Understanding SCSI – CompTIA A+ 220-801: 1.5

| October 3, 2012


The SCSI standard has been around for a long time, and it’s now finding new life due to virtualization technologies. In this video, you’ll learn about the SCSI standard and how SCSI devices are configured and connected together.

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The abbreviation SCSI is pronounced “scuzzy.” And the stands for Small Computer Systems Interface. It was a method of connecting storage devices– at that time were small computer systems or personal computers. And it was one of the very first ways that we had standardized storage on our computer systems.

We were able to do quite a bit with this. We could string a number of devices together. They could be hard drives. They could be optical storage devices.

And we could put up to 16 devices on one of these SCSI chains, using a single cable and a single SCSI controller. So you were able to just put one interface in your computer and simply hang off the back of that interface, 16 different devices.

Through the years, there have been a number of different kinds of SCSI formats. You may see it called Fast SCSI, Ultra SCSI. There’s Ultra Wide SCSI. There’s Ultra2 SCSI and so on.

All of these formats tend to be backwards compatible. But the best possible performance you’re going to be, would be to get a storage device and a controller that were using the same type of SCSI interface.

Before USB came along, SCSI was really the standard for connecting our peripherals together. We would plug in printers, scanners, tape drives, CD-ROM drives. Anything that needed in an external connection, SCSI was the format that we were using.

On the SCSI buses, we had different sizes. We had a narrow bus SCSI. Some SCSI was a wide bus. The narrow bus could plug in eight devices that we could string onto an individual controller. The wide bus allowed us to have 16 different devices on a single controller.

The part of SCSI that really makes it powerful is that the interface and the communication itself is very, very intelligent. It’s very powerful. We were able to do a lot of different communication over that SCSI bus. And it was very simple to plug in.

Even though we were able to control so many diverse kinds of devices, it was a single type of interface. We assign a number to it and now we’ve got a SCSI connection on our computer.

SCSI has been around for a very, very long time. But even today, especially in large server farms, we see SCSI interfaces for hard drive controllers. And especially on virtualized environments, we see a lot of SCSI being used. Even though it’s not a physical SCSI connection, we’re still using that SCSI standard of protocols to communicate over these virtualized systems.

One particular piece of SCSI that was a very important, especially early on in SCSI, was the termination. You could plug many devices in. But the last device had to have what we called a terminator on the end of the chain. That way, the SCSI communication knew exactly where the signal ended. Without a terminator, none of these devices would be able to operate properly.

The newer SCSI devices tend to have these terminators built into the device. And you can enable or disable the terminator just by clicking a button. But the older devices needed a physical terminator that you would connect to the interface that was on back of the SCSI device.

There are many different types of interfaces that you’ll see for SCSI. They’ve changed quite a bit through the years.

Here’s an example of one. You can see this one happens to be an Ultra3 SCSI connection. And you can see a PATA connection right next to it. They’re almost the same size. The SCSI might be a little bit shorter.

This is a view from above. If we were to look at it from the side, you can see the interface connector is a little bit different than the PATA or the SATA connections that we’ve used before. This SCSI connection has quite a number of pins associated with it.

The cable that we would use to connect this would be something like this, where we would plug into the motherboard. And then there might be many different connections on that same cable where we can use all of these different SCSI devices and plug them all in at the same time.

If we were to look closer, you could really see a lot of different pins being used. And on this particular SCSI connection, it was a higher bandwidth so it needed a lot more pins to be able to communicate.

But it may not be this type of connection that you would see with SCSI. Because they change so much through the years, you might see a DB25 connection that looks like a serial or maybe even an older parallel connection. They all look very similar. But these are actually SCSI connections. You have to look very carefully to see the differences.

Some connections are very clear that they’re SCSI. This high density 50-pin connection has locks on the side of it that connect in. That’s very, very exclusive to SCSI. And some of these higher density connections as well really make you see that very quickly that’s a SCSI connection. There’s nothing else that looks quite like those connectors.

