A vision testing device for measuring single or multiple visual functions consisting of an internally illuminated hand-held instrument, vision charts, vision correcting and occluding elements, a testing frame for holding combined vision correcting and blocking elements, and modular accessories. The input is supplied to each sun gear in equal magnitude but opposite direction to account for driving the left or right wheels. When used alone or combined together, these DFD commands will detect or locate physical failures in the DFT modules in the integrated circuit on an evaluation board or system using a low-cost DFT debugger.