• IDC header - 50 pin insulation displacement connector (IDC) used on ribbon cables for internal SCSI cabling. Female type used on cables, male on deice or host adapter. (veiw connector)
  
• 50 Pin Centronics - SCSI Alternative 2, A-cable - This connector is a 50-pin version of the 36-pin connector used for parallel printer ports for years. Specified in SASI and still an acceptable connector for NARROW SCSI. Male connector used for external cables, female for external devices. Uses bail fasteners on the device-mounted female connector to secure the male connector. Being replaced by the 50-pin male high-density micro-D connector because of smaller footprint and more reliable connection. (veiw connector)
  
• 50 Pin Micro-D (narrow high-density) - Also called Alternative 1, A-cable in SCSI-2 it is replacing the SUB-D 50-pin Alternative 1 connector defined in SCSI-1. A smaller connector than the Centronics type with pin-and-socket connections and a more reliable connection. Specified to have spring-latch fasteners but may be found with screw locks in rare cases. Male connector used for external cables, female for external devices. (veiw connector)
  
• 68 Pin Micro-D (wide high-density) - Used on the P-cable for 16-bit WIDE SCSI. 68-pin version of the 50-pin micro-D high-density connector. Although the thumbscrew fastener is specified in SCSI-3, some manufacturers have used the spring-latch fastener. Male connector used for external cables, female for external devices. (veiw connector)
  
• DB-25 - Not specified in the SCSI specifications, this 25-pin connector can support SLOW 8-bit (NARROW) SCSI only and then only with very short cables. Macintosh version became a de facto standard . Male connector on cable; female on devices. (veiw connector)
  
• 30 Pin HDI (PowerBook) - A non-standard connector created by Apple for reduced mounting space on their PowerBook notebooks. Not suitable for multiple SCSI devices or long cables because there are only 30 pins. (veiw connector)
  
• 60 Pin High-Density - A non-standard connector used by IBM. Early in the process of writing the SCSI-2 specification, the ANSI X3T9.2 committee specified a 60-pin connector for 8-bit SCSI which was later abandoned. IBM, however retained this connector for their PS/2 systems. Rare fellow indeed! (view connnector)

Connect
The function that occurs when a SCSI initiator selects a SCSI target to start an operation, or when a SCSI target selects an SCSI initiator to continue an operation.

Converter
Powered device designed to convert between single-ended and differential SCSI signals or HVD to LVD. Allows placement of differing devices on a bus connected to a host and vice versa. Properly designed converters may be used back-to-back to extend single-ended or differential SCSI up to 61 meters (200 feet). Termed an "expander" in SCSI-3. See Rancho Technology (http://www.rancho.com) or Paralan (http://www. Paralan.com) for your best selection of this type of equipment.

Data rate
The rate at which data is transferred across the SCSI bus during the data transfer phase. The data rate achieved is dependent upon the bus width (8-bit or 16-bit) and the transfer rate. For example an 8-bit bus operating at 10 Mhz (or Fast) has a data rate of 10 Megabytes per second (See Chart #1).

Device
Any single unit on the SCSI bus identifiable by a unique SCSI address. In the context of SCSI, the term device can be applied to hosts as well as peripherals. A SCSI device can act as an initiator or a target.

DIFFSENSE
A signal on differential SCSI cabling (Pin 21 on NARROW and pin 16 on WIDE) that is used as an active HIGH enable for differential drivers. If a single-ended device is connected to the bus, this line is pulled low. This disables the differential drivers to protect them from trying to drive signals into ground. Used by Universal transceivers (LVD/SE) to determine if the SCSI device is connected to single-ended or differential cabling.

Differential interface
A SCSI bus configuration in which each signal is sent on two wires. The signal is derived by taking the difference in voltage between the two wires, effectively eliminating unwanted noise in the wire. A differential interface is designed to be more (as compared to a single-ended configuration) noise immune and supports longer, up to 25 meter, SCSI bus lengths. Differential interfaces are generally used to externally interconnect systems and peripheral devices.

