On RAID 3 systems, datablocks are subdivided (striped) and written in parallel on two or more drives. An additional drive stores parity information. You need at least 3 disks for a RAID 3 array.
Since parity is used, a RAID 3 stripe set can withstand a single disk failure without losing data or access to data.
Advantages
RAID-3 provides high throughput (both read and write) for large data transfers.
Disk failures do not significantly slow down throughput.
Disadvantages
This technology is fairly complex and too resource intensive to be done in software.
Performance is slower for random, small I/O operations.
Ideal use
RAID 3 is not that common in prepress.
Wednesday, October 22, 2008
RAID 1: mirroring
Data are stored twice by writing them to both the data disk (or set of data disks) and a mirror disk (or set of disks) . If a disk fails, the controller uses either the data drive or the mirror drive for data recovery and continues operation. You need at least 2 disks for a RAID 1 array.
RAID 1 systems are often combined with RAID 0 to improve performance. Such a system is sometimes referred to by the combined number: a RAID 10 system.
Advantages
RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single disk.
In case a disk fails, data do not have to be rebuild, they just have to be copied to the replacement disk.
RAID 1 is a very simple technology.
Disadvantages
The main disadvantage is that the effective storage capacity is only half of the total disk capacity because all data get written twice.
Software RAID 1 solutions do not always allow a hot swap of a failed disk (meaning it cannot be replaced while the server keeps running). Ideally a hardware controller is used.
Ideal use
RAID-1 is ideal for mission critical storage, for instance for accounting systems. It is also suitable for small servers in which only two disks will be used.
RAID 1 systems are often combined with RAID 0 to improve performance. Such a system is sometimes referred to by the combined number: a RAID 10 system.
Advantages
RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single disk.
In case a disk fails, data do not have to be rebuild, they just have to be copied to the replacement disk.
RAID 1 is a very simple technology.
Disadvantages
The main disadvantage is that the effective storage capacity is only half of the total disk capacity because all data get written twice.
Software RAID 1 solutions do not always allow a hot swap of a failed disk (meaning it cannot be replaced while the server keeps running). Ideally a hardware controller is used.
Ideal use
RAID-1 is ideal for mission critical storage, for instance for accounting systems. It is also suitable for small servers in which only two disks will be used.
RAID 0: striping
In a RAID 0 system, data are split up in blocks that get written across all the drives in the array. By using multiple disks (at least 2) at the same time, RAID 0 offers superior I/O performance. This performance can be enhanced further by using multiple controllers, ideally one controller per disk.
Advantages
RAID 0 offers great performance, both in read and write operations. There is no overhead caused by parity controls.
All storage capacity can be used, there is no disk overhead.
The technology is easy to implement.
Disadvantages
RAID 0 is not fault-tolerant. If one disk fails, all data in the RAID 0 array are lost. It should not be used on mission-critical systems.
Ideal use
RAID 0 is ideal for non-critical storage of data that have to be read/written at a high speed, e.g. on a PhotoShop image retouching station.
Advantages
RAID 0 offers great performance, both in read and write operations. There is no overhead caused by parity controls.
All storage capacity can be used, there is no disk overhead.
The technology is easy to implement.
Disadvantages
RAID 0 is not fault-tolerant. If one disk fails, all data in the RAID 0 array are lost. It should not be used on mission-critical systems.
Ideal use
RAID 0 is ideal for non-critical storage of data that have to be read/written at a high speed, e.g. on a PhotoShop image retouching station.
RAID levels
RAID stands for Redundant Array of Inexpensive Disks. A RAID system consists of two or more disks working in parallel. They appear as one drive to the user, and offer enhanced performance or security (or both).
The software to perform the RAID-functionality and control the hard disks can either be located on a separate controller card (a hardware RAID controller) or it can simply be a driver. Both Windows NT 4 and 2000 include a software RAID solution. Hardware RAID controllers cost more than pure software but they also offer better performance.
Most RAID-systems are based on SCSI, although implementations using IDE disks or FC (fibre channel) disks also exist. There are even systems that use IDE disks internally but that have a SCSI-interface for the host system.
There are different RAID levels, each suiting specific situations. RAID levels are not standardized by an industry group. This explains why companies are sometimes creative and come up with their own unique implementations.
