SCSI

In this video from ITFreeTraining, I will be having a more detailed look into SCSI. Nowadays, SCSI is obsolete, so you won’t come across it unless you are working with an old computer. There are some SCSI concepts that we still use today and also some new technology that is based on the original SCSI design.

Download PowerPoint: https://itfreetraining.com/handouts/a...

Small Computer System Interface (SCSI)
The Small Computer System Interface or SCSI was first developed in 1986. It defines the commands, protocols, electrical signals, optical signals and the interface that SCSI uses. SCSI was originally designed to connect devices like hard disks, scanners and printers together.

Although SCSI is obsolete nowadays, I will have a quick look at how it works and what parts of SCSI we still use today and how we use them.

SCSI-1
To start with, I will have a look at SCSI-1 first released in 1986. For the CompTIA A+ exam, you won’t need to know a lot about SCSI. I will do an overview of SCSI simply to give you a starting point, so you know where to look if you have to support an old SCSI system.

SCSI-1 uses a 50-pin connector. The connector was quite large by today’s standards, but was very durable. This version of SCSI uses an 8-bit parallel interface. This means that it can transfer eight bits of data at once.

The original version of SCSI signaling was called singled ended or SE. The SE connection was utilized a lot by Apple, so if you are working on an old Apple system it is probably has an SE connection.

In some cases, your SCSI devices and cables may be labeled as SE, in other cases they may not. Before I go into more detail about the different types of signaling used in SCSI, I will first have a look at the different types of connectors.

SCSI Connectors
There are a lot of different SCSI connectors available. Shown here are a few of them. This is not a complete list of all the different connectors. Although there are a lot, most of the time, it is a matter of matching the connector to the correct plug.

Shown here is a SCSI cable with the same connector on each end. SCSI allows multiple devices to be connected together sharing the same bus. Notice, however, the cables do not have to have the same connectors.

If you purchase a pre-made cable, you won’t need to worry about compatibility issues since they are made with compatibility in mind; however, there are some things that you should be aware of.

Signaling
SCSI has three different types of signaling. The first is single ended or SE. This was the first signaling type used by SCSI. If you are working on an old Apple computer, it is most likely using this connector. In some cases, the cable or the plug will be labeled with SE.

The next signaling type is Low Voltage Differential or LVD. This worked similarly to SE in that it uses two wires – one data wire and one ground wire for each bit of data that is transferred. Having multiple groups of two wires allows data to be transferred in parallel. More on that later in the video.

Since SE and LVD use similar signaling, LVD is backwards compatible with SE. Keep in mind, however, that if you put an LVD and SE device on the same SCSI bus the whole SCSI bus will change to SE. This allows for compatibility, but SE is generally slower than LVD.

The last signaling type is High Voltage Differential or HVD. This type of signaling still uses two wires, however there is no ground wire. Instead, the wires are used for data and the difference in signal between the two is measured in order to determine if a zero or a one bit has been sent.

Description too long for YouTube. Please see the following link for the rest of the description. https://itfreetraining.com/ap/3b45


References
“The Official CompTIA A+ Core Study Guide (Exam 220-1001)” Chapter 3 Paragraph 163-171
“CompTIA A+ Certification exam guide. Tenth edition” Pages 302-303
“SCSI” https://en.wikipedia.org/wiki/SCSI
“SCSI FAQ” http://www.paralan.com/scsifaq/scsifa...
“Picture: Follow the rules” https://pixabay.com/photos/board-empt...
“Picture: Picture Frame” https://unsplash.com/photos/22ikHjykXg4
“Video: Cat putting head in jar” https://www.pexels.com/video/animal-p...
“Picture: SCSI ID” https://commons.wikimedia.org/wiki/Fi...
“Picture: SAS Cable” https://en.wikipedia.org/wiki/Serial_...
“Picture: SAS Raid” https://commons.wikimedia.org/wiki/Fi...

Credits
Trainer: Austin Mason http://ITFreeTraining.com
Voice Talent: HP Lewis http://hplewis.com
Quality Assurance: Brett Batson http://www.pbb-proofreading.uk