Disconnect
The action that occurs when a SCSI target releases control of the SCSI bus allowing the bus to go to the Bus Free phase.

FCP - Fibre Channel Protocol
See Fibre Channel

FC-AL - Fibre Channel Arbitrated Loop
See Fibre Channel

FPT - Force Perfect Terminator
See Terminators

Fast
A term used to describe a SCSI-2 defined synchronous transmission rate of between 5 MHz and 10 MHz.

Fast-20
A simile for Ultra SCSI.

Fast SCSI
A term used to describe an 8-bit wide bus operating Fast (between 5 and 10 MHz). The maximum data rate of a Fast SCSI device or bus is 10 Mbytes/sec.

Fast and Wide SCSI
A term used to describe a 16-bit wide bus operating Fast (between 5 and 10 MHz). The maximum data rate of a Fast and Wide SCSI device or bus is 20 Mbytes/sec.

Fibre Channel
An ANSI standard that specifies high-speed serial communication between devices. Fibre channel supports serial communications across both copper and optical connections. Fibre Channel is used as one of the serial bus architectures supported by SCSI-3.

Firewire
Apple's name for their implementation of IEEE 1394

HVD - High Voltage Differential
Differential SCSI scheme that has been in use for years. Terminators run on 5 VDC. See also Differential and LVD (Low Voltage Differential).

High Performance Serial Bus (IEEE 1394)
Serial SCSI in SCSI-3 will include mappings for IEEE 1394 as well as FC-AL and SSA. Designed by Apple as a serial replacement for parallel SCSI and called "Firewire" by them. Uses three twisted pair copper cables and, like other serial SCSI schemes has no terminators and no IDs to contend with. Logically it looks like a bus, just like parallel SCSI. It supports isochronous transfers so it is very attractive to time-dependent data applications such as video and audio. Currently supports transfers at 100 Megabits/second (about 10 MBytes/second) but devices are under development to increase this to 200 or 400 Megabits/second. Cable lengths can be up to 5 meters "per hop" with up to 63 nodes or devices. Just starting to show up in consumer electronics such as home entertainment systems with VCR, video cameras, etc.

Host
A processor or system usually consisting of a CPU and memory. A host communicates with other devices such as peripherals and other hosts on a SCSI bus. On the SCSI bus, a host is known by its SCSI address or SCSI ID.

ID
The unique address of a SCSI device. 8-bit SCSI can have up to eight IDs; 16-bit up to sixteen IDs; 32-bit up to 32 IDs. There must be a minimum of one target and one initiator on the bus. SCSI IDs range from 0 to 7 for 8-bit, 0 to 15 for 16-bit and 0 to 31 for 32 bit systems. The host is usually assigned ID #7.

Initiator
A SCSI device (usually a host system) that requests an operation to be performed by another SCSI device (target).

I/O
Input / Output.

IEEE
Institute of Electrical and Electronics Engineers. An organization that promotes electrical and electronics standards.

(IEEE 1394)
Serial SCSI in SCSI-3 will include mappings for IEEE 1394 as well as FC-AL and SSA. Designed by Apple as a serial replacement for parallel SCSI and called "Firewire" by them. Uses three twisted pair copper cables and, like other serial SCSI schemes has no terminators and no IDs to contend with. Logically it looks like a bus, just like parallel SCSI. It supports isochronous transfers so it is very attractive to time-dependent data applications such as video and audio. Currently supports transfers at 100 Megabits/second (about 10 MBytes/second) but devices are under development to increase this to 200 or 400 Megabits/second. Cable lengths can be up to 5 meters "per hop" with up to 63 nodes or devices. Just starting to show up in consumer electronics such as home entertainment systems with VCR, video cameras, etc.

LVD - Low Voltage Differential
Refer to "LVD" under "Terminator".

