i want to know how i will interface it in ros.
One of the factors that will influence this aspect will be what you are currently using to interface with the scara, and whether the controller is programmable (enough). If it has any serial or ethernet ports, you might be able to write a ROS node that can function as a bridge, relaying information between the scara controller and the ROS system. Packages like rosserial might help.
If the controller is already PC(-like), you might be able to run ROS on it directly. You could then use packages like ros_control to directly control the joints of the manipulator.
You might also be interested in ROS-Industrial, which has packages that are geared towards usage with industrial manipulators (industrial_robot_client fi).
Edit:
I want to ask you the best microcontroller that i can use to support 5 motors and decoders. I' m looking at arduino, rasperry pi and single PC. Please can you advise me on the best one to go with in designing the controller for the robot before integrating it to ROS. I appreciate you input.
As to what the 'best one' would be I cannot comment: I'd say this depends on your personal experience with those platforms, and the interfaces supported by your scara. The PC will probably give you the most flexibility, and development will probably be easiest. Any of the embedded options will likely involve cross-compilation and building / setting up build infrastructure. This can become involved, especially if you're just starting out with ROS.
