SCSI DEFINED

What is SCSI?

Pretty much entirely replaced by SATA as is IDE (PATA). Now under the heading "Remember When"!


SCSI (pronounced scuzzy) stands for Small Computer Systems Interface. It is typically used for the connection of mass storage devices to a host computer, however, in concept, it can be used to interface any device such as a printer or modem. Most SCSI devices are hard disk drives and tape drives. The original implementation of SCSI allowed for data to be passed in parallel along an 8-bit data path at speeds up to 5MB/sec.

What is SCSI 2?
SCSI 2 is a second implementation of SCSI. Additional commands were added such as tagged command queuing. SCSI 2 is a subset of SCSI.

What is SCSI 2 Fast?
SCSI 2 Fast is an implementation of SCSI 2. It allows for transfer rates of up to 10MB/sec. across the SCSI bus. This allows for much better throughput across the SCSI bus.

What is SCSI 2 Wide or Ultra-Wide?
A standard SCSI bus has only 8 data lines for the transfer of data across the SCSI bus. SCSI 2 Wide had 16 data lines which allows you to double the amount of data across the SCSI bus. SCSI 2 Ultra-Wide allows transfer rates of 20MB/sec with SCSI 2 Fast and Wide and 40MB/sec. with SCSI 2 Fast and Ultra-Wide.

Breakdown of speeds with associated names
SCSI-1 5MB/Sec 8 bit SCSI Bus

SCSI-2 5MB/Sec 8 bit SCSI Bus

SCSI-2 Fast 10MB/Sec 8 bit SCSI Bus
SCSI-2 Fast Wide 20MB/Sec 16 bit SCSI Bus
SCSI Ultra 20MB/Sec 8 bit SCSI Bus

SCSI Ultra Wide 40MB/Sec 16 bit SCSI Bus

Ultra2 Wide 80MB/sec 16 bit SCSI Bus

Ultra160 (Ultra3) Wide 160MB/sec 16 bit SCSI Bus

Why is there no performance difference between a single SCSI drive and a single SCSI 2 Fast drive?
The reason for this is because the drive is the bottle neck, not the SCSI bus. SCSI 2 Fast allows for more data to be sent at a faster rate across the SCSI bus, however, a single SCSI drive is not able to saturate the potential of the SCSI 2 Fast bus. Where you will see an improvement is when you have multiple drives on the same bus. In this configuration, multiple drives are supplying the data to the SCSI bus. You can think of it as there being more package handlers putting boxes on the conveyor belt versus only one.

What is SCSI termination?
The SCSI bus needs to have both ends of the SCSI bus terminated to facilitate the transfer of data across the SCSI bus. It is very much like terminating a coax Ethernet segment. Failure to properly terminate a SCSI bus can, but not always, cause data transmission issues. Many times even though the SCSI bus is not terminated properly, it will work just fine. This is a testament to the robustness of SCSI. However, as the data transfer rates increase, the number of devices on the bus increases and the length of the SCSI bus increase, termination becomes more and more important.

What is a SCSI terminating resistor?
A SCSI terminating resistor is basically a pull up resistor network. It is used to condition the SCSI bus to facilitate data transmission across the SCSI bus.

What is active termination versus passive termination?
Passive termination is just a series of resistors and there are no active components. All components are passive. Active termination takes the +5 volts from the SCSI bus, uses a voltage regulator (active component) and regulates the power used by the resistor network. This provides a much more stable termination for the SCSI bus. Passive termination is primarily used on older SCSI buses. Mainly because the termination was not as critical. Active termination is can be used with standard SCSI, however, it is mostly used with SCSI 2, SCSI 2 Fast and SCSI 2 Fast and Wide.

How do you properly terminate a SCSI bus?
In most cases, you will terminate the devices at each end of the SCSI bus. If you have a host bus adapter with two or more devices connected to the external connector of the host adapter, the host adapter will have it terminators installed and turned on, the drive at the other end of the SCSI bus will also have its terminators installed and turned on. The SCSI devices in the middle of the bus, between the last SCSI device and the host bus adapter will have their terminators removed and or turned off.

If you have only one SCSI device attached the to the external connector of the SCSI host bus adapter, that SCSI device and the host bus adapter will both have the termination resistors installed and turned on.

If you have two or more SCSI devices attached to the internal SCSI connector on the host bus adapter and nothing attached to the external SCSI connector, the SCSI host bus adapter will have the its terminating resistors installed and turned on. The SCSI device at the other end of the cable will also have its terminators installed and turned on. The device(s) in between will have their terminators removed and or turned off.

If you have SCSI devices attached to the SCSI host bus adapter's internal SCSI connector and external SCSI connector, the last drive on the end of the external SCSI cable will have its terminators installed and turned on and the last drive on the internal SCSI cable will have its terminators installed and turned on. All other devices in between, including the SCSI host bus adapter, will have their terminators removed and or turned off.

What is the maximum bus length for SCSI?
The maximum bus length for SCSI is 6 meters, or about 20 feet. For SCSI 2 it is 3 meters or about 10 feet. These numbers not only include the actual cable length, they also include the stub lengths of each SCSI device attached to the bus as well as the effect that connectors on the bus cause. You can typically add one foot of cable for each SCSI device and connector in the SCSI chain.

Does placement of the type of SCSI devices on my SCSI bus matter?
Yes, absolutely. Typically tape drives are only SCSI and at most SCSI 2 which means that they are only transferring data at 5MB/sec. At this slower rate, SCSI bus termination is not that much of an issue, however if you are mixing SCSI 2 Fast devices with standard SCSI devices, we would recommend that you put the SCSI 2 Fast devices towards the end of your SCSI bus nearest the termination and put the standard SCSI devices in the middle of the SCSI bus. The SCSI termination is best nearest the terminators and gets worse towards the middle of the bus. By putting the SCSI 2 Fast devices nearest the terminators, they are getting the best possible termination, which they need because they are transferring data at twice the speed that the standard SCSI devices are. The faster the transfer rate, the more critical the SCSI termination becomes.

What is Single Ended SCSI? What is Differential SCSI?
Single-Ended signal transmission is a scheme whereby a signal is transmitted down the SCSI bus by a bus driver on a single signal wire, with return current on a ground wire. A single-ended receiver looks only at the signal wire as an input. The transmitted signal will arrive at the receiving end of the bus on the signal wire somewhat distorted by signal reflections. The length and loading of the bus determine the magnitude of this distortion.

Differential signal transmission uses TWO wires to transmit a signal down the SCSI bus. The two wires are driven by a differential driver that places a signal on one wire (+SIGNAL) and it's "mirror" image (-SIGNAL) on the other wire. At the receiving end, a differential receiver looks at the two wires and generates an active signal output only when the two inputs are different. The intended signal is always received as two opposite (different) signals. However, other effects such as signal reflections occur virtually the SAME on both wires. Thus, signal reflections are not seen by the receiver, because it only sees differences on the two wires. Thus, although signal reflections still exist, they are effectively canceled out at the differential receiver.

An indication of the non-critical nature of differential signal transmission is that Differential Terminators are PASSIVE. In single-ended transmission, ACTIVE terminators are recommended in the SCSI 2 specification for FAST transmission.

Also, the SCSI 2 specification specifies a maximum single-ended FAST cable length of only 3 meters (approx. 10 feet), while specifying 25 meters (approx. 80 feet) for differential FAST cable length. The 25 meters limit is determined by the round trip signal time on the cable. Beyond this length, SCSI 2 signal protocol timing specifications cannot be met. It takes too long for a signal to travel down the cable and for a response to return (about 240 nsec round-trip time).

 

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