SPI-2 is also called Ultra2 SCSI, was officially published as ANSI X3.302-1998, and adds several features to the prior versions:

• Fast-40 (Ultra2) speeds (40MBps or 80MBps)

• Low Voltage Differential signaling

• Single Connector Attachment (SCA-2) connectors

• 68-pin Very High Density Connector (VHDC)

The most notable of these is a higher speed called Fast-40, which is commonly called Ultra2 SCSI and runs at 40MHz. On a narrow (8-bit) bus, this results in 40MBps throughput, whereas on a wide bus (16-bit), this results in 80MBps throughput and is commonly referred to as Ultra2/Wide.

To achieve these speeds, a new electrical interface called LVD must be used. The slower single-ended electrical interface is only good for speeds up to Fast-20; Fast-40 mode requires LVD operation. The LVD signaling also enables longer cable lengths up to 12 meters with multiple devices or 25 meters with only one device.

LVD and SE devices can share the same cable, but in that case, the bus runs in SE mode and is restricted in length to as little as 1.5 meters in Fast-20 mode. LVD operation requires special LVD-only or LVD/SE multimode terminators. If multimode terminators are used, the same terminators work on either SE or LVD buses.

The SPI-2 standard also includes SCSI Interlink Protocol (SIP) and defines the Single Connector Attachment (SCA-2) 80-pin connector for hot-swappable drive arrays. There is also a new 68-pin Very High Density Connector (VHDC), which is smaller than the previous types.

SCSI Signaling

"Normal," or standard, SCSI uses a signaling technique called single-ended (SE) signaling. SE signaling is a low-cost technique, but it also has performance and noise problems. Single-ended signaling is also called unbalanced signaling.

Each signal is carried on a pair of wires, usually twisted to help reduce noise. With SE, one of the pair is grounded—often to a common ground for all signals—and the other carries the actual voltage transitions. It is up to a receiver at the other end of the cable to detect the voltage transitions, which are really just changes in voltage.

Unfortunately, this type of unbalanced signaling is very prone to problems with noise, electromagnetic interference, and ground leakage; these problems get worse the longer the cable is. This is why Ultra SCSI was limited to such short maximum bus lengths—as little as 1.5 meters, or 5 feet.

When SCSI was first developed, a signaling technique called High Voltage Differential signaling was also introduced into the standard. Differential signaling, also known as balanced signaling, is still done with a pair of wires. In fact, the first in the pair carries the same type of signal that single-ended SCSI carries.

The second in the pair, however, carries the logical inversion of that signal. The receiving device detects the difference between the pair (hence the name differential). By using the wires in a balanced pair, the receiver no longer needs to detect voltage magnitude, only the differential between voltage in two wires.

This is much easier for circuits to do reliably, which makes them less susceptible to noise and enables greater cable length. Because of this, differential SCSI can be used with cable lengths of up to 25 meters, whereas single-ended SCSI is good only for 6 meters maximum, or as little as 1.5 meters in the faster modes.

Unfortunately, the original standard for HVD signaling called for high-voltage differentials between the two wires. This meant that small, low-power, single-chip interfaces using HVD signaling could not be developed. Instead, circuits using several chips were required.

This worked at both ends, meaning both the host adapter and device circuitry had to be larger and more expensive. Another problem with HVD SCSI is that although the cables and connectors look (and are) exactly the same as for SE SCSI, both types of devices can't be mixed on the same bus.

If they are, the high voltage from the HVD device burns out the receiver circuits on all SE devices attached to the bus. In other words, the result is smoked hardware—not a pretty sight. Because SE SCSI worked well enough for the speeds that were necessary up until recently, HVD SCSI signaling never really caught on.

It was used only in minicomputers and very rarely, if at all, in PCs. Because of this fact, the extra cost of this interface, and the fact that it is electrically incompatible with standard SE SCSI devices, HVD signaling was removed from the SCSI specification in the latest SCSI-3 documents. So, as far as we are concerned, it is obsolete.

Still, a need existed for a more reliable signaling technique that would allow for longer cable lengths. The answer came in the form of LVD signaling. By designing a new version of the differential interface, it can be made to work with inexpensive and low-power SCSI chips.

Another advantage of LVD is that because it uses low voltage, if you plug an LVD device into an SE SCSI bus, nothing will be damaged. In fact, as an optional part of the LVD standard, the LVD device can be designed as a multimode device, which means it works on both SE and LVD buses.

In the case of installing a multimode LVD device into an SE bus, the device detects that it is installed in an SE bus and defaults to SE mode. Therefore, all multimode LVD/SE SCSI devices can be used on either LVD or SE SCSI buses. However, when on a bus with even one other SE device, all the LVD devices on the bus run only in SE mode.

Because SE mode supports only SCSI speeds of up to 20MHz (Fast-20 or UltraSCSI) and cable lengths of up to 1.5 or 3 meters, the devices also work only at that speed or lower; you also might have problems with longer cables.

Although you can purchase an Ultra3 SCSI multimode LVD/SE drive and install it on a SCSI bus along with single-ended devices, you will certainly be wasting the capabilities of the faster device.

Note that all Ultra2 and Ultra3 devices support LVD signaling because that is the only way they can be run at the Ultra2 (40MHz) or Ultra3 (80MHz) speeds. Ultra SCSI (20MHz) or slower devices can support LVD signaling, but in most cases, LVD is synonymous with Ultra2 or Ultra3 only.

Because the connectors are the same for SE, HVD, LVD, or multimode SE/LVD devices, and because putting an HVD device on any bus with SE or LVD devices causes damage, it would be nice to be able to tell them apart. One way is to look for a special symbol on the unit; the industry has adopted different universal symbols for single-ended and differential SCSI.

If you do not see such symbols, you can tell whether you have a High Voltage Differential device by using an ohmmeter to check the resistance between pins 21 and 22 on the device connector:

• On a single-ended or Low Voltage Differential device, the pins should be tied together and also tied to the ground.

• On a High Voltage Differential device, the pins should be open or have significant resistance between them.

Although you will blow up stuff if you plug HVD devices into LVD or SE buses, this generally should not be a problem because virtually all devices used in the PC environment are SE, LVD, or LVD/SE. HVD has essentially been rendered obsolete because it has been removed from the SCSI standard with Ultra3 SCSI (SPI-3).