Small Computer Systems Interface (SCSI)
Over the past decades, computer’s industry has seen radical change in key components. Limitations in speed, bandwidth, and distance have driven evolution and advancements of the technology.
In terms of hardware interface, the development of the Small Computer Systems Interface (SCSI), in the mid 80’s, was a considerable advancement because SCSI was the first intelligent interface. SCSI is a standard interface that supports all devices on a single one adapter. Indeed prior to SCSI, interfaces were designed for specific devices. One hard disk interface for hard drive, one tape interface for tape drive, etc…The development of Fibre Channel started in the late 80’s in order to provide high-speed via high-bandwidth, and became an approved ANSI (American National Standards Institute) standard in 1994.
Now, storage needs are booming with today’s exceptional growth of data-intensive applications such as decision support system, audio, video and Internet/intranet. Fueled by this demand, Fibre Channel is gaining popularity over SCSI however SCSI continues to be the most common interface, and will remain appropriate for some time and some applications. Today’s period is a transition period where Fibre Channel and SCSI cohabit in the market and in configuration. Hybrid product (Fibre Channel for host and SCSI for drive) is the step before full Fibre Channel.
SCSI Universal symbol icons
SCSI Terminators
SCSI terminators must be placed onto or just beyond the last SCSI device on a SCSI chain. SCSI controlleRs usually have automatically enabled/disabled termination or offer jumper pin caps that can be placed onto pins to enable/disable termination. “High-byte” terminators are not the same as terminators because they go somewhere in the middle of your SCSI chain and terminate only 18 of the 68 wires in a 68-pin SCSI cable/device and allow the other 50 wires to continue to the end of your SCSI chain. More details below.
Passive Terminators -
Used in SCSI-1 Cabling when only one or two SCSI devices are on the SCSI chain.
Active Terminators -
Used in SE SCSI Cabling. Active terminators are SE terminators.
FPT Terminators ( Forced Perfect Terminators ) -
Often used instead of active terminators in Narrow SE SCSI cabling.
HVD Terminators (High Voltage Differential Terminators) -
Often called "Differential" terminators, HVD terminators MUST BE USED with HVD SCSI devices and ONLY with HVD SCSI devices.
Active Negation Terminators -
These terminators offer better termination than active terminators.
Feed-through Terminators (Pass-through Terminators ) -
Used when there is no place to attach a terminator at the end of a SCSI chain. Placed between SCSI cable and last SCSI device. Feed-Through terminators can be passive, active, SE, LVD, etc.
LVD Terminators (Low Voltage Differential Terminators ) -
Required for LVD SCSI cabling (see LVD SCSI chart). LVD/SE terminators will automatically work in either SE or LVD SCSI mode, however ALL devices on a SCSI chain must be LVD to achieve LVD benefits (again, see the LVD SCSI chart). LVD ONLY terminators will not work in SE mode and will shut down a SCSI chain. Important Notes: To achieve Ultra2 LVD or Ultra3 LVD speeds, you must use a corresponding Ultra2 LVD or Ultra3 LVD terminator.
High Byte Terminators (50-Pin/68-Pin SCSI Adapters) –
refer to “68-Pin to 50-Pin or 25-Pin Adapters” and “High-Byte Termination Situations”.
Narrow SCSI
Narrow SCSI means an 8-bit data bus and uses a 25-pin or 50-pin connector.
Wide SCSI
Wide SCSI means a 16-bit data bus and uses a 68 or 80-pin connector.
Speed
68-pin and 80-pin devices cannot reach maximum speed when they are adapted to 25-pin or 50-pin connectors or if the SCSI controller only offers a 25-pin or 50-pin connector.
68-Pin to 50-Pin or 25-Pin Cables
68-Pin to 50-Pin external SCSI cables will work ONLY when the 25-pin or 50-pin side is towards the SCSI controller and the 68-pin side is towards the SCSI device to be connected. To connect a 25-pin or 50-pin external SCSI device to either a 68-pin SCSI controller or to a 68-pin SCSI device, will be need a 68-Pin/50-Pin SCSI adapter with a high-byte terminator built in plus a 50-pin to 50-pin cable or 50-pin to 25-pin SCSI cable.
68-Pin to 50-Pin or 25-Pin Adapters
When connecting 68-Pin devices onto a 25-pin or 50-pin SCSI cable, a 68-pin to 50-pin adapter (not an adapter with high-byte termination) is needed. However often a 68-Pin/50-Pin SCSI adapter with a built-in high-byte terminator will be required. SCSI adapters with high-byte terminators are needed only used when using a 68-pin connector on your SCSI controller card and want to connect 25-pin or 50-pin devices onto the end of your 68-pin SCSI chain. If using a 50-pin connector on your SCSI controller, you do not have to worry about high-byte termination. Externally, high-byte terminators are required when attaching 50-pin SCSI devices to a 68-pin SCSI controller card or a 68-pin SCSI device. Internally, high-byte terminators are required when attaching a 50-pin internal cable to a 68-pin SCSI controller card or when the last device on a 68-pin internal cable is a 50-pin SCSI device. Internally, it is okay to use a standard 50-pin/68-pin adapter without high-byte termination when placed in the middle of your SCSI chain and your last device on the 68-pin internal cable is a 68-pin device with termination. The point is this: A high-byte terminator will terminate the 18 wires that will not continue on to your 50-pin SCSI device(s).
80-Pin SCSI Adapters
SCA80 adapters are often used to connect an 80-pin SCSI drive to either a 50-pin or 68-pin internal SCSI cable.
SCSI IDs
Each device on the SCSI chain must have a unique SCSI ID on the SCSI chain. You can set up the ID on each device by using tiny jumper pin caps that are usually provided on SCSI devices. SCSI controllers are usually set to an ID of 7. For 25-pin or 50-pin SCSI devices, your IDs must be between 0 and 6. For 68-pin or 80-pin devices, an ID between 0 and 17 must be used but NOT 7 because this is usually reserved for the SCSI controller.
Technical Specifications
In the computer industry, the SCSI interface is found on every type of device: disks, host adapters, complex storage controllers, tape, scanners, printers and so. As the current storage interface, SCSI is a bus parallel architecture where a host system communicates with storage devices via a parallel bus. SCSI is an intelligent, parallel interface, which allows transfer of data over multiple pieces of the same medium simultaneously.
In consequence, the main issue with SCSI is to be sure that data sent over this medium, but in each individual parallel wire of it, arrives at the same time at the target. If it is not the case, data corruption will occur. Restrictions and limitations of SCSI come from this transfer protocol. In order to avoid data corruption, speed of data transfer and cable length are reduced.
Essential feature of SCSI is its ability to process multiple overlapped commands. Also known as multi-tasking support, this allows SCSI drives to fully overlap their read and write operations with other drives in the system. Commands can be processed concurrently rather than serially from different SCSI drives. The data can then be buffered and transferred over the SCSI bus at very high speeds with other data in the system.