A callback function is a developer-supplied function that executes in addition to or in place of one of the Microsoft-supplied Unidriver functions. A developer must provide a callback function whenever the Unidriver does not provide functionality that a printer requires.
For example, the Unidriver expects the topmost addressable pin on a 9-pin dot-matrix printer to correspond to bit-seven in a byte of graphics data, and the bottommost addressable pin to correspond to bit-zero. However, some printers (such as the older Okidata dot-matrix printers) reverse this ordering. To solve this problem, a developer must provide a callback function.
For a description of the callback functions, see the UNITOOL.HLP help file.