SCSI has been an industry standard for decades. In this video, you’ll learn to recognize SCSI cables and the difference between SCSI connections over parallel and serial links.
SCSI or what we refer to in the industry as SCSI, is the Small Computer Systems Interface. This is a standard interface that can be used to connect to storage devices, scanners, printers, and other devices on your network. This was originally designed so that you can have many devices connected to one single interface on your computer by daisy chaining multiple devices together. And many of the SCSI standards support up to 16 devices in one of those single SCSI chains.
This is a standard that’s been around for quite some time. So you’ll find a number of different SCSI standards with different types of interfaces and different types of cables for each one of these standards. One of the newest standards is iSCSI or SCSI over IP. And certainly, you’ll find a lot of SCSI implementation in virtual networks. And if you look at some of the other connectivity that we use for storage devices, such as PATA, which is a parallel connectivity, and SATA, which is serial connectivity, SCSI can support both parallel and serial connectivity, depending on which type of format you’re using for that SCSI connection.
Before we had the SATA standard or was able to use USB to be able to connect our peripherals, we used SCSI. SCSI was used to connect our scanners and our CD-ROM drives and our tape backup systems. And we had SCSI connectivity inside of our computers and external interfaces that you could also use. And as I mentioned earlier, you can daisy chain these devices. So a single interface on your computer might support eight different SCSI devices on a narrow bus, or up to 16 if you’re using a wide bus version of SCSI.
One of the benefits of SCSI and the protocols that are used to communicate over this SCSI connection is that it is a very intelligent form of connectivity. You simply provide the basic connectivity, and SCSI takes care of identifying what devices are in that chain and how to access those devices over the SCSI connection. And although SCSI doesn’t have the popularity that it had in the past, you mat still find systems that have SCSI drives or other types of SCSI connectivity. And SCSI as a standard is one that’s used extensively on our virtual systems.
Here’s a somewhat dated motherboard that does have a SCSI connector on it. This motherboard also has a floppy drive connector and an integrated drive electronics, or IED interface. Today we refer to these interfaces as PATA interfaces. On the left side is the SCSI interface. This is an Ultra 3 interface. It has 68 pins on this connector. If we turn the motherboard on its side, we can see exactly the pins that are used for this particular cable. The cable itself is a 68 pin cable. You can see where it plugs into the motherboard. And you can see along the length of this ribbon cable are connections that you can use to attach other SCSI devices.
That’s 68 pin connector that we see on the motherboard is just one of many types of SCSI connections. You can see that they have changed quite a bit through the years. And you may find that one or more of these connectors is being used by SCSI devices that might be in your data center. When you have a single interface on your computer that you’re connecting multiple devices to, you need some way to be able to differentiate between all of these separate devices you’re connecting to the same cable.
In SCSI we’re able to accomplish this by using a SCSI ID number. So every device you would connect to that particular SCSI cable would all have a separate ID number associated with it. So your computer might be a SCSI ID 0. You might be connecting to your computer a hard drive on that cable. That can be assigned an ID number of 2. And you might have another device on that cable, such as CD-ROM, and you would assign a separate SCSI ID to that device, for example SCSI ID 3.
If you think about connectivity to a drive array, it’s a single chassis with many different drives inside of it. Because of this, SCSI has a method, not only to identify the chassis of that drive array through a SCSI ID, but we can identify individual drives by a logical unit or LUN. That logical unit allows you to reference a single drive within a much larger SCSI ID device. It’s also very common on this cable that at the end of the cable you place a SCSI terminator. The SCSI terminator allows you to have multiple devices on this wire and still be able to communicate across all of those devices simultaneously.
One of the more recent standards for SCSI is a serial attached SCSI or SAS. These devices are much simplified over the older SCSI configurations that require you to manually set a SCSI ID and to always have a terminator on the connection. With these newer SAS devices, you don’t have to worry about setting jumpers for SCSI IDs. You don’t have to install any terminators or make any additional settings to be able to use those serial attached SCSI devices.
Here’s the back of an external SCSI storage device. You see we have a spot for power. Looks like there’s a fan inside of this device or some type of cooling. And here are the SCSI interfaces, one that is a SCSI out and one that is a SCSI in. This is how you would be able to daisy chain this particular device along with other SCSI devices on the same cable. This SCSI device has been assigned a SCSI ID of 6. And you can see there are buttons on this device that allow you to modify the SCSI ID to prevent any conflicts on that same cable.
So if you look inside of a computer that has a SCSI controller and multiple SCSI hard drives, you’ll probably see the SCSI controller set to a SCSI ID of 0. And then there’s probably a cable from that device that goes to the first hard drive. And that drive is daisy chained to, in this case, a second hard drive. And then at the end of that cable is a SCSI terminator. This is what the SCSI terminator looks like that is connected to the end of that SCSI cable.
So if you look inside one of these computers with SCSI interfaces, you may find a cable that looks something like this. You can see that it plugs into the motherboard. It’s got a long set of cables. Here’s one connector for a storage device. Another connection, a third, a fourth, and then finally a terminator at the end of this cable. So inside of the computer we would use a cable like that to connect to the motherboard. And then one of those other connectors would connect to the SCSI drives that are inside of our system.
Here’s a view of the SCSI connection outside of a computer case. You can see the motherboard is on the top with that SCSI connector. And that single ribbon cable wraps around. And one of the interfaces on that ribbon cable connects to a storage drive. You can see there are multiple interfaces on this cable. So we can continue to add storage drives as long as we have connectors on this cable.
In our next video, we’ll talk about how we moved from the PATA or parallel communication to SATA or serial communication. And SCSI has had a very similar evolution. One of the more recent standards for SCSI is the serial attached SCSI where we went from parallel communication to a serial communication. This not only increased the total throughput that we could have in our systems, but it greatly simplified the implementation in our computers. Because this is a point to point connection between the motherboard and the drive.
We no longer have these cables with multiple interfaces or devices with multiple SCSI interfaces, because there’s no more daisy chains. We also removed the need to have a terminator, because the only thing on the serial attached SCSI connection is the controller on one side and the hard drive or storage device on the other. This gives us the control and management of a well-established SCSI set of protocols, but it also provides us with the speeds we need for our most modern storage devices.
Here’s a serial attached SCSI drive. You can see it looks very similar to a SATA drive. But the connectors on the back are slightly different, and they do require a different set of connectors. An example of this very large connector that is a high density internal SAS connector that you would use inside of your computer. And of course, there are also serial attached SCSI arrays so that you can connect many different serial attached SCSI drives into this single chassis.