Logical Unit
A virtual device addressable through a target SCSI device. A physical SCSI device can have more than one logical unit. Logical units are supported only by SCSI targets.

LUN
Logical Unit Number. The value used to identify a logical unit of a SCSI device. In the SCSI-2 specification, there may be up to 8 logical units for each SCSI device address. These logical units are numbered from 0-7.

MByte or Megabyte
A measurement of approximately one million bytes.

MHz
Megahertz. A measurement of one million cycles per second. Used to describe the cycle rate of the SCSI bus.

Multi-threaded SCSI
The ability of SCSI to initiate and complete more than one task at a time. A result of the "intelligence" of SCSI that allows a device to disconnect from the bus while it is performing a task, such as locating some information at a specific address, and, once it has located the information, reconnect to the bus in order to complete its task. Optimizes the use of the bus bandwidth. Refer to disconnect and reconnect.

Narrow
A term used to describe a SCSI bus that is 8-bits wide.

P1394
An IEEE standard serial bus also known as FireWire. The intent of P1394 is to provide a physical bus that was competitive with SCSI parallel buses in both cost and speed, had no configuration switches, no terminators, a flexible topology, and deterministic latency.

P-cable
A 68-wire cable used for 16-bit SCSI-3 buses. P-cables can be used with Q-cables for 32-bit SCDSI-3 buses.

P-to-A transition cable
An adapter used to connect 8-bit SCSI-1 devices using A-cables to a 16-bit or 32 bit SCS-3 device using P-cables.

Parallel
The transfer of bits over multiple wires at one time. Accomplished by devoting a separate wire for each bit of a byte.

Parity checking
A simple way of detecting errors in SCSI data that is required to be built into all SCSI-2 devices and will be continued in future issues of SCSI. Counts the number of 1's in a byte of data and sets a parity bit so that the number is always odd or even. SCSI uses odd parity. You can use parity checking only if all devices on the bus use parity checking.

Peripheral Device
A device that can be attached to a host computer or server using a SCSI bus. Typical peripheral devices include disk drives, tape drives, scanners, and printers.

Port
A connection to a bus. The single-byte (narrow or 8-bit) SCSI bus (SCSI-1) allows eight ports. The 16-bit wide SCS-2 Wide bus allows sixteen ports.

Q-cable
A 68-wire cable used in conjunction with a P-cable for 32-bit SCSI buses.

RAID
Redundant Array of Inexpensive Disks or Redundant Array of Independent Disks

Reconnect
The concept of disconnect-reconnect is what provides the ability of SCSI to initiate and complete more than one task at a time. A result of the "intelligence" of SCSI that allows an initiator or target to disconnect from the bus while it is performing a task, such as locating some information at a specific address, and, once it has located the information, reconnect to the bus in order to complete its task. Optimizes use of the bus bandwidth. See also "disconnect".

SSA
An open serial interface standard developed by IBM called Serial Storage Architecture that has been submitted to the ANSI X3T10.1 subcommittee for approval as an ANSI standard. Incorporates a dual port full-duplex module capable of maintaining four conversations simultaneously for a total of up to 80 MBytes/sec. Includes multiple signal paths for fault tolerance and provides hot plugging and automatic configuration when nodes are added. With shielded twisted pair cable, nodes can be up to 20 meters (65.6 feet) apart. The optical fiber implementation extends this to 2.5 Km. A loop configuration can support up to 127 nodes. Uses 9-pin miniature D-shell connectors with two twisted wire pairs. Requires modified system firmware when interfacing to SCSI. Mapping for SSA will be included in SCSI-3.

SASI
Shugart Associates Standard Interface. An interface designed by Shugart Assoc. in 1980. It was the predecessor of SCSI.

SCSI
An acronym for Small Computer System Interface (pronouced "skuzzy"). An Industry standard for connecting peripheral devices and their controllers to a microprocessor. The SCSI defines both hardware and software standards for communication between a host computer and a peripheral. Computers and peripheral devices designed to meet SCSI specifications should work together. See also SCSI-1,SCSI-2, or SCSI-3.

