Page 46 - Hacking Roomba

Chapter 2 — Interfacing Basics 27

There is no way to determine which state the robot is in. If you are unsure, explicitly set its state
to what you want.

If the Roomba battery is removed or when the battery is too low for Roomba to operate reliably,
the system will switch to the off state.
One undocumented event that has happened a few times to me is an over-current event. If the
Roomba battery is low and the robot attempts to do something that uses a lot of current (like
drive across thick carpet), it appears to trigger an over-current sensor that switches Roomba out
of safe or even full mode and into passive mode.

ROI Command Structure

All ROI commands start with a command opcode, a single byte with a value between 128 (0x80)
and 143 (0x8F), inclusive. Only 16 opcodes have been published, but these 16 allow almost
complete control. There may be more, unpublished opcodes.
In addition to the command opcode byte, an ROI command may include one or more data bytes
that are arguments to the command. Many commands, like the START command, consist of only
the command opcode byte and no data bytes.The PLAY command is an example of a command
that takes one extra command byte, a data byte with the number 1–16 of the song to play. All
commands except the SONG command have a fixed number of data bytes.The SONG command
has a variable number of data bytes, depending on the length of the song, N.The number of data
bytes is determined by the formula 2+2N. Therefore, if the song to be sent is one note long
(N=1), the number of data bytes to send is 4. For a 10-note song (N=10), 22 data bytes are sent.
To send a complete ROI command, send the appropriate command byte and the appropriate
data bytes, if any, from the serial port of your controlling computer and to the RXD pin of the
ROI port.
Table 2-3 shows the opcodes and the number of data bytes for each command. Hexadecimal
representation of the opcode values is useful since each hexadecimal digit represents exactly
four bits, so two hex digits exactly represent 8 bits, or one byte.

Table 2-3 Command Opcodes and Data Bytes
Command Opcode Hexadecimal Values Number of Data Bytes
START 128 0x80 0
BAUD 129 0x81 1
CONTROL 130 0x82 0
SAFE 131 0x83 0
FULL 132 0x84 0
POWER 133 0x85 0

Continued