SIO

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.

Command Frame

Offset	Name	Description
$00	DDEVIC	Device ID
$01	DCOMND	Command
$02	DAUX1	Auxiliary byte 1
$03	DAUX2	Auxiliary byte 2
$04		Checksum of previous bytes

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.

Device IDs

Floppy		Printer		Cassette		RS232
ID	Device	ID	Device	ID	Device	ID	Device
$31	D1:	$40	P1:	$60	C: (dummy)	$50	R1:
$32	D2:	$41	P2:			$51	R2:
$33	D3:	$42	P3:			$52	R3:
$34	D4:	$43	P4:			$53	R4:
$35