SCSI-1
The first version of SCSI as defined by ANSI - X3.131-1986.

SCSI-2
The successor to SCSI-1 as defined by ANSI - X3.131-1992. SCSI-2 is upward compatible from SCSI-1. SCSI-2 specification allows for faster transmission rates, a wider bus (up to 32-bit wide), increased device addressing, and improved functionality.

SCSI-3
The SCSI-3 specification is designed to further improve functionality and accommodate high-speed serial transmission interfaces. To do so, SCSI was effectively "layered" logically. This layering allowed the software interfaces to remain relatively unchanged while accommodating new physical interconnect schemes based upon serial interconnects such as Fibre Channel and Serial Storage Architecture (SSA).

SCSI Address
is an octal representation of the unique address assigned to a SCSI device. Also called a SCSI ID.

SCSI ID
See SCSI address.

Serial
The transfer of bits over a single wire or fibre optic in a sequential fashion.

Serial Storage Architecture (SSA)
An architecture defining a serial physical interface and supporting protocol developed by IBM and now supported by the ANSI X3T10.1 committee. SSA supports point-to-point, loop, and multiple-ring topologies. Each node of an SSA topology can support up to 2 full duplex ports operating at 20 MBytes/sec per port. SCSI commands, data transfers, and responses are transported across an SSA topology.

Single-Ended Interface
One of the SCSI bus electrical signal configurations. On a single-ended configuration, each signal is carried by a signal wire. Single-ended buses are more susceptible to noise than differential buses. Traditionally, a single-ended interface was designed to use fewer (as compared to a differential configuration) pins, chips and PCB area. It cost less, was smaller, but it also is more sensitive to noise, more restrictive in physical devices spacing, and more restrictive in length (up to 6 meters). Single-ended interfaces are generally found inside subsystems that use SCSI to interconnect internal components (for example RAID arrays).

Stub
An un-terminated SCSI bus segment branching off the main SCSI bus. The SCSI specification dictates that a stub is to be no longer than 0.1 meters (4 inches) for single-ended SCSI and no longer than 0.2 meters (8 inches) for differential SCSI. Stubs cause lots of difficult-to-trace problems. Stubs are unavoidable but keep them to a minimum and remember that SCSI devices have stubs that cannot be removed (the PC board traces from the connector on a disk drive to its electronic circuitry are stubs).

Synchronous Transmission
A transmission in which the sending and receiving devices operate continuously at the same frequency. Synchronous transmission allows many bytes of data to be sent before acknowledgment is received from the target. Data can be transferred synchronously faster than is can be transferred asynchronously. In SCSI, only data can be sent synchronously. Commands, messages, and status are sent asynchronously.

Tagged Command Queuing
A SCSI-2 feature that is used when the initiator wants to send multiple commands to the same SCSI address or LUN. Tagged queues allow the target to store up to 256 commands per initiator. Without tagged queues, targets could support only one command per LUN for each initiator on the bus. Per the SCSI-2 specification, tagged queue support by targets is optional.

Target
A SCSI device (usually a peripheral) that performs an operation as requested by a SCSI initiator.

Terminator (Terminations)

Electrical circuitry at the end of a cable to prevent the reflection of electrical signals when they reach the end of the cable. In SCSI systems, this electrical circuitry is called a terminator. It should be noted that any SCSI bus segment requires two terminators and only two terminators. Not one, not three, but two terminators. Also, the terminators must be installed at the very ends of the SCSI cable, not at devices in the middle of the bus. Terminators require power that is usually provided by the host adapter on the TERMPWR line(s) on the bus. Many SCSI devices power their own terminators. There are five basic types of SCSI termination: Active, active negation, FPT, LVD (including LVD/SE) and passive.

