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.