Sometimes disks in a RAID system are defined as JBOD, which stands for 'just a bunch of disks'. This means that those disks do not use a specific RAID level and are used as if they were stand-alone disks. This is often done for disks that contain swap files or spooling data.
The software to perform the RAID-functionality and control the hard disks can either be located on a separate controller card (a hardware RAID controller) or it can simply be a driver. Both Windows NT 4 and 2000 include a software RAID solution. Hardware RAID controllers cost more than pure software but they also offer better performance.
Most RAID-systems are based on SCSI, although implementations using IDE disks or FC (fibre channel) disks also exist. There are even systems that use IDE disks internally but that have a SCSI-interface for the host system.
There are different RAID levels, each suiting specific situations. RAID levels are not standardized by an industry group. This explains why companies are sometimes creative and come up with their own unique implementations.
Sometimes disks in a RAID system are defined as JBOD, which stands for 'just a bunch of disks'. This means that those disks do not use a specific RAID level and are used as if they were stand-alone disks. This is often done for disks that contain swap files or spooling data.
RAID TECHNOLOGY.
Short for Redundant Array of Independent (or Inexpensive) Disks, a category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren't generally necessary for personal computers.
There are number of different RAID levels:
• Level 0 -- Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.
• Level 1 -- Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.
• Level 2 -- Error-Correcting Coding: Not a typical implementation and rarely used, Level 2 stripes data at the bit level rather than the block level.
• Level 3 -- Bit-Interleaved Parity: Provides byte-level striping with a dedicated parity disk. Level 3, which cannot service simultaneous multiple requests, also is rarely used.
• Level 4 -- Dedicated Parity Drive: A commonly used implementation of RAID, Level 4 provides block-level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks.
• Level 5 -- Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID.
• Level 6 -- Independent Data Disks with Double Parity: Provides block-level striping with parity data distributed across all disks.
• Level 0+1 – A Mirror of Stripes: Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them. Used for both replicating and sharing data among disks.
• Level 10 – A Stripe of Mirrors: Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.
• Level 7: A trademark of Storage Computer Corporation that adds caching to Levels 3 or 4.
• RAID S: EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems.
There are number of different RAID levels:
• Level 0 -- Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.
• Level 1 -- Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.
• Level 2 -- Error-Correcting Coding: Not a typical implementation and rarely used, Level 2 stripes data at the bit level rather than the block level.
• Level 3 -- Bit-Interleaved Parity: Provides byte-level striping with a dedicated parity disk. Level 3, which cannot service simultaneous multiple requests, also is rarely used.
• Level 4 -- Dedicated Parity Drive: A commonly used implementation of RAID, Level 4 provides block-level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks.
• Level 5 -- Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID.
• Level 6 -- Independent Data Disks with Double Parity: Provides block-level striping with parity data distributed across all disks.
• Level 0+1 – A Mirror of Stripes: Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them. Used for both replicating and sharing data among disks.
• Level 10 – A Stripe of Mirrors: Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.
• Level 7: A trademark of Storage Computer Corporation that adds caching to Levels 3 or 4.
• RAID S: EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems.
SCSI Buses - An introduction to SCSI technology
Introduction
Credit for this information should go to Dinah McNutt.
SCSI is an 8-bit parallel I/O bus.
It comes in 3 flavours, SCSI-1, SCSI-2 and SCSI-3. SCSI-1 is the only one which is an official standard, as of the middle of 1994. The current SCSI-1 standard was finalized in 1986.
Incompatibility between SCSI host adapters and SCSI devices is still a monumental problem. I speak from experience. Many manufacturers who produce allegedly SCSI-2 devices actually produce incompatible SCSI-2 devices.
Worse, SCSI hard disk prices are 2 or 3 times the price is same-size IDE hard disks.
Devices
Up to 8 devices can be attached to a single SCSI bus. One of these is the host adapter, leaving room for 7 'real' devices.
No matter how many devices are actually connected, only one pair of devices can communicate with each other at any one time.
Addresses
SCSI uses a 3-bit addressing scheme, where each device is assigned an address ranging from 0 to 7.
Device 7 has the highest priority, so the host computer is usually assigned to be device 7.
Initiator/Target = Client/Server
Communication occurs when the initiator, which is typically the host computer, originates a request, and the target (eg a device controller) performs the request.
