HID Usage Tables Document 1.1 - C. Scott Ananian

Universal Serial Bus HID Usage Tables 109Appendix C: Physical Descriptor ExamplePhysical descriptors allow a device to identify how the user physically interacts with the device. These areparticularly useful for devices such as ergonomically designed flight simulator throttle controls.Attaching a designator to a control is as simple as adding a usage. The Designator Index is used to access aPhysical descriptor in a physical descriptor set. In this example, the stick is designed to be held in eitherhand. However, the way that the user accesses the buttons will change depending on which hand is used.Consider the joystick below. When the joystick is held by a right-handed person, the thumb falls on the leftbutton (2). It would make sense to assign this button to a function that requires quick access or a fast repeatrate, while the button on the right (4) would be assigned a function that does not. This is because a righthandeduser must stretch the thumb from the resting position to touch button 4. If a left-handed person heldthe device, the reverse would be true because the thumb would naturally rest on the button on the right (4).These considerations result in the Effort values that are declared for the right-hand bias physical descriptorset (1) below. Buttons 2, 3, 4, and the hat switch are accessed by the user’s thumb. The Effort assignmentsare Button 2 = 0, Hat switch = 1, Button 3 = 2, and Button 4 = 3. In the case of the Hat switch and Button 3,the thumb has to stretch the same amount. The user must, in essence, “heel and toe” the two controls withthe thumb. The Hat switch receives the lower Effort value because the tip of the thumb (toe) is considered amore effective manipulator than the first joint of the thumb (heel).The left hand of a right-handed user normally manipulates the throttle, while a left-handed user must let goof the stick and use the index finger to manipulate it. This is why the Physical descriptor for both righthandedand left-handed users indicates the left index finger. However, for the left-handed user, the Effort ishigher.Figure 17: Joystick Button LayoutButton 2HatswitchButton 4Button 3Button 1 (Trigger,behind stick)ThrottleIn the following Report descriptor example, Physical descriptor 1 is attached to the throttle, Physicaldescriptor 2 to the stick, and so on. Two physical descriptor sets are provided: right and left hand. Thephysical descriptor set that is actually referenced depends on whether the user is right- or left-handed. It isassumed that the orientation of the user is stored in the user’s profile on the system.Version 1.1 April 8, 1999