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Port Configuration                                                      Page 1

                          Configuring the I/O Ports

If  you  are  writing  a program which performs I/O to one of the devices that
connect to the back of the ST (i.e., a printer or a modem), you will  probably
want to set the configuration at some time or other.  If you are writing a GEM
application  and you are content to use the desk accessories to allow the user
to configure the  ports,  you  can  skip  this  section.   However,  the  desk
accessories don't always set up the configuration properly, so be careful!  In
any  case,  if  you  want  to  set  the configuration of the RS232 port or the
parallel port, you need to know a few calls:

Set the printer configuration.

We'll investigate the printer  configuration  first,  since  there  are  fewer
parameters  to  explain.  The following XBIOS call allows you to configure the
printer:

FUNCTION setprt( config : integer ) : integer ;
  XBIOS( 33 ) ;

In order to set or get the current printer configuration, you should use  this
call.  If config is -1, the current configuration is passed back as the return
value.   Otherwise, config specifies the desired configuration of the printer.
The various bits within config specify the configuration as follows:

bit#  when 0            when 1
----  ----------------  -----------------
  0   dot matrix        daisy wheel
  1   color printer     monochrome
  2   Atari printer     Epson compatible
  3   draft mode        final mode
  4   parallel port     RS232 port
  5   continuous paper  single sheet
  6               reserved
  7               reserved
  8               reserved
  9               reserved
 10               reserved
 11               reserved
 12               reserved
 13               reserved
 14               reserved
 15   MUST BE ZERO!


Configure the RS232 port.

OK, on to the RS232 configuration.  The following XBIOS call sets the  various
parameters controlling the RS232 port:

PROCDURE rsconf( speed, flowctl, ucr, rsr, tsr, scr : integer ) ;
  XBIOS( 15 ) ;

If  any  of  the  parameters  is -1, the corresponding RS232 parameter is left
unchanged from its previous value.  You will mostly be  dealing  with  setting
the baud rate, which is governed by the speed parameter:








Port Configuration                                                      Page 2

speed   rate
-----  -----
  0    19200
  1     9600
  2     4800
  3     3600
  4     2400
  5     2000
  6     1800
  7     1200
  8      600
  9      300
 10      200
 11      150
 12      134
 13      110
 14       75
 15       50-

The last value, 15, may not generate an accurate (as if you'll ever need it!).
You  may also need to change the flow-control option of the RS232 port.  It is
specified in the flow parameter as follows:

flow  flow-control
----  ------------
 0    No flow control
 1    XON/XOFF (control-S/control-Q)
 2    RTS/CTS
 3    XON/XOFF and RTS/CTS

The value 3 doesn't represent a very useful condition,  but  it  should  work.
The  other  four  parameters  set  registers within the 68901 chip (for a more
complete, but still sketchy, discussion, see the book  ST  Internals).   These
registers perform the following functions:

register  function
--------  --------
  ucr     USART control register
  rsr     Receiver status register
  tsr     Transmitter status register
  scr     Synchronous character register

If  you  are  transmitting in asynchronous mode (i.e, almost always), you will
probably only use the ucr parameter, which has the following meanings:

ucr bits   function
--------   --------
   0       unused
   1       parity type: 0=odd 1=even
   2       parity enable: 0=no parity 1=parity
  4,3      0,0 -> synchronous mode (all others asynch)
           0,1 -> 1 start bit, 1 stop bit
           1,1 -> 1 start bit, 2 stop bits
  6,5      number of data bits
           0,0 -> 8 bits
           0,1 -> 7
           1,0 -> 6







Port Configuration                                                      Page 3

           1,1 -> 5
   7       transmit and receive frequency
            0 -> divide by 1 (synchronous only)
            1 -> divide by 16

If you want to use the rsr, tsr, or scr registers, please refer elsewhere  for
more documentation on the 68901 chip (ST Internals, for example).