The SCSI lets all devices communicate with each other, but some devices are implemented in such a way that the cannot initiate communications.
Electrical Specifications
There are 2 electrical specifications for SCSI: single-ended and differential.
Single-ended is limited to a maximum cable length of 6m.
Differential supports cables up to 25 m in length. Differential is much more expensive to design and manufacture.
Protocols
SCSI devices can use either asynchronous or synchronous communication protocols.
In the original SCSI specification, synchronous communication allowed speeds of up to 5 Mb/sec. Note that if your SCSI bus is short, asychronous communication can be fast.
SCSI-2
SCSI-2 extends the original Common Command Set (CCS) to support CD-ROMs, scanners, communications devices and optical memory drives (eg WORM and erasable media).
SCSI-2 supports 8-bit, 16-bit and 32-bit buses; also it supports up to 10 Mb/sec on 8-bit buses, and thus up to 40 Mb/sec on 32-bit buses.
Fast-Wide SCSI-2
5 Mb/sec 10 Mb/sec
8-bit Fast
16-bit Wide Fast-Wide
32-bit Wide Fast-Wide
The SCSI-2 specification allows fast SCSI only on differential SCSI buses, but in practice vendors sell single-ended fast SCSI devices.
SCSI-3
SCSI-3 is planned to support optical fibre, longer cables, and more that 8 targets per bus.
Also, it will run at up to 20 Mb/sec.
SCSI-3 will include a specification for serial SCSI over fibre-optic or high-speed copper cabling. Speeds aimed for are 51 Mb/sec up to 1 Gb/sec. These serial proposals are called IEEE P1394 (nicknamed FireWire), Fibre Channel and Serial Storage Architecture (SSA).
Another advantage of serial is that fewer wires are required, simplifying cabling. Frankly though, mixing serial SCSI and parallel SCSI looks like a recipe for disaster
Credit for this information should go to Dinah McNutt.
SCSI is an 8-bit parallel I/O bus.
It comes in 3 flavours, SCSI-1, SCSI-2 and SCSI-3. SCSI-1 is the only one which is an official standard, as of the middle of 1994. The current SCSI-1 standard was finalized in 1986.
Incompatibility between SCSI host adapters and SCSI devices is still a monumental problem. I speak from experience. Many manufacturers who produce allegedly SCSI-2 devices actually produce incompatible SCSI-2 devices.
Worse, SCSI hard disk prices are 2 or 3 times the price is same-size IDE hard disks.
Devices
Up to 8 devices can be attached to a single SCSI bus. One of these is the host adapter, leaving room for 7 'real' devices.
No matter how many devices are actually connected, only one pair of devices can communicate with each other at any one time.
Addresses
SCSI uses a 3-bit addressing scheme, where each device is assigned an address ranging from 0 to 7.
Device 7 has the highest priority, so the host computer is usually assigned to be device 7.
Initiator/Target = Client/Server
Communication occurs when the initiator, which is typically the host computer, originates a request, and the target (eg a device controller) performs the request.
The SCSI lets all devices communicate with each other, but some devices are implemented in such a way that the cannot initiate communications.
Electrical Specifications
There are 2 electrical specifications for SCSI: single-ended and differential.
Single-ended is limited to a maximum cable length of 6m.
Differential supports cables up to 25 m in length. Differential is much more expensive to design and manufacture.
Protocols
SCSI devices can use either asynchronous or synchronous communication protocols.
In the original SCSI specification, synchronous communication allowed speeds of up to 5 Mb/sec. Note that if your SCSI bus is short, asychronous communication can be fast.
SCSI-2
SCSI-2 extends the original Common Command Set (CCS) to support CD-ROMs, scanners, communications devices and optical memory drives (eg WORM and erasable media).
SCSI-2 supports 8-bit, 16-bit and 32-bit buses; also it supports up to 10 Mb/sec on 8-bit buses, and thus up to 40 Mb/sec on 32-bit buses.
Fast-Wide SCSI-2
5 Mb/sec 10 Mb/sec
8-bit Fast
16-bit Wide Fast-Wide
32-bit Wide Fast-Wide
The SCSI-2 specification allows fast SCSI only on differential SCSI buses, but in practice vendors sell single-ended fast SCSI devices.
