Atari 400 Technical Reference Manual page 133

Command Frame
The serial bus protocol provides for three types of commands: 1) data
send, 2) data receive and 3) immediate (no data -- command only).
There is a common element in all three types, a command frame
consisting of five bytes of information sent from the computer
while the COMMAND- line is held low. The 40rmat of the command
frame is
shown below:
+----------------+
device ID
+----------------+
command
+----------------+
I auxiliary #1
+----------------+
: auxiliary #2
+----------------+
checksum
+----------------+
Figure 9-5 Serial Bus Command Frame Format
The device ID specifies that of the serial bus devices is being
addressed (see Appendix I for a list of device IDs).
The command byte contains a device-dependent command (see
Appendix I for a list of device commands).
The auxiliary bytes contain more device-dependent information.
The checksum byte contains the arithmetic sum of the first four
bytes (with the carry added back after every addition).
Command Frame Acknowledge
The peripheral being addressed would normally respond to a
command frame by sending an ACK byte ($41) to the computer; if
there is a problem with the command frame, the peripheral should
not respond.
Data Frame
OPERATING SYSTEM C016555 --Section 9
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