The SIO protocol description


             Written by Draco, 18Sep1996 Last update 7Nov1996
                                                                            

What is the SIO?
----------------
The SIO stands for the "Serial Input/Output".  It is a data transfer 
protocol used by the Atari 8-bit machines to communicate with a printer, 
disk drive, modem etc. via the SIO connector.  These transfers are 
controlled by the PokEY chip.

                        Is this something like RS-232?

Basically, yes.  It is possible to connect the SIO port to a RS-232 port, 
the only one you should change is the voltage: the SIO uses TTL-signals.  
Serial transfer parameters usually are as follows:

             * 1 start bit
             * 1 stop bit
             * 8 data bits
             * No parity control
             * 19200 bits per second


The last parameter may vary, as for example so called "cassette recorders" 
(who can remember such data storage?) use 600 baud; also some disk drives 
may operate at 38400, 52000, 67000, 96000 or 125000 baud.

SIO commands:
-------------
The command block contains 4 (four) bytes:

          * CDEVIC - device ID, e.g. $31 for the DI:, $32 for the D2: etc.
          * CCMND - command
          * CAUXI

          * CAUX2 - sector number (low/high)

Most commonly used commands (DCMND values) for disk drives are as follows:

          * $50 (P) - put a sector (128 or 256 data bytes to the drive)
          * $52 (R) - read a sector (128 or 256 data bytes from the disk 
                      drive)
          * $53 (S) - read status (4 data bytes from the drive)
          * $57 (W) - write sector (the same as P command, but with verify)
          * $21 (!) - format disk (128 data bytes from the drive, 18 
                      sectors per track)
          * $22 (") - format disk in the medium density (128 data bytes 
                      from the drive, 26 sectors per track)
          * $4E (N) - read configuration (12 data bytes from the drive)
          * $4F (O) - set configuration (12 data bytes to the drive)


Communication:
--------------
          * first, the computer sets the COMMAND line at the SIO connector
          * then sends the command block (4 bytes + checksum)
          * then waits for a response (1 byte without a checksum):
            + $41 (A) = Acknowledge, the command is valid and will be 
                        executed
            + $4E (N) = Negative, the command is invalid.
          * if the device responds "N", the transfer is aborted.
          * otherwise ("A") the data block is transferred with the 
            checksum.
          * now the computer waits for a final acknowledge (1 B without a 
            checksum):
            + $43 (C) = Complete, operation completed successfully.
            + $45 (E) = an Error has occurred.