

                    SAVING DF0: - BOOT UP FROM DF1: 

                                                  by Les Ayling


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    Several of my friends who are Amiga owners have had problems with the
   internal drive. This modification will overcome that problem by
   allowing Amiga 500 owners to boot from df1:.

    This mod is also useful for people who own the Commodore 5 1/4 inch
   drives and wish to boot from them. More importantly, people who have
   added an 80trk 5 1/4 inch drive (such as myself) as described in the
   file ADD_5.25_DRIVE can boot from it. As 5 1/4 inch disks can be
   bought for as low as $5.95 for 10 in some places, it is well worth
   considering as a cheap backup library option OR indeed as an
   alternative primary storage medium.

      You will need the following tools and parts:

      * Low wattage soldering iron, suitable for PCB's
      * DPDT switch mini toggle variety is fine
      * 1 foot of ribbon cable
      * solder
      * small snippers
      * hand drill
      * tool to unscrew the @#$%^& non-standard case screws
      * Exacto knife or similar

    Start by unscrewing the 6 hex style screws around the edge of the
   case (Goodbye warranty!).  Do not remove the three phillips screws as
   they are holding the drive in place.  Turn the machine back the right
   way up and remove the lid by pressing around the edge to ease off the
   catches.  Remove the keyboard making careful note of the orientation of
   the connector on the main board!!!!!  Now remove the further 4 hex
   screws which are holding the top rf shield in place and remove the
   shield.  Unplug the disk drive POWER cable from the drive end only.

    Remove the 34-way disk drive patch lead taking note of its orientation
   (This saves so much time when you have forgotten later on!)  Unscrew
   the Disk drive unit and remove it.

    The main PC board should now come free with the lower RF shield.
   Unscrew the hexagonal socket retainers on the I/O connectors at the
   rear of the board and this shield should come free.

    You are now ready to start!

    Locate connector 11 (CN11) on the component side, this is the socket
   that you just unplugged the disk drive patch lead from.  In front of it
   should be U8 which is an 8520 chip.

    CN11 is 2 rows of 34 pins.Count along the front row from the right
   hand side until you see the 5th and 6th pins (pins 9 and 11 of the
   connector respectively).  There should be two circuit traces that lead
   from these pins and disappear under U8.  Using the exacto knife cut
   both of these traces leaving no stray copper. A 1mm cut should be
   sufficient.

    Turn the board over to the solder side.
    Leave the board for a moment and get the DPDT switch.
    For ease I will label its pins as such:

        A   B

        C   D

        E   F

    Cut a piece of ribbon cable about 1' long with 4 conductors.
    Strip and tin the wires then connect them to A, B, C and D.
    Cut two small pieces of wire and connect A-F and E-B.

    Tin and strip the other ends of the ribbon cable.Using the snippers
   trim these tinned ends until there is about 1 1/2 to 2mm showing. Any
   longer is unnecessary and can lead to solder blobs etc.

    Grab the Amiga pc board again. On the solder side locate pins 9 and
   11 of CN11 as described earlier. Solder the lead from A to pin 9 and
   the lead from B to pin 11.

    Locate pins 13 and 14 of U8.

    If you are unsure of pin numbering on IC's refer to one of the data
   sections of DSE's or ROD Irving catalogues.  Otherwise looking at the
   chip there is a notch at the right hand end of the board. Pin 1 is
   behind this notch (relative to the pc board) and the numbers increase
   in an anti-clockwise fashion around the chip.  Solder the lead from C
   to pin 13 of U8 and the lead from D to pin 14 of U8.  That completes
   the soldering side of things.

    All that remains is to drill a hole for the switch and put the rest
   back together. There is a convenient spot to the right of the joystick
   socket that will house 3 or 4 switches comfortably.

    PLEASE PLEASE PLEASE be careful of the orientation of the connectors
   when assembling your amiga.

    To use the mod:
             1. Remove ALL disks from ALL drives.
             2. Flick the switch to the NORMAL position.
             3. Turn the machine ON or reset if already ON.
             4. If you wish to boot from drive DF1: now is the time to
                flick the switch to the NON-standard position.
             5. Insert the appropriate disks in the appropriate drives.

    HOW it works.
   The mod works by switching the drive select lines SEL_0 and SEL_1.
   SIMPLE!

    IMPORTANT:!!
   The switch must be in the NORMAL position when you either turn the
   machine on or reset.The reason for this is that after a cold or warm
   boot the KERNEL roms go looking for what equipment is connected to the
   system. A part of the GARY chip which is triggered by the SEL_0 signal
   provides seperate MOTOR_ON signals for the internal and external
   drives. When the switch is in the ALTERED position the GARY chip is
   fed by the SEL_1 signal and surprise surprise! the KERNEL roms won't
   find the internal drive!  However is is PERFECTLY safe to change the
   switch at ANY time the power is on (without disks in the drives!). So
   if the switch is changed after the roms have found what drives are
   online, the drives 0 and 1 will operate fine only reversed.  More
   circuitry could have been added to swap the motor_on signals so that
   this extra switching wasn't required, however most people would
   probably like to keep their mods to the main pcb to a MINIMUM!

    *NOTE*
   If the switch is changed while disks are inserted in the drives,
   AmigaDOS will become confused as to what volumes are on line and if
   your write-protect tags are OFF, chances are your directories will be
   CLOBBERED!!

    The mod is relatively simple, however if you are unsure of your
   ability to perform it.. DON'T. get some hardware oriented person to
   help or drop me a line at PARAGON bbs if in Sydney, or via Megadisc and
   I may be able to help or do it for you at a small nominal cost.
   Naturally I cannot guarantee your labour content so therefore I cannot
   make any warranties, implied or otherwise, as to the suitability or
   performance of this modification.  However I can say that I am not too
   worried about my software collection as I have a set of bootable 5 1/4"
   backups!!

    Good luck and I hope that this is of some help.


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