• Active - Termed "Alternative 2" in SCSI-2 - Because fluctuations in the TERMPWR voltage supplied to passive terminators shows up as fluctuations in signal levels, active terminators include a voltage regulator to reduce the effect of fluctuations in TERMPWR to insignificance. Uses only a 110 ohm resistor from the regulator to the signal line which is a much closer match to the SCSI cable impedance. Results in more stable SCSI signals, less signal reflection and fewer data errors.
  
• Active Negation - A method of providing single-ended termination that uses voltages to drive the bus signal not just high as in active termination, but high and low. Results in faster switching which is necessary for FAST and Ultra-SCSI speeds.
  
• Force Perfect Termination (FPT) - Single-ended termination method utilizing diode switching and biasing to actively compensate for impedance mismatches between the SCSI cabling and the peripheral device. You should be aware that there are several designs of FPT that may not be totally compatible. Also, our customers have found that, generally speaking, FPT likes to "talk" only to FPT.
  
• HVD - High Voltage Differential - Differential SCSI scheme that has been in use for years. Terminators run of 5 VDC. See also LVD.
  
• LVD - Low Voltage Differential - A method of driving SCSI cables that will be formalized in the SCSI-3 specifications. Lower power consumption than the current differential drive (HVD), is less expensive and will allow the higher speeds of Ultra-2 SCSI. Requires 3.3 VDC instead of 5 VDC for HVD.
  
• LVD/SE - LVD that uses "Universal" transceivers. Depending on the voltage level appearing on the DIFFSENSE pin of the cable, the Universal transceivers of LVD/SE will be automatically configured for LVD or single-ended (SE). Most new SCSI designs will include Universal transceivers.
  
• Passive - Termed "Alternative 1" in SCSI-2 - The simplest form of terminator consisting of a 220 ohm resistor from TERMPWR to the signal line and a 330 ohm resistor from the signal line to ground. Low cost but has the disadvantage that any fluctuations in the TERMPWR voltage will show up on the signal lines of the bus which may cause data errors. SCSI-2 recommends the use of active terminators whenever possible for single-ended SCSI. Differential SCSI uses only passive terminators.
  
• Universal - See LVD/SE.

Term power
The voltage (+5 VDC) placed on the TERMPWR line(s) of the SCSI bus used to power terminators. SCSI requires that the host adapter provide term power. Many peripheral devices are also capable of providing term power. Having more than one device on the bus providing term power does no harm and is often desirable to reduce problems of voltage "droop" caused by IR losses in long SCSI cables. LVD requires a 3.3 VDC TERMPOWER instead of 5 VDC.

Ultra
A term used to describe a SCSI defined synchronous transmission rate of between 10 MHz and 20 MHz.

Ultra SCSI
A term used to describe an 8-bit wide bus operating Ultra (between 10 MHz and 20 MHz). The maximum data rate of an Ultra SCSI device or bus is 20 Mbytes/sec. Sometimes referred to as Fast-20.

Ultra2
A term used to describe a SCSI defined synchronous transmission rate of between 20 MHz and 40 MHz.

Ultra2 SCSI
A term used to describe a 8-bit wide bus operating Ultra (between 20 MHz and 40 MHz). The maximum data rate of a Ultra SCSI device or bus is 40 Mbytes/sec.

Wide
A term used to describe a SCSI bus that is 16-bits in width.

Wide Ultra SCSI
A term used to describe a 16-bit wide bus operating Ultra (between 10 MHz and 20 MHz). The maximum data rate of a Ultra SCSI device or bus is 40 Mbytes/sec.

Wide Ultra2 SCSI
A term used to describe a 16-bit wide bus operating Ultra (between 20 MHz and 40 MHz). The maximum data rate of a Ultra SCSI device or bus is 80 Mbytes/sec.

X3.131-1986
The document describing the specifications of the SCSI-1 standa
rd.

X3.131-1994
The document describing the specifications of the SCSI-2 standard.

0
The ANSI committee responsible for organizing, defining, and promoting SCSI standards.

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