Chapter 13: Human Interface Devices

Chapter 13

Human Interface Devices

A Human Interface Device (HID) is one that communicates with the host computer using structured reports. The HID class of devices consists primarily of devices that end users use to interact with computers. The class includes keyboards, mice, and game controllers of all kinds, but it can include any imaginable knob, switch, button, slider, exoskeletal device, or other type of control that a user might use to control a computer. Noninteractive devices such as bar code readers and measuring instruments might also be designed to fit into the HID class. In addition, HID devices can incorporate user-information components such as lights, displays, force feedback, and other indicators.

HID devices built for the Universal Serial Bus conform to the Device Class Definition for Human Input Devices. Additional specifications relevant to force feedback are in the USB Device Class Definition for Physical Interface Devices. HID relies on extensive sets of numeric constants, whose definition can be found in the HID Usage Tables specification. All of these specifications are available for free download from www.usb.org.

Although the HID specifications are oriented toward USB implementations, any sort of device can function, in whole or in part, as a HID device. The important characteristic of a HID device, once again, is that the host is able to perform input and output operations using report packets that conform to an extremely flexible structure definition, called the report descriptor.

Applications access HID-compliant keyboards and mice only indirectly, by handling window messages whose character and content haven t fundamentally changed in over twenty years. Applications access other sorts of HID device through COM interfaces that are part of the DirectX component of Windows and through Win32 API calls.