SCSI-3
SCSI-3 is planned to support optical fibre, longer cables, and more that 8 targets per bus.
Also, it will run at up to 20 Mb/sec.
SCSI-3 will include a specification for serial SCSI over fibre-optic or high-speed copper cabling. Speeds aimed for are 51 Mb/sec up to 1 Gb/sec. These serial proposals are called IEEE P1394 (nicknamed FireWire), Fibre Channel and Serial Storage Architecture (SSA).
Another advantage of serial is that fewer wires are required, simplifying cabling. Frankly though, mixing serial SCSI and parallel SCSI looks like a recipe for disaster
SCSI TERMINATION
On SCSI drives, the Host Adapter also has resistors. These are needed to terminate both ends of the bus. Since the SCSI bus can have up to 7 devices attached to it, only the Host Adapter and the device farthest from it will retain the resistors. All devices in-between should have theirs removed. With the advent of increased use of SCSI for peripherals comes the chance that one day you'll need an SCSI terminating resistor. Prepare for a shock, because you might be very surprised at the prices charged, for what you get. Many Manufacturers still have SCSI peripheral hardware priced for the Workstation market, not the PC market. We may see these prices erode as more PCs adopt SCSI as their disk interface of choice, but for now be prepared to pay a premium for anything to do with SCSI. So here you are, with a disk drive mounted internally, and a CD-ROM hanging off the back of the PC. Everything looks great, but it just doesn't work... Maybe it doesn't even recognise the CD-ROM. You've checked the connectors, and everything looks good... So what's the problem? Well, did you check the terminators? (Say Whaaat??) Improper termination of an SCSI bus can raise havoc with the Host Adapter's interface circuit, and result in missing peripherals, or intermittent operation and possible loss of data. Well, here's a way to build an inexpensive terminator that will connect to the second SCSI connector on many SCSI peripherals.
SCSI termination is very important, there must be two terminator on each SCSI chain, one at the beginning and one at the end device chain. The terminator must be within 10 centimeters (4 inches) of each end of the chain on the bus. Take a look at figure 1 to see what is acceptable and what's not when terminating SCSI devices.
The following ARE acceptable:
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A Unconnected Unconnected Unconnected DEVICE B DEVICE C
Terminated (adapter-Terminated)
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A Unconnected DEVICE B Unconnected Unconnected DEVICE C
Terminated (adapter) Terminated
+------------+----------+-----------+-----------+---------+
| | | | | |
Terminated DEVICE A DEVICE B Unconnected Unconnected DEVICE C
(adapter) Terminated
The following ARE NOT allowed:
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A DEVICE B DEVICE C Unconnected Unconnected Unconnected
Terminated (adapter) Terminated
+------------+----------+-----------+-----------+---------+
| | | | | |
Unconnected DEVICE A DEVICE B DEVICE C Unconnected Unconnected
Terminated (adapter) Terminated
Figure 1: Correct and wrong way of connecting SCSI devices
Note: When putting the host adapter in the middle of the chain,
you must disable the host terminators. If this is impossible you MUST put the host adapter at either end of the cable.
SCSI CONNECTORS
Names
External Connector Types (not to scale)
Maximum
Speed
(MB/s) Number of
Devices (A,B)
Max Total
Length of Cable Uses Comments
SCSI-1
50-pin low-density (SCSI-1, Centronix, A-Cable)
>Apple DB-25C
SCSI-1 5 8 6 mD
Scanners, Zip drives, hard drives, etc. The DB-25 version is standard on Macintosh® computers.C
Many peripherals are available.
SCSI-2
SCSI-2 (Fast SCSI, Fast Narrow)
50-pin low-density (SCSI-1, Centronix, A-Cable)
50-pin high-density (SCSI-2)
10 8 ²4 devices: 3 m
5+ devices: 1.5 m Desktop hard drives, scanners, CD-ROM drives, CD-Recordable drives, and removables Backward-compatible with SCSI-1. Many products available.
"Wide" products use 16-bit instead of 8-bit data paths.
Fast Wide SCSI (Wide SCSI) 68-pin
20 16 ²4 devices: 3 m
5+ devices: 1.5 m Faster hard drives, workstations, servers and RAID systems Backward-compatible with SCSI-1. Many products available.
