Is ROS compatible with all robots?

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In the abstract, ROS could be compatible with any robot which has a computer interface. However, to be compatible, there needs to be a program (or piece of hardware) which sits between ROS and the robot hardware to translate ROS messages/services, etc into commands that the robot hardware can understand, and vice versa. This is generally called a driver - it subscribes to and advertises ROS topics, and communicates with the robot over USB, serial, EtherCAT, etc.

Many robots already have ROS support. An extensive, though incomplete, list can be found on the ROS Wiki.