Parallel SCSI

The original SCSI standard, SCSI-1, was derived from the Shugart Associates System Interface (SASI) and formally adopted in 1986 by ANSI. SCSI-1 features an 8-bit parallel bus (with parity), running asynchronously at 3.5 MB/s, or 5 MB/s in synchronous mode, and a maximum bus cable length of 6 metres (20 ft), significantly longer than the 18 inches (0.46 m) limit of the ATA interface also popular at the time. A rarely-seen variation on the original standard featured high-voltage differential signaling and supported a maximum cable length of 25 metres (82 ft).

NCR 53C94 SCSI-2 controller in PLCC-84 package

SCSI-2 was introduced in 1994 and gave rise to the Fast SCSI and Wide SCSI variants. Fast SCSI doubled the maximum transfer rate to 10 MB/s while retaining the same 50-pin cables, while Wide SCSI doubled the bus width to 16 bits on top of that to reach a maximum transfer rate of 20 MB/s, using new 68-pin cables. However, these improvements came at the cost of reducing the maximum cable length to three meters. SCSI-2 also specified a 32-bit version of Wide SCSI, which used two 16-bit cables per bus. The 32-bit implementation was largely ignored because it was deemed expensive and unnecessary, and was officially retired in SCSI-3.

SCSI-2 expanded the command set with the CCS (Common Command Set) for better support of devices other than disk drives, introduced command queueing (up to 256 commands for one device) and tightened up the requirements on some features that were optional in SCSI-1; parity was now mandatory and the host adapter was required to provide termination power in order to support active termination. SCSI-1 devices would generally remain compatible while simply ignoring the new features.[10]

An HVD mode that was incompatible with standard SE was introduced to accommodate longer bus lengths.

Before Adaptec and later SCSITA codified the terminology, the first parallel SCSI devices that exceeded the SCSI-2 capabilities were simply designated SCSI-3. These devices, also known as Ultra SCSI or Fast-20 SCSI, were introduced in 1996. SCSI-3 itself is not as much a single document as a collection of various standards that have received updates at different points in time.

The bus speed was doubled again to 20 MB/s for narrow (8-bit) systems and 40 MB/s for wide (16-bit). The maximum cable length stayed at 3 meters but single-ended Ultra SCSI developed an undeserved reputation for extreme sensitivity to cable length and condition (faulty cables, connectors or terminators were often to blame for instability problems).

Unlike previous SCSI standards, SCSI-3 (Fast-20 speed) requires active termination.

This standard was introduced c. 1997 and featured a low-voltage differential (LVD) bus. For this reason, Ultra-2 is sometimes referred to as LVD SCSI. LVD's greater resistance to noise allowed a maximum bus cable length of 12 meters. At the same time, the data transfer rate was increased to 80 MB/s. Mixing earlier single-ended devices (SE) and Ultra-2 devices on the same bus is possible but connecting only a single SE device forces the whole bus to single-ended mode with all its limitations, including transfer speed. The standard also introduced VHDCI, a very small connector that allows placement of four wide SCSI connectors on the back of a single PCI card slot. Ultra-2 SCSI actually had a relatively short lifespan, as it was soon superseded by Ultra-3 (Ultra-160) SCSI.

This standard featured five new optional features, which lead manufacturers to decide on two substandards to ensure consistent performance. First introduced as Ultra-160 toward the end of 1999, this iteration improved on the Ultra-2 standard by doubling the transfer rate to 160 MB/s through the use of double transition clocking, and offered new features such as CRC, a robust error-correcting process more suited for high-speed operation than the parity checking used previously, and domain validation for negotiating maximum performance for each device on the chain.[11]

However, Ultra-160 merely represents a subset of Ultra-3, lacking its two other new features, packetization and quick arbitration and selection (QAS). Packetization refers to a protocol with a reduced number of bus communication phases for less command and protocol overhead, while QAS reduces arbitration time by eliminating bus free time. Devices supporting these features were marketed as Ultra-160+ or U3. 8-bit bus width as well as HVD operation were eliminated starting with Ultra-3.[10]

Ultra-320 included the Ultra-160+ features as mandatory, doubled the clock to 80 MHz for a maximum data transfer rate of 320 MB/s, and included read/write data streaming for less overhead on queued data transfers, as well as flow control.[10] The latest working draft for this standard is revision 10 and is dated May 6, 2002. Nearly all SCSI hard drives being manufactured at the end of 2003 were Ultra-320 devices.

