ATARI 8 bit Serial Input/Output (SIO)

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      Most input and output with the Atari computer passes through the 
      serial I/O bus.  The SIO interface is rather complicated but you are 
      unlikely to need to use it directly.  CIO usually handles SIO for you. 
      However, if you want to design your own I/O device and it's associated 
      handler, you need to know how to use the SIO. 
      SIO transfers data at a rate of 19,200 baud on separate input and 
      output lines.  The data is sent one byte at a time, LSB first, in an 
      asynchronous format.  There are also clock-in and clock-out lines. 
      There is a signal on the clock-out line but it is not used by any 
      present devices.  The clock-in line is available for synchronous 
      transfer but is not used by the OS.  The signal on the clock-out line 
      goes high at the leading edge of each bit and goes low in the middle 
      of each bit. 
                              One byte of SIO data 
                     +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ 
                     | | | | | | | | | | | | | | | |        clock 
        -------------+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +------ 
        ---------+       +---+   +-------+       +-------- 
                 |     0 | 1 | 0 | 1   1 | 0   0 | 1        data 
                 +-------+   +---+       +-------+ 
                   |                                    | 
               start bit                            stop bit 
      The SIO interface is used much like the resident disk handler.  In 
      fact, it uses the same device control block as the resident disk 
      handler.  After the control block parameters are set, a JSR is made to 
      the SIO entry vector, SIOV, at $E459 (58457). 
                         Device control block (for SIO) 
      DDEVIC [$0300 (768)] 
           Serial bus I.D.  Set by handler or program. 
      DUNIT  [$0301 (769)] 
           Device number if more than one. 
      DCOMND [$0302 (770)] 
           Device command byte. 
      DSTATS [$0303 (771)] 
      Before the SIO call, this byte tells whether the operation is read, 
      write or that there is no data transfer associated with the command. 
      After the call this byte will hold the status (error/no error code) of 
      the operation. 
                          DSTATS format before command 
                7 6 5 4 3 2 1 0 
               |W|R| not used  | 
      If both W and R are 0, there is no data transfer. 
      DBUFLO [$0304 (772)] 
      DBUFHI [$0305 (773)] 
      Points to the data buffer for either input or output. 
      DTIMLO [$0306 (774)] 
      Timeout value (response time limit) in 64/60ths of a second to be set 
      by handler or program. 
      DBYTLO [$0308 (776)] 
      DBYTHI [$0309 (777)] 
      Number of bytes to be transferred, set by handler or program.  This 
      parameter is not required if the DSTATS specifies no data transfer. 
      DAUX1  [$030A (778)] 
      DAUX2  [$030B (779)] 
      These parameters are sent to the device as part of the command frame. 
      All commands on the serial bus must originate from the computer.  The 
      peripherals will present data on the bus only when commanded to do 
      Any operation on the serial bus begins with a five byte command frame. 
      While the command frame is being sent, the command line of the serial 
      connector is 0. 
                              Command frame format 
                $xx  DDEVIC 
                $xx  DCOMND 
                $xx  DAUX1 
                $xx  DAUX2 
                $xx  checksum 
      The first four bytes of the command frame come from the device control 
      block.  the checksum is the sum of the other four bytes with the carry 
      added back after each addition. 
      If both R and W of the DSTATS are 0, no data is sent to, or expected 
      from the peripheral, after a command frame is sent.  However, the 
      device is usually expected to send an ACK byte ($41) after the command 
      frame is sent.  If the command frame is invalid, an NAK byte ($4E) 
      should be sent. 
      If the operation is output (W = 1) the computer will send a data frame 
      after it receives the ACK of the command frame.  It then expects an 
      ACK after the data frame is sent. 
      If the operation is an input (R = 1) the computer expects a data frame 
      from the peripheral after the ACK.  With either input or output, a 
      "complete" code ($43) should be sent to the computer when the 
      operation is finished.  The "complete" code would follow the ACK of 
      the data frame with an output operation. 
      If the operation is not completed for some reason, the peripheral 
      should send an error code ($45) instead of "complete". 
                                 SIO data frame 
          byte 1     $xx\ 
                         > data bytes 
          byte n     $xx/ 
          byte n+1   $xx   checksum 
                                  SIO commands 
      READ      $52 
      WRITE     $57 
      STATUS    $53 
      PUT       $50 
      FORMAT    $21 
      DOWNLOAD  $20 
      READADDR  $54 
      READ SPIN $51 
      MOTOR ON  $55 
       SECTOR   $56 
                            Present SIO device I.D.s 
      DISK      $31 - $34  (D1 - D4) 
      PRINTER   $40 
      RS-232-C  $50 - $53  (R1 - R4) 
      The serial connectors on the computer and all peripherials are 
      identical.  Nearly all peripherials have two serial connectors. 
      Either connector may be used for any connection.  The serial bus is 
      designed so that peripherials can be daisy-chained together.  The 
      following is a diagram of the serial connector. 
                          The serial connector pin-out 
                             1 1 
                     2 4 6 8 0 2 
                    /o o o o o o\ 
                   /o o o o o o o\ 
                    1 3 5 7 9 1 1 
                              1 3 
       1  clock in (to computer) 
       2  clock out 
       3  data in 
       4  GND 
       5  data out 
       6  GND 
       7  command (active low) 
       8  cassette motor control 
       9  proceed (active low) 
      10  +5V/ready 
      11  audio in 
      12  +12V (400/800) 
      13  interrupt (active low) 
      Proceed goes to pin 40 (CA1) of the PIA.  It is not used by present 
      Interrupt goes to pin 18 (CB1) of the PIA.  It is not used by present 
      Pin 10 doubles as a 50mA +5V peripharal power supply and a computer 
      ready signal. 
                    Useful database variables and OS equates 
      SIOV   $E459      (58457): serial port handler entry 
      DDEVIC $0300        (768): device ID 
      DUNIT  $0301        (769): device number 
      DCOMND $0302        (770): command byte 
      DSTATS $0303        (771): status byte 
      DBUFLO $0304        (772): data buffer pointer 
      DBUFHI $0305        (773): 
      DTIMLO $0306        (774): timout value 
      DBYTLO $0308        (776): number of bytes to transfer 
      DBYTHI $0309        (777): 
      DAUX1  $030A        (778): sent to device 
      DAUX2  $030B        (779): sent to device 


Category:Serial Connectors


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