Parallel SCSI
[a] The Symbios Logic 53C810 chip is an example of a PCI host interface that can act as a SCSI target.
The SCSI-3 standard then split the framework into separate layers which allowed the introduction of other data interfaces beyond parallel SCSI.
The iSCSI protocol doesn't describe a data interface but uses any IP network, usually run over Ethernet.
Individual vendors and the SCSI Trade Association have given names to specific combinations of capabilities.
Similarly, no version of the standard requires Low-voltage differential signaling (LVD), but products called Ultra-2 SCSI include this capability.
This terminology is helpful to consumers because Ultra-2 SCSI device has a better-defined set of capabilities than simply identifying it as SCSI-3.
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).
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.
The 32-bit implementation was largely ignored because it was deemed expensive and unnecessary, and was officially retired in SCSI-3.
[6] A high-voltage differential (HVD) mode that was incompatible with standard single-ended (SE) was introduced to accommodate longer bus lengths.
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.
Unlike previous SCSI standards, SCSI-3 (Fast-20 speed) requires active termination.
LVD's greater resistance to noise allowed a maximum bus cable length of 12 meters.
Not all phases will occur in all cases: The above list does not imply a specific sequence of events.
Specific types of connectors for parallel SCSI devices were developed by vendors over time.
Parallel SCSI buses must always be terminated at both ends to ensure reliable operation.
A positive DC termination voltage is provided by one or more devices on the bus, typically the host adapter.
This is called a diode-OR circuit, designed to prevent backflow of current to the supplying device.
In current practice most parallel SCSI buses are LVD and so require external, active termination.
There is a special case in SCSI systems that have mixed 8-bit and 16-bit devices where high-byte termination may be required.
Different SCSI transports, which are not compatible with each other, usually have unique connectors to avoid accidental mis-plugging of incompatible devices.
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.
Drivers for existing PCMCIA interfaces were not produced for newer operating systems.
Ratoc produced USB and FireWire to parallel SCSI adaptors, but ceased production when the integrated circuits required were discontinued.
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.
