Serial Input/Output (SIO) Interface
The Atari 1200XL communicates with peripheral devices over an asynchronous serial port
(19.2K Baud RAte Max). Data is transmitted and received as 8 bits of serial data (LSB sent
first) preceded by a logic zero start bit and succeeded by a logic one stop bit. The serial data out
is transmitted as positive logic. The serial DATA OUT line always assumes its new state when
the serial CLOCK OUT line goes high; CLOCK OUT goes low in the center of the DATA
OUT time.
The bus protocol specifies that all commands must originate from the computer, and that
peripherals present data on the bus only when commanded to do so. Every bus operation goes
to completion before another bus operation is initiated (no overlap). An error detected at any
point in the bus operation will abort the entire sequence. A bus operation consists of the
following elements:
Command Frame (From Computer)
Acknowledge Frame (From Peripheral)
Optional Data Frame (To or From Computer)
Complete Frame (From Peripheral)
Command Frame
The serial bus protocol provides for three types of commands:
1) Data Send
2) Data Receive
3) Immediate (No Data-Command Only)
1200XL Home Computer
Field Service Manual 1-12
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