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You have chosen to modify manually the batch file used to start AutoCAD.

You need to modify two enviroment variables in that file (default name
ACADR12.BAT in the root directory) to tell AutoCAD where to find the driver.

Example: (sample additions in lower case)

     SET ACADDRV=C:\ACAD\DRV;c:\acad\drv\hpgl2
     set rhpadi=c:\acad\drv\hpgl2\rhprtl.exp