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SCSI
The Bus and Network
Big Three
• The Big three of SCSI are:– Adaptec– Adaptec– Adaptec
• And then comes everybody else
The Copier Analogy
Historical
• 1979 – Shugart, an early pioneer in computer technology, introduces SCSI
• System-independent: Not Mac- or PC-specific
• Network within your computer
• Common devices: hard drives, tape backup, removable hard drives, scanners, CD-ROM drives, printers
SCSI Chains
• SCSI manifests itself as a chain or series of devices working together through a host adapter.
• The host adapter provides the interface/link between chain and PC
Host Adapter
• Also called a SCSI controller
• Usually has two connectors – one for internal devices and one for external
Connections
• For Wide SCSI-2• Internal devices: 68-pin ribbon cable• External devices: 50-pin, High Density
connector• Plug it in backwards and you fry the device and
the host controller– Cables have “key” on connector and device has slot
for the key
• Connect device to device – daisy chain up to 15 devices in addition to host controller
SCSI IDs
• Host adapter has to be able to address each device on the cable(s)
• We use IDs for this address: 0-15• A SCSI device can (theoretically) have any
address• Can’t share address/ID• Most times host adapter is 7• Addresses don’t have to be ordered by position
on the cable• We use Binary to set IDs
Counting Revisited
ID A0 = 1 A1 = 2 A2 = 4 A3 = 80 Off Off Off Off
1 On Off Off Off
2 Off On Off Off
3 On On Off Off
4 Off Off On Off
5 On Off On Off
6 Off On On Off
7 On On On Off
8 Off Off Off On
9 On Off Off On
10 Off On Off On
IDs and Booting
• Host controller manual will tell you which ID can boot; often you can change that
• Usually, ID 0 is used for boot device
• Have to set CMOS to boot to SCSI first – if you can!
Logical Unit Number (LUN)
• Used in drive arrays – RAID
• One SCSI ID, different LUNs
• Have not seen one of these (I think)
Termination
• We need a solution for “bounce back”
• Usually pull-down resistors
• Only at the ENDS of SCSI chain
SCSI - 1
• More a group of suggestions than standard
• Hence, most vendors took liberty with the suggestions and thus devices would not interoperate, or cooperate
• PC use was “me too!” from Mac use (Apple was much more clear about SCSI)
• 8-bit, parallel, 5MHz, 8 devices (counting host controller)
SCSI - 2
• Finalized in 1990; showed up in 1994
• Much improved over -1 standard
• 18 commands in Common Command Set (CCS)– Added CD-ROM, tape and scanners– Command Queuing: multiple incoming
commands stored for execution
10
SCSI -2, cont.
• 8-bit “Narrow”
• 16-bit “Wide
• 5 MHz standard, 10 MHz “Fast”
Electricity 401
• One wire for each bit
• Common ground back to power supply
• “Noise” develops, looks like data, forces a retransmission of data
• This is called “Single-Ended” or SE
• Limits SCSI chain to max of 6 meters (standard) or 3 meters (fast)
8
Variations
• HVD: High Voltage Differential – 25 meters
• LVD: Low Voltage Differential – 12 meters
• Both use negative/inverse matching to distinguish data from noise
• HVD is bad boy, on its own
• LVD can live with SE devices
SCSI-3
• A whole group of standards
• Ultra, Ultra2 and Ultra3 for 20, 40 and 80 MHz speeds; Fast-20, Fast-40, Fast-80 per Adaptec
• Hot swap: to unplug a drive without rebooting or resetting the chain
6
More Termination
• Passive – resistors – used on SCSI-1
• Active – voltage regulators – used on SCSI-2
• Forced Perfect – diodes
• LVD active for LVD chains (even with SE)
Bus Mastering
• SCSI devices are “intelligent” devices
• Host adapter sets up conversation then drops out of the loop
• Devices use SCSI bus and not system bus
4
SCSI Cabling - Internal
• A 50-wire cable for 8-bit called A cables
• 16-bit uses 68-wire cables: P cables
External Cables
• 50-pin HD DB for SCSI-2
• 68-pin HD DB for wide SCSI-2 and -3
• Relic: 25-pin D-shell, just like parallel port
2
SCSI and IDE
• Usually, it’s an “OR” situation
• Can be “AND” with boot (IDE) drive providing SCSI drivers. IDE gets drive letters first (so what?) then SCSI
Troubleshooting
• Most problems are at installation: cables, connectors, IDs and termination
• SCSI scan should report all devices
• Remember: the interface is different, but the drive itself is the same
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