A Device Group may own up to 16 input devices. An application program may request input from any combination of the available input devices belonging to the active Device Group.
DFSETUP restricts the values that input devices may take to powers of two. You are prompted for input device names and these names will be available to you in a QA Build application that has selected the owner Device Group. A DP4 C program must reference the numeric value (preferably using a #define name - however you will have to create these for yourself as DFSETUP does not have a facility to generate them for you).
Powers of two are offered as input device values to enable you to combine input sources in a bit mask. The advantage of using these values is that your application may combine them to specify multiple input sources. The combined value is passed from your application to your OSDI. For example, suppose the keyboard is defined as the value 1, and the scanner as value 2. You can select just an individual input device in your application - choose 2 to select the scanner only. If you want to select both the scanner and the keyboard you would pass the value 3 (2+1).
In the OSDI you can isolate which input devices have been selected using the bitwise AND (&) operator on the mask. Further detail on selecting input devices from applications is given in Enabling Input Devices (C Programs) and Enabling Input Devices (QAB).
The DP4 Till will use either a keyboard or a scanner, an application using the Till may request input from both simultaneously.