Simple Hard Disk Selector Circuit Diagram

 
In the last few years, the available range of operating systems for PCs  has increased dramatically. Various free (!) operating systems have been  added to the list, such as BeOS, OpenBSD and Linux. These systems are  also available in different colours and flavours (versions and  distributions). Windows is also no longer simply Windows, because there  are now several different versions (Windows 95, 98, ME, NT, XP, Vista  and 7). Computer users thus have a large variety of options with regard  to the operating system to be used. One problem is that not all hardware  works equally well under the various operating systems, and with regard  to software, compatibility is far from being universal. In other words,  it’s difficult to make a good choice.

Hard Disk Selector Circuit Diagram

Switching from one operating system to another - that’s a risky business, isn’t it? Although this may be a bit of an exaggeration, the safest approach is still to install two different operating systems on the same PC, so you can always easily use the ‘old’ operating system if the new one fails to meet your needs (or suit your taste). A software solution is often used for such a ‘dual system’. A program called a ‘boot manager’ can be used to allow the user to choose, during the start-up process, which hard disk will be used for starting up the computer. Unfortunately, this does not always work flawlessly, and in most cases this boot manager is replaced by the standard boot loader of the operating system when a new operating system is installed.

In many cases, the only remedy is to reinstall the software. The solution presented here does not suffer from this problem. It is a hardware solution that causes the primary and secondary hard disk drives to ‘swap places’ when the computer is started up, if so desired. From the perspective of the computer (and the software running on the computer), it appears as though these two hard disks have actually changed places. This trick is made possible by a feature of the IDE specification called ‘CableSelect’. Every IDE hard disk can be configured to use either Master/Slave or CableSelect. In the latter case, a signal on the IDE cable tells the hard disk whether it is to act as the master or slave device. For this reason, in every IDE cable one lead is interrupted between the connectors for the two disk drives, or the relevant pin is omitted from the connector.

This  causes a low level to be present on the CS pin of one of the drives and  a high level to be present on the CS pin of the other one (at the far  end of the cable). The circuit shown here is connected to the IDE bus of  the motherboard via connector K1. Most of the signals are fed directly  from K1 to the other connectors (K2 and K3). An IDE hard disk is  connected to K2, and a second one is connected to K3. When the computer  is switched on or reset, a pulse will appear on the RESET line of the  IDE interface. This pulse clocks flip-flop IC1a, and depending on the  state of switch S1, the Q output will go either high or low. The state  on the Q output is naturally always the opposite of that on the Q  output. If we assume that the switch is closed during start-up, a low  level will be present on D input of IC1a, so the Q output will be low  following the reset pulse.