SCSI Implementations

So exactly how do you put together a SCSI system? Well, the essential things you need are a host adapter, some SCSI devices (hard drives, CD-ROMs, etc.), and of course cables. Setting up the SCSI bus is fairly straight forward when it comes to small systems. The bus is basically arranged in a straight line. All the devices are connected to at most 2 devices. The only real concern you need to address is having proper termination. Like I said earlier, we don't want the signals to be reflected back and cause interference. Each end of the bus must have a terminator, which can be either SCSI devices that are part of the chain or actual terminators. So the order of the devices in the chain doesn't really matter. However, depending on the physical arrangement of the system and case, the order should be apparent. Although you can have up to 16 devices on a single chain, practically speaking most people can't fit that many devices in their cases. The complexity level increases if you add more devices.

Another thing you have to worry about when putting together a SCSI system is giving each device their own address. Depending on the bus width, you may be limited to only 8 devices on a given SCSI chain. Each device is usually assigned a number from 0 to 7 for narrow devices (8-bit), or 0 to 16 for wide devices (16-bit). Narrow devices won't be able to access any devices with IDs over 7, so you should keep that in mind as well. Care should be taken when mixing different bus widths on the same SCSI chain. You need to make sure that your host adapter hardware properly supports this and that you have the proper cables for everything. For the average individual, you probably shouldn't have that much trouble setting up a SCSI system, provided you have the right hardware. Larger systems will be more complex, and probably have more issues. In high end servers, RAID SCSI implementations are usually used. This combines the performance and data reliability features of RAID with the performance and features of SCSI, which is usually desired with high end machines.

Conclusion

As you can see, SCSI is not just your average interface. It's actually a very complex and advanced system level bus. Although this isn't by far the most intensive guide to SCSI, I tried to do as much justice as I could to the subject. The future still holds many possibilities for the SCSI-3 standard since it's not finished yet. I mean it even has Fibre Channel and IEEE-1394 (FireWire) in the spec, so who knows what else might pop up. If you're thinking of switching to SCSI, it really depends on your needs and type of system you want. SCSI implementations can get very expensive, so you have to decide if the performance is worth it. If the work you're doing isn't very complicated or doesn't involve large amounts of I/O activity, you probably don't need to waste money on a SCSI system. Setting it all up can also be a pain if you don't know what you're doing. Everything should be straight forward as long as you have the proper hardware. I hope you've enjoyed reading this article and maybe learned a thing or two about SCSI.