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Easy to Build Personal Computer PC Multiple Hard Drive Selector

OddMix.com - PC Technical Note - PCN0702 - Karl Nagy

All Personal Computers [PC] have at least one hard drive. Most standard store bought, factory built desktop computers have only one hard drive. Often that drive is internal and not easily removable. The hard drive is the computer's largest storage device. That is a relatively recent development, because in the past, the CD [Compact Disc] - and more recently - DVD drives were larger in capacity than the available hard drives. The purpose of the PC's hard drive is to store all operating and application software, along with much different kind of data. It is highly beneficial to have more then one hard drive. It is even more useful to be able to select the operating hard drive(s) quickly and easily from the front panel. This project describes a simple arrangement, which does all of that.

Picture 1. PC Hard drive selector front view [5 KB]
Picture 1. PC Hard drive selector front view
As some personal computer users had discovered already, PC BIOS allow selection of different drives, and drive parameters. However, switching via the BIOS is somewhat difficult. The BIOS information is usually cryptic, requires repeated rebooting, entering or selecting data, or other inconveniences just to include or exclude a certain drive. Additionally there is only so much selection is permitted or possible. This switch is more versatile than most BIOS selection arrangements, yet it is very simple to make. Only an empty drive bay or a drive bay-cover (or a suitable place for the switches), a few switches, jumpers and some connection wire are necessary to build this project.

The project takes advantage of the fact that two different protocols are available for jumpering EIDE devices. The older protocol is a master-slave and the newer one is a cable select. Beware that protocols cannot be mixed on the same data (ribbon) cable, but they are mixable using different data cables.
Figure 1. PC Hard Drive Switch Wiring Diagram [2 KB]
Figure 1. PC Hard Drive Switch Wiring Diagram

On Picture 1 is the image of a 5.25 inch drive frame installed into one vacant large drive bay. The black front panel has the two switches and serves as the control panel. On the left side, next to the original red LED is the hard drive's power switch, labeled SW1 on the schematic.

Picture 2. Hard drive jumper selector block connections [6 KB]
Picture 2. Hard drive jumper selector block connections
On the right side of the control panel is a three-position drive selector switch - SW2 on the schematic. Top position marked with a "C" is "Cable Select", when it is selected the drive is mapped according to the location it occupies on the ribbon cable - either master or slave. The drive plugged into the free end connector of the ribbon data cable would be "Master". If it plugs into the middle connector it will then becomes a "Slave". The center position on the switch is marked "1" on both the control panel and on the schematic. That position is used when only a single active drive is available. That configuration is the default single drive (no jumper) position on many hard drives. No jumpering takes place when "1" is selected. The bottom position "M" of the switch is a forced "Master" for the drive, regardless of what is the position the drive occupies on the ribbon cable. This switch position overrides all other protocols and settings.

Figure 1 is the schematic for the project. Although a large drive bay was used, an unused small drive bay cover is just as good, or even better, to hold the drive control switches and optional lights. In many desktop computers there is more than one small (3 1/2 in.) floppy drive bay, but usually only one floppy drive. Similarly, it is also possible to drill some holes onto the computer's front panel, or any other desired place on the computer. Or if none of these locations are available or desirable, then the control panel can be built into a suitable small external box. It may even be infrared, or RF coupled, to make the control totally wireless. Commercially made remote drive control boxes are also available.

The jumper block is at the backside of the hard drive. The arrow in the picture - Picture 2 - points to this jumper selector block. The two pairs of wires connect to the CS "Cable Select" and to the MS "Master" position. The possible selections are often posted on the drive's top. Some drives have markings at the jumper selector block as well. Inspect the drive for any markings in all places than decide. Picture 3 depicts a current 80 GB Western Digital hard drive jumper block. The drive has a white jumper, positioned at the CS position.

Modern and preferred selection is done by a method called "Cable Select" [CS]. Prior protocol - hard coded - drive selection is still honored by many manufacturers. This versatility made it possible to construct this switcher. A hard drive can be made a "Master" [MS] or a "Slave" [SL] by placing a jumper to a certain position. If a drive is hard jumpered, the jumper selection takes precedence over the CS protocol.

Picture 3. Hard drive jumper selector block closeup [3 KB]
Picture 3. Hard drive jumper selector block closeup
Six or Ten Pin Jumper Block 
SelectionConnect pins
"1" Single StdNone
"M" Master5-6
"S" Slave3-4
"C" Cable Select1-2
All is required for our project is to connect two pair of wires to the MS and CS connections at the back of the hard drive. If the two wires of the MS jumper connections are connected together through the front panel switch, then the drive will be a Master. Likewise, if the other CS pair is shorted via the same switch, then the drive will obey the standard CS protocol, and whatever position it occupies on the ribbon cable will determine its operational position.

Picture 4. SW1 and LED [3 KB]
Picture 4. SW1 and LED
In Picture 4 the power switch is in the left front, its wiring and the LED connection is visible. The LED is strictly optional. Currently it is used to show when the drive is powered up. It can also be used to show when the drive is selected and active. Both of this information is somewhat redundant as it is available already on the computer's front panel, so there is no real need for this LED.

Hard drive requires two different power sources, a +5V and a +12V. Both of this power lines can be switched "ON" or "OFF" with a single mechanically connected switch. This switch is a DPDT type. It is possible to interrupt only the +5V line and to use a simple SPST switch, but it is safer to switch all power sources together at the same time. In our project SW1 does just that.

Picture 5. Hard drive selector assembly top view [7 KB]
Picture 5. Hard drive selector assembly top view
Picture 5 shows the completed project from the top, before it was installed into the computer. A special slim hooded fan (invisible on the picture) is attached to the bottom side of the hard drive. Since hard drives are the second largest power guzzler in a typical PC (the first is the CPU), the small fan is a good investment. It provides much needed airflow to remove internally generated heat, to extend the drive's life and improve its reliability.

Since current PC use pollute as much as 3,5 Million cars yearly (and that is a somewhat dated statistics), computer makers were directed to cut power when the machines are idle. All current operating systems provide some sort of computer power management. It is a good idea to take advantage of this power management. Not only power management saves wasted power, but also it allows time periods for the CPU and hard drives to cool off.


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