FIFO
      control register.  Writing bits 6 & 7 will set the RX FIFO trigger
      level (number of bytes received before an interrupt is generated).

      Bit 7  Bit 6   16550 Trigger  16650 Trigger  16750 Trigger
       0      0        1 byte          8 bytes        1 byte
       0      1        4 bytes        16 bytes       16 bytes
       1      0        8 bytes        24 bytes       32 bytes
       1      1       14 bytes        28 bytes       56 bytes

      The TX FIFO level can also be set in the 16650 by setting bits 4 & 5.
      The 64-byte FIFO mode on the 16750 can be enabled by setting bit 5 in
      the FCR. See the 16650 & 16750 data sheets for more details.

      REG 2 : Enhanced Feature Register (EFR) [16650 ONLY]

      The EFR can only be accessed after writing a BF to the LCR, after
      which the advanced features on the 16650 are enabled by setting bit 4
      of the EFR. For more details, see the 16650 data sheet.







     RS-232 Serial Communications                               Page 7