scsi

scsi

Post by Michael Smit » Mon, 05 Apr 1999 05:00:00



[ scsifaq.htm 18K ]

SCSI FAQ

Frequently Asked Questions

1. What is SCSI?
2. What is SCSI 2?
3. What is SCSI 2 Fast?
4. What is SCSI 2 Wide and Ultra Wide?
5. Why is there no performance difference between a single
SCSI drive and a single SCSI 2 Fast drive?
6. What is SCSI termination?
7. What is a SCSI terminating resistor?
8. What is passive termination versus active termination?
9. How do you properly terminate a SCSI bus?
10. What is the maximum bus length for SCSI?
11. Does it matter where the SCSI devices are placed on the SCSI bus?
12. What are single-ended SCSI and differential SCSI?

 

1. What is SCSI?

SCSI (pronounced "scuzzy") stands for Small Computer Systems Interface. SCSI is typically used for the connection of mass storage devices to a host computer. However, in concept, SCSI 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.

 

2. What is SCSI 2?

SCSI 2 is a second implementation of SCSI that includes additional commands such as tagged command queuing. SCSI 2 is a super set of SCSI.

 

3. What is SCSI 2 Fast?

SCSI 2 Fast is an implementation of SCSI 2 that allows for transfer rates of up to 10MB/sec. across the SCSI bus, which translates to much better throughput across the SCSI bus.

 

4. What is SCSI 2 Wide and Ultra Wide?

A standard SCSI bus has only 8 data lines for the transfer of data across the SCSI bus. SCSI 2 Wide has 16 data lines, which allows double the amount of data across the SCSI bus. SCSI 2 Ultra Wide is a SCSI bus with 32 data lines. Combining the fast attributes of SCSI 2 with the wider data paths allows transfer rates of 20MB/sec. with SCSI 2 Fast and Wide and 40MB/sec. with SCSI 2 Fast and Ultra Wide.

 

5. Why is there no performance difference between a single SCSI drive and a single SCSI 2 Fast drive?

Because the drive is the bottleneck, 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 supply data to the SCSI bus. You could think of it as being similar to using more package handlers to put boxes on a conveyor belt.

 

6. What is SCSI termination?

The SCSI bus needs to have both ends 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 cause data transmission issues. Still, even though the SCSI bus is not terminated properly, it will work just fine, which is a testament to SCSI's robustness. However, proper termination becomes increasingly important as data transfer rates increase, the number of devices on the bus increases and the length of the SCSI bus increases.

 

7. 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.

 

8. What is passive termination versus active termination?

Passive termination is just a series of resistors; there are no active components (i.e., 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 used primarily on older SCSI buses, mainly because the termination was not as critical. Active termination can be used with standard SCSI; mostly, however, it is used with SCSI 2, SCSI 2 Fast and SCSI 2 Fast and Wide.

 

9. How do you properly terminate a SCSI bus?

In most cases, you 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 its terminators installed and turned on and 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 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.

 

10. 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. You can typically add one foot of cable for each SCSI device and connector in the SCSI chain.

 

11. Does it matter where the SCSI devices are placed on the SCSI bus?

Yes. Typically, tape drives are only SCSI -- at most SCSI 2 -- which means that they only transfer data at 5MB/sec. At this slower rate, SCSI bus termination is not much of an issue. However, if you are mixing SCSI 2 Fast devices with standard SCSI devices, we recommend putting the SCSI 2 Fast devices toward the end of your SCSI bus nearest the termination and putting the standard SCSI devices in the middle of the SCSI bus. The SCSI termination works best nearest the terminators and gets worse toward the middle of the bus. Putting the SCSI 2 Fast devices nearest the terminators provides the best possible termination, which the SCSI 2 devices need because they are transferring data at twice the speed of the standard SCSI devices. The faster the transfer rate, the more critical the SCSI termination becomes.

 

12. What are single-ended SCSI and 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 arrives 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 its "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; it only sees differences on the two wires. So, 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 (approximately 10 feet), while specifying 25 meters (approximately 80 feet) for differential FAST cable length. The 25-meter limit is determined by the roundtrip signal time on the cable. Beyond this length, SCSI 2 signal protocol timing specifications cannot be met, because it takes too long for a signal to travel down the cable and for a response to return.


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