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General Purpose Input/Output (GPIO) Controllers

General Purpose Input/Output (GPIO) Controllers provide a standardized way to interact with I/O signals exposed by a robot or device. These signals are commonly used to control or monitor external hardware such as grippers, sensors, indicators, safety devices, or auxiliary tooling.

GPIO controllers operate on input and output signals, providing dedicated handling of digital (binary) and analog (non-binary) I/O within the context of a controller. This specialization differentiates them from motion-oriented controllers and keeps GPIO interaction simple, deterministic, and easy to integrate into higher-level application logic.

GPIO controllers enable applications to:

  • Read GPIO state (e.g., sensor feedback, limit switches)
  • Command GPIO outputs (e.g., enable a tool, toggle a relay)
  • Interact with external devices with low latency and predictable behavior

AICA provides generic GPIO controllers that are specialized for a single responsibility:

  • Broadcasting controllers expose GPIO state to the AICA application
  • Output controllers command GPIO values on the hardware

GPIOs in robot models​

In ROS 2, GPIOs are declared in the robot’s URDF using dedicated GPIO tags. These tags define:

  • The GPIO name and direction (state or command)
  • Its association with a robot or device
  • The command or state interface used to access it

By relying on the robot description, GPIO controllers can automatically discover available I/O signals and expose them consistently, without hard-coded assumptions about the underlying hardware.

info

Some specialized controllers (for example, controllers for specific robot arms) may combine motion control and I/O handling within a single controller.

For example, a typical URDF that contains a GPIO command interface will look something like the following:

<gpio name="sample_gpio_group_1">
  <state_interface name="output_register" />
</gpio>

and one that contains a GPIO state interface:

<gpio name="sample_gpio_group_2">
  <command_interface name="input_register" />
</gpio>

or you may see both command and state interfaces bundled under the same group, as follows:

<gpio name="sample_gpio_group">
  <state_interface name="output_register" />
  <command_interface name="input_register" />
</gpio>

Using GPIO controllers in the AICA framework​

In a ROS 2 control setup, and by extension in AICA Core, GPIO controllers operate on GPIO interfaces defined in the URDF and exposed by the underlying hardware interface.

Two GPIO controllers are bundled by default, each aligned with a specific GPIO role:

  • GPIO Broadcaster Controller: reads and publishes GPIO state through a general-purpose state interface
  • GPIO Output Controller: commands GPIO values through a general-purpose command interface

Applications interact with these controllers through standard ROS 2 communication mechanisms, without direct access to low-level drivers.

GPIO Broadcaster Controller​

The GPIO Broadcaster Controller is responsible for observing GPIO state and making it available to the rest of the AICA application.

It requires two parameters:

  • GPIO group name (for example sample_gpio_group_1 in the URDF snippet above)
  • State interface (for example output_register in the URDF snippet above)

Using this information, the controller retrieves the GPIO state and, for binary interfaces, publishes Is high and Is low predicates. The raw state value is also exposed through a signal for direct consumption.

GPIO Output Controller​

The GPIO Output Controller is responsible for commanding GPIO values on the hardware.

It requires the following parameters:

  • GPIO group name (for example sample_gpio_group_2 in the URDF snippet above)
  • Command interface (for example input_register in the URDF snippet above)

The parameter Command specifies the value written to the interface and can be dynamically updated at runtime to trigger the desired hardware behavior.