"Wide" products use 16-bit instead of 8-bit data paths.
SCSI-3
50-pin low-density (SCSI-1, Centronix, A-Cable)
50-pin high-density (SCSI-2)
68-pin
Ultra SCSI
(SCSI-3, Fast-20, Ultra Narrow) 20 8 1.5 mD,LVD
Newer and faster hard drives, performance desktop, workstations, servers, RAID systems Backward-compatible with previous SCSI versions.
SCSI-3 specification still under consideration, but products are available that meet current specifications. "SCAM" specification allows for devices to set own ID automatically.
Some variations of SCSI-3 are hot-pluggable.
Ultra2 devices available now.
Ultra3 devices under development.
Wide Ultra SCSI
(Fast Wide 20) 40 16 1.5 mD,LVD
Ultra2 SCSI 40 8 12 m
Wide Ultra2 SCSI 80 16 12 m
Ultra3 SCSI 80 8 12 m
Wide Ultra3 SCSI 160 16 12 m
(A): Including Host Adapter
(B): Per channel
(C): The Apple DB-25 is not an official standard. It uses SCSI-2 protocols, but at SCSI-1 speeds.
(D): Differential allows for cable lengths of up to 25 meters.
(E): The DB-9 Fibre Channel connector is the industry standard, but only the four outer pins are live.
SCSI termination is very important, there must be two terminator on each SCSI chain, one at the beginning and one at the end device chain. The terminator must be within 10 centimeters (4 inches) of each end of the chain on the bus. Take a look at figure 1 to see what is acceptable and what's not when terminating SCSI devices.
The following ARE acceptable:
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A Unconnected Unconnected Unconnected DEVICE B DEVICE C
Terminated (adapter-Terminated)
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A Unconnected DEVICE B Unconnected Unconnected DEVICE C
Terminated (adapter) Terminated
+------------+----------+-----------+-----------+---------+
| | | | | |
Terminated DEVICE A DEVICE B Unconnected Unconnected DEVICE C
(adapter) Terminated
The following ARE NOT allowed:
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A DEVICE B DEVICE C Unconnected Unconnected Unconnected
Terminated (adapter) Terminated
+------------+----------+-----------+-----------+---------+
| | | | | |
Unconnected DEVICE A DEVICE B DEVICE C Unconnected Unconnected
Terminated (adapter) Terminated
Figure 1: Correct and wrong way of connecting SCSI devices
Note: When putting the host adapter in the middle of the chain,
you must disable the host terminators. If this is impossible you MUST put the host adapter at either end of the cable.
SCSI CONNECTORS
Names
External Connector Types (not to scale)
Maximum
Speed
(MB/s) Number of
Devices (A,B)
Max Total
Length of Cable Uses Comments
SCSI-1
50-pin low-density (SCSI-1, Centronix, A-Cable)
>Apple DB-25C
SCSI-1 5 8 6 mD
Scanners, Zip drives, hard drives, etc. The DB-25 version is standard on Macintosh® computers.C
Many peripherals are available.
SCSI-2
SCSI-2 (Fast SCSI, Fast Narrow)
50-pin low-density (SCSI-1, Centronix, A-Cable)
50-pin high-density (SCSI-2)
10 8 ²4 devices: 3 m
5+ devices: 1.5 m Desktop hard drives, scanners, CD-ROM drives, CD-Recordable drives, and removables Backward-compatible with SCSI-1. Many products available.
"Wide" products use 16-bit instead of 8-bit data paths.
Fast Wide SCSI (Wide SCSI) 68-pin
20 16 ²4 devices: 3 m
5+ devices: 1.5 m Faster hard drives, workstations, servers and RAID systems Backward-compatible with SCSI-1. Many products available.
"Wide" products use 16-bit instead of 8-bit data paths.
SCSI-3
50-pin low-density (SCSI-1, Centronix, A-Cable)
50-pin high-density (SCSI-2)
68-pin
Ultra SCSI
(SCSI-3, Fast-20, Ultra Narrow) 20 8 1.5 mD,LVD
Newer and faster hard drives, performance desktop, workstations, servers, RAID systems Backward-compatible with previous SCSI versions.
