Screen display and timing synchronization on the Apple IIe and Apple Ilgs

The logic of the vertical blanking interval (VBL) Softswitch in the Apple Ilgs is the reverse of the logic of the VBL in the Apple IIe. Programs written for the Apple IIe that synchronize the videoscan with the experiment control program timing by sensing the VBL pulse will therefore function incorrectly when run on an Apple Ilgs. Procedures for modifying existing synchronization routines are discussed. A flexible alternative program that senses the type of computer on which it is running and accordingly adjusts the logic of the synchronization routine is described.