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

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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)

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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

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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

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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

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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