When you’re physically connecting SCSI devices to the SCSI cable that you have, whether they are inside of the computer or outside of the computer, you have to give each one of them a completely unique number. This is called a SCSI ID. So on that single bus, on that single connection coming out of an interface of the computer, we would provide different IDs.

For instance, if this was your boot disk, maybe we’ll give that SCSI ID 0. If there’s a floppy disk or scanner or an optical device, we might assign that a number 2. If there’s a CD-ROM, we might assign that with a the ID of 3.

There’s also a concept within SCSI connections called a logical unit. So within each physical SCSI ID on that interface, you might have logical units inside of this. This was very common to see.

If you were plugging in an external SCSI storage device that had multiple hard drives inside of it, your would give the storage device itself a SCSI ID. And then there would be logical units assigned to the individual drives within that single physical device. You also see this quite a bit when you’re working with virtualized systems.

As I mentioned earlier, the signal that is at the very end of the SCSI bus has to be terminated. And there might be a physical terminator or it might be a termination functionality that’s built into the hardware itself.

If you ever see what we call SCSI attached devices that are serial attached SCSI, they don’t have jumpers, they don’t have terminators, they don’t have settings. A lot of the more modern storage devices that we are plugging in over something called serial attached SCSI are completely automated in the way that they configure themselves on that SCSI bus.

Here’s an example of a legacy SCSI device. I think this is an optical device. But it could be a hard drive. It could be a scanner. It could be anything else.

Generally, we see two different types of connectors on the back of the device. One of them generally is the connection going in. You can see it written on the back of the device, where we would plug in the in connection from our computer. If we had additional SCSI devices, we would use this out connection to keep the string going. To keep that daisy chain going throughout the connection.

If this was the end of the line, if this really was the last device in this SCSI chain, this is where we would plug in the terminator. If this was a device that had the termination built in, there would probably be a connector on this device or a button you could push to say, this was the terminating place.

And you can also see the SCSI ID number built right into this as well. There’s little buttons that would allow you to change that SCSI ID. So when this device plugged in and communicated across the SCSI bus, everyone know that this device was SCSI the ID number 6.

Once you have everything plugged in, it looks a little like this where you have a SCSI controller. That might be ID 0. Here’s SCSI ID 1. That’s a hard disk. Here’s another hard disk.

And they’re all chained together with different SCSI IDs. And at the end of the chain, is a terminator. Usually these terminators even have a little light, so you can see that they are active. Here’s a picture of what a SCSI terminator might look like. So you can look at the back of your devices and see if you have one of those.

That term “daisy chain” actually came from this idea of taking daisies and connecting them to each other. And you could wear them as bracelets or wear them on your head. And that same daisy chaining was how we came up with this idea of plugging in all of these devices, one to the other, to create that chain of devices.

The back of the SCSI drive looks very similar to the back of any other hard drive. It’s just using a very specialized SCSI connection on the back. Usually you have jumpers that can help you designate what the SCSI ID of this drive happens to be.

And here’s traditional power that you might find, so that drive can be powered up on the inside of your computer. This was an external device. Then you have external power. You usually have options for setting your SCSI IDs that we saw on the SCSI device earlier. And the interfaces for going in and out of that SCSI connection.

The connection of the SCSI device on your motherboard looks very similar to PATA in that we’ve got a connection on the motherboard and multiple interfaces on SCSI. But of course with PATA, we are limited to two individual devices. Whereas with SCSI, especially the wide bus SCSI, I can connect up to 16 devices. And you would simply find the closest connector to you and plug it in.

It doesn’t matter where along this particular cable we plug in our SCSI device. The determination of how this device communicates is associated with the ID that you assign to the SCSI device. So you simply plug into the closest connector to you and plug in all of your different SCSI devices, make sure they have different SCSI IDs, and your SCSI communication is now complete.

Once you follow those rules, SCSI is very simple to get running. And it’s going to provide you with a lot of flexibility to plug in multiple storage devices.

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Category: CompTIA A+ 220-801

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