SCSI Drive Cables – CompTIA A+ 220-1001 – 3.1

| January 14, 2019


SCSI interfaces have been around for many years, and the standard has continued to evolve through the years. In this video, you’ll learn about traditional SCSI connections and the newer serial attached SCSI standard.

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The SCSI standard has been around for a very long time, and you’ll probably find some variant of SCSI inside of your modern data center. The SCSI standard is an abbreviation for Small Computer Systems Interface. This standard was designed to connect many different kinds of devices together into the standard format. So you might have hard drives, optical drives, scanners, and other devices all along the same SCSI bus. In many versions of the SCSI standard, you can support up to 16 devices chained together one after the other.

Because the standard has evolved so much through the years, you’ll find that there are many different ways to communicate over SCSI using many different interfaces, and there are both parallel and serial versions of SCSI available.

These days if we want to connect a peripheral to our computer, we generally would use a USB connection. But when SCSI was introduced, there was no USB. SCSI was the way that we were able to connect many different devices together. You can connect up to eight devices together on what we call the SCSI narrow bus, and up to 16 devices together on the SCSI wide bus.

One of the nice things about SCSI is there wasn’t a lot of configuration that you as the administrator would have to do. You would simply assign an ID number to a device connected to the SCSI bus, and then SCSI would take care of all of the devices communicating between each other. SCSI is one of those formats that continues to be used in today’s data centers. You’ll certainly find SCSI being used in virtual machines and even physical drive arrays.

Here’s a motherboard that has one of these older SCSI connections on it. This is the 68-pin Ultra III SCSI connection. You can see that the size of the SCSI connection is very similar in size to a PATA drive connection.

Here’s a view from the front of this 68-pin connection. This is where you would plug into the motherboard, and then have a cable that would then connect to many other devices along that chain.

This is the cable that you would use for that 68-pin connection, but SCSI has many different types of interfaces. So depending on the type of interface you have and the type of devices that you have, you might be using one of many different kinds of SCSI connections.

When you connect a device onto a legacy SCSI bus, you assign it a separate ID number. It’s very common for the SCSI controller to have an ID of 0, and then you might assign a hard drive to have an ID of 2 or an optical drive to have an ID of 3. Within these IDs, there’s also the concept of logical units. So you might have separate drives within a drive array. The drive array would be assigned an ID number, and then the individual drives within that drive array may have separate logical unit numbers.

To signal the end of this SCSI bus, you use a terminator. Some devices have a terminator built into those devices, but sometimes you might use an external terminator to connect onto the end of the cable. The idea of having a SCSI ID number and these logical units and being able to terminate across the SCSI bus is a concept used with these older SCSI devices.

Our modern data centers probably won’t have these older devices that have separate IDs or logical unit numbers. Instead, they’re probably using a newer style of SCSI called serial attached SCSI where there’s no terminator, it’s a one-to-one connection, and you don’t have to worry about configuring any of that ID information. We’ll talk about serial attached SCSI later in this video.

Here’s the back of an external storage drive that connects with SCSI. You can see the SCSI connectors, and one of those connectors is labeled in and the other is labeled out. And you can see the SCSI ID configuration is made right here on the back of the drive. This one is configured with a SCSI ID of 6.

This in and out connection is used so that you can use a single cable from the computer and simply daisy-chain additional devices along the same connection. This daisy-chain is something that’s commonly seen with the older style SCSI where you might have the SCSI controller as your SCSI ID 0, it would have a single cable coming out plugging into a hard drive, maybe you have a separate drive, another external drive connected to that, and so on.

And then you would terminate that entire daisy-chained bus by either having a physical terminator on the end, such as this one, or it would be internally terminated inside of the last device.

Here is the type of cable you might see on those older daisy-chained SCSI devices. You have the connection on the motherboard, and then you’d be able to connect one, two, three, four separate SCSI devices, and this cable has a terminator built in to the end of the cable.

Here’s the interface on the back of a legacy SCSI drive. This one has that 68-pin connection and a connection for Molex power on this drive as well. You would use the connection coming from your motherboard, you would connect your SATA cable, plug in the SATA cable connection anywhere along the line, and then at the end of the cable would be a terminator.

Just as our parallel ATA moved to serial ATA, we also saw that SCSI moved to a serial attached SCSI. This increased the amount of throughput and added flexibility for connecting these devices in our enterprise. This is a point-to-point connection, so serial attached SCSI doesn’t have any of those concerns with using daisy-chains or terminators.

You simply have a host device on one end of the connection and the storage drive on the other end of the connection. This allows you to have the same control and management that you have with the SCSI protocol, but instead you’re using this on a high-speed serial connection.

The drive itself looks very similar to a traditional SATA drive, but you can see that the connectors, although very similar, do have differences between them. You use a different type of serial attached SCSI cable to connect to this type of drive. There are many different kinds of serial attached SCSI connections. Here’s one for an internal high-density connection for serial attached SCSI drives.

And in your enterprise, you’re probably using serial attached SCSI drives in something like a drive array. There’s 24 drives on the lower part of this array and 24 drives across the top of this array all connecting internally with serial attached SCSI.

Category: CompTIA A+ 220-1001

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