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Hot-swap cheap HDD trays with USB2 interface

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May 17, 2001
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how to swap a hdd diode over

One of the problems of the external hard disks is their price
(you buy one interface per hard disk) or performance (external disks
are often 2.5" disks). The removable trays adapters for internal 3.5"
hard disks offer cheapness and performance but you have to
switch the computer off to swap disks which can be very annoying
if you have other work going on in the background.

One simple solution that solves all problems is to interface one of the
cheap HDD tray adapters to a USB2 interface. The project described here
does this. It uses a standard HDD tray adapter slightly modified and
a commercial USB/IDE adapter from a miniature 2.5" removable
disk enclosure (cutie type, very common).

Mods : The pin 20 from the IDE cable on the rack adapter is removed but
its padstack is available and even drilled. Put back one pin in the pin20
hole and connect this pin to the switched +5V in the rack adapter.
This connection appears as a pink wire on the given pictures.
The on/off switch in the rack adapter switches the +12 volt and a
transistor with its collector connected to +5V supply and the base driven
by the swithched +12V gives the +5V switched to the disk drive bay.
We use this switched +5V to supply the USB2 interface and a relay
that connects the USB lines to the USB interface circuit. This relay
may be unnecessary but it depends on the chip used in the
interface. A schottky diode on the relay coil makes sure the coil
does not drive the +5V switched under -0.7V when the drive is
switched off. The diode used is overkill but was available.

Three small wires as short as possible (USB2 rates and impedance
are no joke) connect the USB2 data and USB2GND lines on the adapter
to the USB2 commercial interface. Keep the wires close to each
other and don't separate the USB data lines on the PCB. I know,
the traces are thin and close but it is on purpose and for your
own good. :wink:

On the cutie interface, there is a landing pattern for a SOT23
double diode which is not installed. The common anode goes to +3.3V
and not used, the other two pins are the USB data pins which go
to the described adapter. Find a ground connection close to the diode
to connect the USB ground from the adapter.

A small 40-pin to 40-pin flat cable is made (with not pin 20 plugs!)
and connects the adapter to the tray adapter. Normally 80-wire
would be required but the cable is so short that it is not mandatory.

Find attached the schematic in pdf, the gerber and excellon files
and the self-explaining pictures.

Have fun with hot-swap cheap HDD trays. 8)

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