Computer Architecture - Hardware - Lesson 14 - SCSI - Eric Vanderburg

1. Computer Architecture:
Core Hardware
Chapter 14
SCSI
Eric Vanderburg
Eric Vanderburg – Core Hardware © 2005

2. SCSI
• Small Computer System Interface
• 15 devices on a controller
• Internal & external devices
• Multiple devices can communicate at
the same time
• Can talk to each other without
utilizing the CPU.
• SCSI host adapter is a layer between
the CPU and the interface controller
Eric Vanderburg – Core Hardware © 2005

3. SCSI
• LUN (Logical Unit Number) – SCSI ID
– Set using jumpers, drivers, or a dial on
the back of the device
• Host adapter
– has highest number (7 or 15)
– Uses DMA, IRQ, & I/O Address
– SCAM (SCSI Configuration Auto-
Matically) – allows PnP SCSI devices
Eric Vanderburg – Core Hardware © 2005

4. Drivers
• Requires disk to install windows
• Some SCSI drivers are in the system
BIOS
Eric Vanderburg – Core Hardware © 2005

5. SCSI Types
• Narrow SCSI
– 8-bit data bus width
– 50 pin cable (A Cable)
• Wide SCSI
– 16-bit data bus width
– 68 pin cable (P Cable)
Eric Vanderburg – Core Hardware © 2005

6. Drivers
• ASPI (Advanced SCSI Programming
Interface)
– Created by Adaptec (major controller
card mfg.)
– WNASPI32.DLL, ASPI32.SYS
– Provides direct support for most CD-ROM
drives, changers and jukeboxes
• CAM (Common Access Method)
– ANSI
– Can be used for several host adapters
– Part of SCSI-3
Eric Vanderburg – Core Hardware © 2005

7. Signaling
• Signaling is separate from the cable
type and cannot be determined by
looking at the cable. Be careful.
• Single Ended (SE) – unbalanced
signaling
– one wire for voltage
and another for
grounding
– Cheaper
– Shorter cable lengths
– Max speed: 20MHz
Eric Vanderburg – Core Hardware © 2005

8. Signaling
• Differential – balanced signaling
– two cables with voltage (difference is used to
get the signal)
– Sends a signal to verify each bit (less errors)
– More expensive
– Longer cable lengths
– HVD (High Voltage Differential)
• cannot be mixed with SE devices because of voltage
• Max speed: 40MHz
– LVD (Low Voltage Differential)
• has a compatible cable called LVD/SE that is shorter
in length but supports both devices
• Max speed: 320MHz
Eric Vanderburg – Core Hardware © 2005

9. Connectors
• 50 pin
• 68 pin
• 80 pin SCA – provides power to the device
• Best case is to use 68 pin and then use
converter to use 50 pin
• Don’t forget the red stripe rule
• Terminating Resistors
– Can be a hardware terminator
– Can be terminated at the
controller through software
or a jumper
– On an end device through a
switch or by applying a
terminator to the device
Eric Vanderburg – Core Hardware © 2005

10. Eric Vanderburg – Core Hardware © 2005
Connectors

11. SCSI Standards
• SCSI 1 (Regular SCSI)
– 8 bit only
– Supports 7 devices & adapter
– 5MBps
• SCSI 2 (Fast SCSI)
– 16 bit
– Supports up to 15 devices
– Mandatory parity checking
– Up to 80MBps
Eric Vanderburg – Core Hardware © 2005

12. SCSI Standards
• SCSI 3 (Ultra SCSI)
– 32 bit
– Supports up to 32 devices
– Supports SCAM
– Ultra2 – 40MBps
– Ultra3 – 80MBps
– Ultra4 – 160MBps
– Ultra5 – 320MBps
**compare to SATA at 150Mbps
• SAS (Serial Attached SCSI) –
compatible with SATA
Eric Vanderburg – Core Hardware © 2005

13. SCSI Summary
• Generally more expensive
• Faster & higher quality
• Supports more devices
• Many compatibility options
Eric Vanderburg – Core Hardware © 2005