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
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.
USING THE SIO INTERFACE
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
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
READ SPIN $51
MOTOR ON $55
Present SIO device I.D.s
DISK $31 - $34 (D1 - D4)
RS-232-C $50 - $53 (R1 - R4)
THE SERIAL CONNECTOR
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
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 clock in (to computer)
2 clock out
3 data in
5 data out
7 command (active low)
8 cassette motor control
9 proceed (active low)
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
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