1. SCSI The Bus and Network

2. Big Three The Big three of SCSI are: –Adaptec And then comes everybody else

3. The Copier Analogy

4. 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

5. 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

6. Host Adapter Also called a SCSI controller Usually has two connectors – one for internal devices and one for external

7. 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

8. 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

9. Counting Revisited IDA0 = 1A1 = 2A2 = 4A3 = 8 0Off 1OnOff 2 OnOff 3On Off 4 OnOff 5OnOffOnOff 6 On Off 7On Off 8 On 9 Off On 10OffOnOffOn

10. 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!

12. Logical Unit Number (LUN) Used in drive arrays – RAID One SCSI ID, different LUNs Have not seen one of these (I think)

13. Termination We need a solution for “bounce back” Usually pull-down resistors Only at the ENDS of SCSI chain

14. 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)

15. 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

16. SCSI -2, cont. 8-bit “Narrow” 16-bit “Wide 5 MHz standard, 10 MHz “Fast”

17. 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

18. 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

19. 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

20. 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)

21. 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

22. SCSI Cabling - Internal A 50-wire cable for 8-bit called A cables 16-bit uses 68-wire cables: P cables

23. 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

24. 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

25. 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