Using Panasonic JU257 Disk Drives on the Amiga


   The Panasonic JU257A 3.5" disk drive is designed for IBM compatible
 computers. With a simple modification, it can also be used on the Amiga.
 To perform the mod, follow these steps:-

1- Disassembly:
    Remove two screws holding the top shield, and remove the shield. Remove
   the three screws holding the circuit board to the chassis. Carefully
   unplug the head and stepper motor cables from the circuit board. The
   board is stuck to the chassis with some double-sided tape, so use a small
   screwdriver to lever the the board away from the chassis. Note that there
   is a cable going to the drive motor board, which must not be broken!

2- Configuration:
    Unsolder the 'resistor' (link) at position R27, and solder it back at R26.
   Similary, move the 'resistor' at R30 to R31. Solder a wire from R30 to
   pin 2 of the 34 pin connector.

    Some older versions of this drive had a different board. In this case
   the principle is the same, but you will have to identify the equivalent
   connection points.

3- Reassembly:
   Reverse step 1 :^)



                          Technical Explanation

 The JU257 was originally designed to be used in a variety of configurations,
selectable with switches on the circuit board. However, since almost all
drives are used in IBM compatible computers, the manufacturer decided to
put links in place of the switches. Thus the drive is set to provide a
DISKCHANGE signal on pin 34, and respond to DRIVE SELECT 1.

 For correct operation on the Amiga, the drive must put DISKCHANGE on pin 2,
READY on pin 34, and be selected by DRIVE SELECT 0. Resistors R26 / R27
select the DRIVE SELECT, and R30 / R31 select the function of pin 34.



                  Why is Amiga Different From IBM ?

 The IBM PC used 5.25" disk drives designed by Shugart. The Shugart disk
drive interface specified a READY signal on pin 34, but IBM decided not
to use it. There were 3 DRIVE SELECT inputs, which logically should be
connected to drives 0,1, and 2. However, IBM decided that it was easier
to make all drives use DRIVE SELECT 1, and twist the cable instead!
When IBM designed the PC-AT, they used a high-density 5.25" drive, and
put a DISKCHANGE signal on the (unused) pin 34. As you can see, IBM was
not interested in following the industry standard. The designers of
the Amiga were not so arrogant.



                                Notes

 You can use this drive to replace a broken JU253, which were often used
in the A500. You should remove the eject knob and mechanism from the old
drive, and install it in the new one. Also, use the mounting rails from
the old drive.

 Somebody recently suggested that Escom modified the WB3.1 ROM so that
they could use a drive without READY in their new A1200. However, I have
discovered that even a WB1.3 A500 works with the READY signal disconnected!
Thus, you could get away with simply swapping pins 34 and 2 in the cable.
Of course, this would not be good enough for some games that access the disk
drive port directly :-(.







                    Bruce Abbott
                    c/o Hastings ComputerWorld
                    Westpoint Plaza
                    1 King Street South
                    Hastings
                    New Zealand

                    ph/fax (646)8786100
                       bbs (646)8786110, 6.00pm to 9.00am

                    email  bhabbott@inhb.co.nz