Setting the VSCAN setting in AutoCAD due to snagging cross hairs.

A.   Why do the AutoCAD cross hairs seem to "snag"?

    1.   With the introduction of very fast PC's into the AutoCAD environment, a problem has surfaced when using
          digitizers in polling mode. This can be corrected by increasing the VSCAN setting in AutoCAD.

     2.   The CalComp digitizer drivers are capable of running in POLLING mode as an option.   
           Polling mode forces frequent scans of the memory page table and can cause the digitizer to display any
           number of different problems, from no tablet response to jittery movement.

B.   What are the symptoms?

     1.   The symptoms are rubber banding effects, snappy or tugging movement, intermittent digitizer communication
           errors and in some instances, a complete digitizer failure upon entering the drawing mode.

C.   What determines the VSCAN setting?

     1.   The performance of the -VSCAN switch is directly related to the amount of memory being managed by the
           VMM(Virtual Memory Manager). The performance is also linked to the computers speed.


D.   How to calculate the VSCAN setting.

     1.   A formula can be used to determine the correct VSCAN setting for a specific combination of processor speed
          and memory on each individual system.  The default setting for the -VSCAN switch in AutoCAD is 20000. For
          simplicity this number is retained in these equations.


     2.   The formula is:

               20000 x MEMORY x Correction Factor

          a.   The first variable in the equation is the correction factor. This correction factor allows for computer speed
                considerations. Refer to the following table to determine the correction factor for different machine speeds:

            Pentium 200:    20000   x Memory  x 11  
            Pentium 166:    20000   x Memory  x 10
            Pentium 150:   20000   x Memory  x 9
            Pentium 133:    20000   x Memory  x 8
            Pentium 120:    20000   x Memory  x 7
            Pentium 100:    20000   x Memory  x 6
            Pentium 90:     20000   x Memory  x 5
            Pentium 66:     20000   x Memory  x 4
            Pentium 60:     20000   x Memory  x 4
            486:                20000   x Memory x   3
            386:                20000   x Memory x   2

               If using a 80486 based processor running faster than 33Mhz, a correction factor of 3.0 is recommended.

          b.   The MEMORY variable refers to the amount of total memory AutoCAD has available to it for managing.
                Normally this is the total amount of memory on the computer. The memory variable is a count of total
               megabytes of memory.  Some typical memory configurations are 4, 8, 16, 20, 32, 64, (all in Megabytes).

               Note:  Use whole numbers.  If a computer has 8192K (8MB)of memory, the memory factor is 8 and not 8192.


     4.   Example of the VSCAN setting for a 486/33 Computer with 16MB of memory.

          20000 x 16 x 3.0 = 960000


E.   Viewing the current VSCAN settings.

     1.   The following command will display all of your VMM settings.  The VSCAN setting should already be defaulted
          to 20000.

          a.   Switch to the directory where the AutoCAD files reside.

               i.    For AutoCAD 12, type CD\ACAD12 [ENTER].

               ii.   For AutoCAD 13, type CD\ACAD13\DOS
                    [ENTER].

                    Note: The directory name may vary with different machines.

     2.   Type:

          cfig386 acad [ENTER]

     3.   The settings should appear on the screen and look like the following:

          -minswfsize 400000
          -swapdefdisk
          -swapchk off
          -demandload
          -vscan 20000

F.   How to enter the new VSCAN setting into AutoCAD.

     1.   Switch to the directory where the AutoCAD files reside.

          a.   For AutoCAD 12, type CD\ACAD12 [ENTER]

          b.   For AutoCAD 13, type CD\ACAD13\DOS [ENTER]

               Note:  The directory name may vary with different machines.

     2.   Type:

          cfig386 acad -VSCAN VALUE

          where VALUE= the VSCAN setting determined in Step D.

          Example:  cfig386 acad -VSCAN 960000

     3.   DOUBLE CHECK THE TYPING FOR ANY MISTAKES!

     4.   Press the [ENTER] key.

     5.   The CFIG settings will be redisplayed.  The list will now show the old VSCAN value followed by the new VSCAN
           value.  Only the new value will be utilized.

G.   In Case of a Mistake

     1.   From time to time, a typo will occur when entering in a VMM switch setting using the CFIG command. Once
          this happens, the old switch settings must be cleared and reentered. The -clear switch will eliminate all previous
          switch settings leaving the VMM at it's default settings.

     2.   To clear the CFIG settings, do the following.

          a.   In the appropriate AutoCAD directory, type:

               cfig386 acad -clear [ENTER]

          b.   cfigphar [ENTER]

          c.   Select 'C' to Begin.

          d.   Right Arrow to 'EXIT' and press [ENTER].

          e.   Select 'END' and save the changes.

          f.   In the AutoCAD directory, type:

               conf386

               This will reset the default values.
    
     3.   Go back to Step E to review the settings then proceed to change the VSCAN parameter to its correct value.