SCSI-3 specification still under consideration, but products are available that meet current specifications. "SCAM" specification allows for devices to set own ID automatically.
Some variations of SCSI-3 are hot-pluggable.
Ultra2 devices available now.
Ultra3 devices under development.
Wide Ultra SCSI
(Fast Wide 20) 40 16 1.5 mD,LVD
Ultra2 SCSI 40 8 12 m
Wide Ultra2 SCSI 80 16 12 m
Ultra3 SCSI 80 8 12 m
Wide Ultra3 SCSI 160 16 12 m
(A): Including Host Adapter
(B): Per channel
(C): The Apple DB-25 is not an official standard. It uses SCSI-2 protocols, but at SCSI-1 speeds.
(D): Differential allows for cable lengths of up to 25 meters.
(E): The DB-9 Fibre Channel connector is the industry standard, but only the four outer pins are live.
CABLING
Internal devices use a 50 pin dual inline connector and a flat ribbon cable. Multiple connectors are usually attached to it in increments of around 12 inches (yet they might be farther apart). You can use any connector on the cable and in any order.
External devices use a different cable and connector for obvious reasons. The cable is usually consisted of 25 twisted pair wires, so you get a total of 50 wires. Each external device has two connectors on it's back side, they are connected to each other pin for pin and are used to connect the next device in the daisy chain or to connect the terminator. This means that you need a cable for each additional external device.
The standard SCSI connector has 50 pins in two rows and is termed as the type A connector from the SCSI specifications, this connector looks like a Centronics printer connector yet has 50 pins instead of 36. Some devices use a regular 25 pin D-shell connector, this connector is good, but can only handle single ended SCSI because they don't have enough connections.
SCSI-2 specified a new cable for wide SCSI, since the original Type A cable does not have enough connections on it. The SCSI-2 specifications indicates the use of a second cable described as the B cable, yet most developers did not follow this path and use a similar cable known as the type P cable. This cable has a 68 pin connector arranged in two rows of 34 pins just like the B cable. The difference is in the pin assignments on the connector. The B cable has since been abandoned and the standard is now the P cable.
External devices use a different cable and connector for obvious reasons. The cable is usually consisted of 25 twisted pair wires, so you get a total of 50 wires. Each external device has two connectors on it's back side, they are connected to each other pin for pin and are used to connect the next device in the daisy chain or to connect the terminator. This means that you need a cable for each additional external device.
The standard SCSI connector has 50 pins in two rows and is termed as the type A connector from the SCSI specifications, this connector looks like a Centronics printer connector yet has 50 pins instead of 36. Some devices use a regular 25 pin D-shell connector, this connector is good, but can only handle single ended SCSI because they don't have enough connections.
SCSI-2 specified a new cable for wide SCSI, since the original Type A cable does not have enough connections on it. The SCSI-2 specifications indicates the use of a second cable described as the B cable, yet most developers did not follow this path and use a similar cable known as the type P cable. This cable has a 68 pin connector arranged in two rows of 34 pins just like the B cable. The difference is in the pin assignments on the connector. The B cable has since been abandoned and the standard is now the P cable.
SINGLE ENDED OR DIFFERENTIAL
The original design of the SCSI standard allowed two different types of buses, the single ended and differential. Single ended SCSI uses a single ended electrical signal (also refered to as unbalanced), this means that a single wire is used as a common ground return path for all signal lines. As of in the differential SCSI (refered to as balanced) each signal line has it's own return path and is also isolated from the reference ground. The cable uses twisted pair wiring, this makes it less prone to electrical interference and to picking up noise. This means that differential SCSI needs more wires and more electronics, and consequently also means higher prices, so as you guest most users systems use single ended SCSI because their cheaper and work good in most applications, like small systems and where electrical noise is not a problem.
Since differential SCSI is less prone to electical interference the cable lengths are greatly improved. On single ended SCSI, the cable length is limited to 6 meters (around 20 feet) and with differential SCSI the cable length can span up to 25 meters (about 82 feet). The shortest cable must not be less than 30 centimeters (12 inches) between each device. For example, if you have 3 device connected to the host adapter, the minimum lenght of the cable should be 1 meter (3 feet).
