Page 44 - Hacking Roomba

Chapter 2 — Interfacing Basics 25

The commands and the responses are binary, not text. Thus communication is not as simple as
connecting to Roomba with a terminal program and typing commands. Determining how to
test the connection after a physical interface has been built will be covered in Chapter 3.
When using the ROI, Roomba exists in one of several modes (or states). These modes repre-
sent both how Roomba behaves and how it responds to subsequent ROI commands. Actions
by Roomba can also change the mode. Several of the ROI commands are dedicated to selecting
the appropriate mode/state because certain commands only work in certain states. The mode
change commands are single bytes with no arguments and are invoked by just sending the
command byte for the desired mode. Other commands have arguments, like how fast to drive
or which vacuum motors to turn on. Those arguments are sent immediately after the command
byte as data bytes with a particular format. The format varies among commands and can either
be a single byte, a 16-bit value represented as two bytes, several binary values (bits) encapsu-
lated in a single byte, or some combination thereof. One of the challenges of using the ROI is
knowing all the data type variations. Only one command causes Roomba to return data, and
the block of bytes it returns must also be carved up into bits, bytes, and 16-bit words.

Roomba Modes
When using the ROI, Roomba can exist in one of five states (see Figure 2-4). The states are:

Sleep (Off): Roomba responds to no commands over the ROI, but can be woken up
and put into the on mode through either the Power button or by toggling the DD hard-
ware line.
On: Roomba is awake and is awaiting a START command over the ROI. In this mode
Roomba is able to operate normally via its buttons or remote control. The only way out
of this mode through the ROI is through the START command.
Passive: Roomba has received the START command. In this mode, sensors can be read
and songs defined, but no control of the robot is possible through the ROI. The
Roomba buttons work as normal. This is the mode to use to spy on Roomba as it goes
about its business. The usual path from this mode is to send the CONTROL command to
enter safe mode.
Safe: Roomba has received the CONTROL command from passive mode or the SAFE
command from full mode. Everything that could be done in passive mode is still possi-
ble, but now Roomba can be controlled. The buttons on Roomba no longer change the
robot’s behavior, but instead their states are reflected in the Roomba sensors data. All
commands are now available, but a built-in safety feature exists to help you not kill your
Roomba. This safety feature is triggered if Roomba detects any of the following:
■ A cliff is encountered while moving forward (or moving backward with a small
turning radius).

■ Any wheel drops.
■ The charger is plugged in and powered.