Ultra-640 (otherwise known as Fast-320) was promulgated as a standard (INCITS 367-2003 or SPI-5) in early 2003. It doubles the interface speed yet again, this time to 640 MB/s. Ultra-640 pushes the limits of LVD signaling; the speed limits cable lengths drastically, making it impractical for more than one or two devices. Because of this, manufacturers skipped over Ultra640 and developed for Serial Attached SCSI instead.

SCSI devices in the same SCSI transport family are generally backward-compatible. Within the parallel SCSI family, for example, it is possible to connect an Ultra-3 SCSI hard disk to an Ultra-2 SCSI controller albeit with reduced speed and feature set.

However, there are some compatibility issues with parallel SCSI buses. Ultra-2, Ultra-160 and Ultra-320 devices may be freely mixed on the parallel LVD bus with no compromise in performance, as the host adapter will negotiate the operating speed and bus management requirements for each device. Single-ended and LVDS devices can be attached to the same bus, but all devices will run at a slower, single-ended speed.

On some host adapters, this problem is solved by using a SCSI bridge to electrically split the bus into an SE and an LVD half, so LVD devices can operate at full speed.[13] Other adapters may provide multiple buses (channels).

The SPI-5 standard (which describes up to Ultra-640) deprecates single-ended devices, so some devices may not be electrically backward compatible.

Both narrow and wide SCSI devices can be attached to the same parallel bus. All the narrow SCSI devices must be placed at one end and all the wide SCSI devices at the other end. The high half of the bus needs to be terminated in between because the high half of the bus ends with the last wide SCSI device. You can get a cable designed to connect the wide part of the bus to the narrow part which either provides a place to plug in a terminator for the high half or includes the terminator itself. This is sometimes referred to as a cable with high-9 termination. Specific capability commands allow the devices to determine whether their partners are using the whole wide bus or just the lower half and drive the bus accordingly.

As an example of a mixed bus, consider a SCSI wide host adapter with an HD-68 male connector connected to a SCSI narrow disk drive with an HD-50 female connector. You might make this connection with a cable that has an HD-68 female connector on one end and an HD-50 male connector on the other. Inside the cable's HD-68 connector, there is termination for the high half of the bus and the cable contains wires for only the low half. The host adapter determines that the disk drive uses only the low half of the bus, so talks to it using only the lower half. The converse example—a SCSI narrow host adapter and SCSI wide disk drive also works.

Alternatively, each narrow device can be attached to the wide bus through an adapter. As long as the bus is terminated with a wide – internal or external – terminator, there is no need for special termination.

Single Connector Attachment (SCA) parallel SCSI devices may be connected to older controller/drive chains by using SCA adapters. Although these adapters often have auxiliary power connectors, caution is recommended when connecting them, as it is possible to damage devices by connecting external power.

Each parallel SCSI device (including the computer's host adapter) must be configured to have a unique SCSI ID on the bus. Another requirement is that any parallel SCSI bus must be terminated at both ends with the correct type of terminator. Both active and passive terminators are in common use, with the active type much preferred (and required on LVD buses and Ultra SCSI). Improper termination is a common problem with parallel SCSI installations. In early SCSI buses, one had to attach a physical terminator to each end, but several generations' SCSI devices often have terminators built in, and the user simply needs to enable termination for the devices at either end of the bus (typically by setting a DIP switch or moving a jumper). Some later SCSI host adapters allow the enabling or disabling of termination through BIOS setup. Advanced SCSI devices automatically detect whether they are last on the bus and switch termination on or off accordingly.

SCSI Configured Automatically (initially Automagically) was an optional method to configure the SCSI ID without requiring user intervention for easier installation and to avoid problems.[14] It was dropped from later standards.

SCSI interfaces had become impossible to find for laptop computers. Adaptec had years before produced PCMCIA parallel SCSI interfaces, but when PCMCIA was superseded by the ExpressCard Adaptec discontinued their PCMCIA line without supporting ExpressCard. Ratoc produced USB and FireWire to parallel SCSI adaptors, but ceased production when the integrated circuits required were discontinued. Drivers for existing PCMCIA interfaces were not produced for newer operating systems. Since 2013, with the release of various ExpressCard and Thunderbolt-to-PCI Express adapters, it is again possible to use SCSI devices on laptops, by installing PCI Express SCSI host adapters using a laptop's ExpressCard or Thunderbolt port.