Since differential SCSI is less prone to electical interference the cable lengths are greatly improved. On single ended SCSI, the cable length is limited to 6 meters (around 20 feet) and with differential SCSI the cable length can span up to 25 meters (about 82 feet). The shortest cable must not be less than 30 centimeters (12 inches) between each device. For example, if you have 3 device connected to the host adapter, the minimum lenght of the cable should be 1 meter (3 feet).
HARDWARE
The basic SCSI bus can connect up to seven devices and 1 host adapter on it, the bus uses 3 address lines which gives you a possibility of 8 addresses. Each device is assigned a SCSI address from 0 to 7 where 7 is reserved as the host adapter. The remaining 7 addresses are used for any device connected anywhere on the bus. Each address is assigned a priority, where 7 (the host adapter) has the top priority and 0 beeing the lowest priority.
Each device can be assigned any address (except for address 7) and is user selected via DIP switches or jumpers. No two devices can have the same address on the same SCSI chain, internal nor external. Usually you can assign any address number to a device as long as the software or drivers for the specific device allow it. Some devices will not let you the freedom of selecting the address, in these cases you must make sure that the predifined address on the device does not conflict with other devices already in use. For example some systems will force you to assign the address 0 on the boot hard drive, this is true for most host adapters that emulate the Western Digital WD1002 controller
Each device can be assigned any address (except for address 7) and is user selected via DIP switches or jumpers. No two devices can have the same address on the same SCSI chain, internal nor external. Usually you can assign any address number to a device as long as the software or drivers for the specific device allow it. Some devices will not let you the freedom of selecting the address, in these cases you must make sure that the predifined address on the device does not conflict with other devices already in use. For example some systems will force you to assign the address 0 on the boot hard drive, this is true for most host adapters that emulate the Western Digital WD1002 controller
APPLE SCSI
Unlike in the PC world, the Apple standardised on one drive interface, SCSI. Also, Apple standardised on a 25 pin connector for external connections. However, Apple decided not to implement the complete ANSI spec., so one must be careful that peripherals used are certified to work with Apple's SCSI bus. Apple also developed it's own pin-configuration. The Apple and Future Domain 25-pin SCSI connectors are as close to "Standards" as there are in the world of PCs. But the real ANSI Standard called for a 50 pin connector commonly referred to as a "Centronics" type (made popular by the Centronics printer company). Instead of the 25 staggered pins of the Apple & Future Domain type connectors, the Centronics type uses 2 parallel rows of 25 pins. This arrangement allows the use of extra grounds for better isolation.
TECH BASICS
SCSI - SCSI stands for Small Computer System Interface.
The normal internal cable for SCSI is a 50 conductor ribbon, with all odd numbered conductors grounded. Two conductors, numbers 25 & 26, are often left not-connected, as they deal with Terminator power, and can be easily shorted by cable reversals. There are no twists in this cable, and it's length may be a maximum of 6 meters. But one is advised to use minimum lengths to improve timing. Up to seven drives, or devices may be attached to an SCSI cable. Each is daisy-chained on the cable, or, when a device has two connectors, another cable may be "spliced" into the chain starting at the second connector, and continued on. Care must be taken to insure that cables and connectors are not reversed, as this would short pin 26 (TERMPWR) to ground, and likely damage the drive or controller. Also, as explained earlier, the terminating resistors should remain only on the controller (Host Adapter) and the LAST drive on the cable, regardless of it's address. The Differential SCSI requires balanced lines, and is used mostly on high-end workstations.
The normal internal cable for SCSI is a 50 conductor ribbon, with all odd numbered conductors grounded. Two conductors, numbers 25 & 26, are often left not-connected, as they deal with Terminator power, and can be easily shorted by cable reversals. There are no twists in this cable, and it's length may be a maximum of 6 meters. But one is advised to use minimum lengths to improve timing. Up to seven drives, or devices may be attached to an SCSI cable. Each is daisy-chained on the cable, or, when a device has two connectors, another cable may be "spliced" into the chain starting at the second connector, and continued on. Care must be taken to insure that cables and connectors are not reversed, as this would short pin 26 (TERMPWR) to ground, and likely damage the drive or controller. Also, as explained earlier, the terminating resistors should remain only on the controller (Host Adapter) and the LAST drive on the cable, regardless of it's address. The Differential SCSI requires balanced lines, and is used mostly on high-end workstations.
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