textfiles/computers/serial.txt

================= PROGRAMMING THE SERIAL PORT ==================

     Here's some bits and pieces that might be of interest.

----------------------------------------------------------------
INT 14 - SERIAL I/O - INITIALIZE USART
   REG AH = 0
   REG AL = INITIALIZING PARAMETERS
   BIT  7 - 6 - 5 4 - 3  2    1 - 0
  -BAUD RATE-    PARITY  STOP    WORD
           BITS  LENGTH
  000  110 BD    00 NONE 0-1   10 - 7
  001  150 BD    01 ODD 1-2   11 - 8
  010  300 BD    11 EVEN
  011  600 BD
  100 1200 BD
  101 2400 BD
  110 4800 BD
  111 9600 BD (4800 ON PCjr)
       DX = PORT NUMBER
----------------------------------------------------------------
INT 14 - SERIAL I/O - TRANSMIT CHARACTER
   REG AH = 1
       AL = CHARACTER
       DX = PORT NUMBER
   ON RETURN:
      REG AH = RS-232 STATUS CODE
           BIT 0 = DATA READY
               1 = OVERRUN ERROR
               2 = PARITY ERROR
               3 = FRAMING ERROR
               4 = BREAK DETECTED
               5 = TRANSMISSION BUFFER REG. EMPTY
               6 = TRANSMISSION SHIFT REG. EMPTY
               7 = TIME OUT
          AL = MODEM STATUS
           BIT 0 = DELTA CLEAR-TO-SEND
               1 = DELTA DATA-SET-READY
               2 = TRAILING EDGE RING DETECTED
               3 = CHANGE, RECEIVE LINE SIGNAL DETECTED
               4 = CLEAR-TO-SEND
               5 = DATA-SET-READY
               6 = RING DETECTED
               7 = RECEIVE LINE SIGNAL DETECTED
----------------------------------------------------------------
INT 14 - SERIAL I/O - RECEIVE CHARACTER
   REG AH = 2
   ON RETURN:
      REG AL = CHARACTER RECEIVED
      REG AH = RS-232 STATUS CODE (SEE ABOVE)
----------------------------------------------------------------
INT 14 - SERIAL I/O - GET USART STATUS
   REG AH = 3
   ON RETURN:
      REG AH = RS-232 STATUS CODE (SEE ABOVE)
      REG AL = MODEM STATUS CODE (SEE ABOVE)

----------------------------------------------------------------
  The following applies to COM1 (COM2 addresses are different).
----------------------------------------------------------------

procedure SendChar(C: byte);
begin
  while (Port[$3FD] and $20)=0 do ;
  Port[$3F8] := C;
end;

Function ReadChar: byte;
begin
  while not odd(Port[$3FD]) do ;
  ReadChar:= Port[$3F8];
end;

procedure SetSerial(BaudRate: Integer);
{ Set serial parameters on to COM1:BaudRate,N,8,1 }
var
  x  : integer;
begin
  x := trunc(115200.0 / BaudRate);
  Port[$3FB] := 128;
  Port[$3F8] := x and 255;
  Port[$3F9] := x shr 8;
  Port[$3FB] := 3;   {Line Control Register}
  Port[$3FC] := 3;   {Modem Control Register}
  Port[$3F9] := 0;   {Interupt Enable Register}
end;

----------------------------------------------------------------

Port(hex)      Description

3F8      I/O   Transmitter Holding Register / Receiver Data Register
3F8/3F9   O    Baud Rate Divisor
3F9      I/O   Interupt Enable Register
3FA       I    Interupt Identification Register
3FB      I/O   Line Control Register
3FC       O    Modem Control Register
3FD       I    Line Status Register  (see "RS-232 STATUS CODE" above)
3FE       I    Modem Status Register (see above for bits)

Programming the 8250 Serial Chip.

1. Set Baud Rate.
  x := trunc(115200.0 / BaudRate);
  Port[$3FB] := 128;
  Port[$3F8] := x and 255;
  Port[$3F9] := x shr 8;

  Take For example 2400 baud; the following would be needed
  in Assembly Language:

           mov   dx,3FBh
           mov   al,128
           out   dx,al
           mov   dx,3F8h
           mov   al,30H
           out   dx,al
           inc   dx
           mov   al,0
           out   dx,al


- You might like to set up a table if more that one baud rate is used.


2. Set LCR ($3FB)
  Bit 7    Normally 0, set to 1 to change baud rate
      6    Normally 0 (Break Disabled).
      5    Normally 0, "Stick Parity" disabled
      4    Parity: 0 = Odd, 1 = Even
      3    Parity: 0 = No parity generated, 1 = Generate parity
      2    Stop Bits: 0 = 1 bit, 1 = 2 bits (or 1.5 if Char len = 5 bits)
    1+0    Character Length:
           00 = 5 bits, 01 = 6 bits, 10 = 7 bits, 11 = 8 bits.

  For 8 Data Bits, No Parity, 1 Stop Bit - value of LCR is 3

3. Set MCR ($3FC)

 Bit 7..5  Always  0
        4  Loop Back (output goes straight to input)
        3  Normally 0, set to 1 for serial interrupt.
        2  Not Used
        1  Activate RTS line
        0  Activate DTR line

  Value of MCR is normally 3.  I won't go into the details of interrupts
  send me a message if you do need the info.


4. Set up IER ($3F9)

  This is normally 0.


That's all that's needed to set it up.


To receive a character:
  A character is ready in the Receiver Data Register  when bit 0 ("Received
  Data Ready") of line status register is 1. See ReadChar function.

To send a character.
  You must wait until the previous character has been sent.
  Bit 5 ("Transmitter Holding Register Empty") will be zero when it is
  O.K for you to send a character. See the SendChar procedure.

                                                          ==== PY ====