Atari 400 Technical Reference Manual page 134

Following the command frame (and ACK) can be an optional data
frame that is formatted as shown below:
+----------------+
data
=
=
bytes
+----------------+
checksum
+----------------+
This data frame can originate at the computer
or
at the device
controller, depending upon the command. Current device
controllers expect fixed-length data frames as does the computer,
where
the data frame length is a fixed function of the device
type and command.
The checksum value in the data frame is the arithmetic sum of all
of the frame data preceding the checksum, with the carry from
each addition being added back (the same as for the command
frame).
In the case of the computer sending a data frame to a peripheral,
the peripheral is expected to send an ACK if the data frame is
acceptable, and send a NAK ($4E), or do nothing if the data frame
is unacceptable. See the first flowchart in Section 9.
Operation Complete
A peripheral is also expected to send an operation-COMPLETE byte
($43) at the time the commanded operation is complete. The
location of this byte in the command sequence for each command
type is shown in the timing diagrams in Section 9. If the
operation cannot go to normal, error-free completion, the
peripheral should respond with an ERROR byte ($45) instead of
COMPLETE.
OPERATING SYSTEM C016555 -- Section 9
149