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Designing a sata drive selector

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SecularSam

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The schematic was done in Visio drawing.

SATA HD SWITCH Specification

A device that switches SATA hard drives at the push of a button.

It will allow the user to select one of three SATA hard drives. The front panel will have only three illuminated buttons. One button will always be illuminated indicating the hard drive in use.

To select a hard drive the user must push a button that will:

A) Blink. The blink acknowledges the selection.
B) Wait five seconds before initiating the computer to shut down. The five-second delay is provided to give the user a chance to cancel the operation.
C) If the operation is to be canceled, the user must push the same button again. Once this is done, the button will no longer blink. The two remaining buttons are disabled while the selected button is blinking.
D) If the operation is to proceed, the user will allow the five-second delay to expire and the computer will commence a shutdown. During this time, the button will continue to blink.
E) Once the computer powers down the blinking will stop and the button will stay illuminated while the previous selected hard drive button will no longer illuminate.
F) Approximately five seconds after shut down, the computer will power back up.

The user will not be able to turn off a hard drive in use. The only way to turn it off is to select another hard drive and allow the sequence described above to be completed.

When the computer is powered down normally the user has the option to select the desired hard drive. Remembering that one button will always be illuminated, whichever of the two hard drives are selected, it will deselect the one previously chosen.

Design Rationale
Motivation:
The design and development of this device is motivated from the desire to:
  1. Run different operating systems on a single computer.
  2. Ability to select the operating systems with ease.
  3. Eliminating multiple steps to accomplish the task.
  4. Eliminate the chance to accidentally switch to another SATA drive during the use of another.
  5. Have a minimal learning curve in order to operate the device.
Hardware Concept:
The device should be modular, self contained with minimal electrical interfacing. Connectors and ports must be easily accessible. The user interface must be easily viewable with controls situated in an unconfused and intuitive manner.

Electrical considerations:
Contemporary computer power supplies provide a +5 VDC when powered down. This voltage is referred to as Stand-by supply. It is present as long as the power supply is connected to the 120 VAC and is not switched off. This feature will be utilized as it will be necessary to maintain control while the computer is powering down during a controlled reset sequence.

TBD
Complete Schematic:

Add power interface.

Component selection:
Locating a:
  1. Project box made to fit into a 5.25 inch bay.
  2. 15 pin SATA power connector mounted either as a right angle board or bulkhead configuration.
  3. I am determined to use illuminated push buttons which are expensive. What's listed in the BOM may change.
 
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Theory of Operation (Work in Progress)

This circuit will control three separate and individual SATA drives containing three different operating systems. In order for any one of these operating systems to function, its assigned SATA hard drive must be enabled (power applied) when the computer is powered up. Each circuit that controls the power to an individual SATA drive is referred to as a channel. Thus there are three channels designated as channel one, two and three.

There are two power up conditions a computer will go through. The first being a soft power up where the computer is attached to the main facility supply referred to the mains. The mains is defined as the power provided a home, office or industrial environment with a voltage that typically ranges anywhere from 110 to 220 AC volts. Once the mains is attached to the computer’s power supply, (and if that power supply has its own power switch which is turned on) that supply becomes operational even though the computer itself is off. The supply generates a +5 volts. This voltage is used to keep an internal clock operational; maintain a charge to it's backup battery and powers the circuitry that senses the push of the computers power button. In contemporary computers, this voltage is usually available through USB ports resident on the CPU board and is known as the standby voltage. It is used as the main voltage source for our circuit.

Voltage Reference
Referring to R28 thru R33 and capacitors C20 thru C22 makes up the voltage reference circuitry. It’s a resistor voltage divider tied between the +5 VDC standby voltage and ground and provides three stable voltages: +2.75, +2.5 and +2.25. C22 and 23 are there to provide stability by nullifying the active effects of the Computer Reset Circuitry. They’re also configured to maintain a +2.5 volts during their initial power up charging times.


Power Up Configuration
The purpose of this circuit is to guarantee that the channel one SATA drive is selected upon the initial power up of the computer. This is done by the use of U23, Q13, Q14 and associated components.
U23 is an analog voltage comparator with an open drain output. When power is applied, a reference +2.5 volts is present at its positive input. C17 is fully discharged resulting with an initial zero volts being applied to the negative input. Thus the comparators output is driven high (+5 volts). This high signal turns on both Q13 and Q14 which brings their respective drains to a near zero volts. These outputs are low for approximately 170ms. When the rest of the circuit is fully powered up, the "Power Config A" signal forces the channel A drive to be enabled while the "Channel Config B" forces the other two channel drives to be disabled when the computer is turned on.

Power Down
The purpose of this circuit is to sense when the computer is powered down. When this happens it initiates the change to another drive and provides a trigger for the computer to power back up.
It consists of just two components, R22 and U24. The open drain comparator U24 compares the supply’s +5 volts to the reference +2.5 volts derived from the standby voltage. When the computer is powered down U24s open drain output (designated as “Power Down”) is driven low. This signal interfaces with each channel and is used to initiate the enabling of the newly selected channel. It also interfaces with the Computer Reset circuitry and initiates the five second delay that will turn the computer back on. When the computer is powered up again and the +5 volts is present, U24's output is pulled high via R22.

Channel Selection
This description will begin with the assumption that the computer just finished an initial power up condition, meaning that only the +5 volt standby is being applied to the device. After 170 milliseconds the Power Up Config circuit has enabled only the channel one drive power to be enabled when the computer is fully powered on.
 
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Design Rationale
Motivation:
The design and development of this device is motivated from the desire to:
  1. Run different operating systems on a single computer.
  2. Be able to select the operating systems with ease.
  3. Use a device that is simple to operate.
  4. Have a minimal learning curve in order to operate the device.
I do this on my office machine by using Linux (Ubuntu), Windows XP and Windows 7 all on the same drive. Installing XP first, Win 7 second and Linux last results in Linux installing a bootloader which allows you to select the operating system on bootup. It also has the advantage that the XP and Win 7 can see each others drives and Linux can see everything.

My other machine (a quad core server) uses Virtualbox running on Linux which will simultaneously run Win XP, Vista, Win7 and Linux on the same machine for testing purposes. I can also remotely access them - it makes life so much easier.
 
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