1. SCSI Richard Goldman April 2000
4/21/2017
The SCSI Interface
Richard Goldman
April 2000
Richard Goldman

2. SCSI Small Computer System Interface
A bus system that allows many different types of devices to operate simultaneously from a single SCSI controller card (Host Adapter).
1 to 4 SCSI host adapters (ISA or PCI) may be used per PC.
Each SCSI host adapter can support up to 7 Standard or 15 Wide SCSI devices.

3. SCSI Communication
Each SCSI device has its own controller which includes a SCSI interface circuit.
The SCSI device communicates with the SCSI host adapter through the SCSI bus
The host adapter communicates with the system through the system bus.

4. Hard Drive Read/Write Heads
Hard Drive Controller
SCSI
Interface
Hard Drive
SCSI Communications
SCSI Bus
SCSI Host Adapter
Mother Board
CPU
System Bus

5. Standard SCSI (50 Pin) Wide SCSI (68 Pin) Host Adapter Host Adapter
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive

6. SCSI ID Number
Each SCSI device must be set to a different SCSI ID number. (0-7 or 0-15; number 7 or 15 is used for the host adapter)
SCSI ID number may be set with either a set of jumpers or a switch.
Some SCSI hard drives are mounted in hot-swap cages that allow the SCSI ID number to be set remotely.

7. SCSI Signaling
Standard SCSI uses Single-ended (SE) signaling. (Signals are transmitted and sensed by a voltage transition.)
SCSI devices that transmit at speeds greater then 20 MHz use Low Voltage Differential signaling (LVD). (Signals are sensed by comparing the difference between two voltages.)
All devices on a SCSI bus must support LVD or it will automatically drop back to SE signaling and lower transmission speeds.

8. SCSI Bus Speeds Bit Transfer Width, Cable Width, & Devices per Controller
8-bit, 50-pin, devices
MB/s
16-bit, 68-pin, 15-devices
Standard 5
Standard/Wide 10
Fast
Fast/Wide 20
Ultra
Ultra/Wide
(7 devices) 40
Ultra2
(LVD)
Ultra2/Wide 80
Ultra3
(LVD – 2X)
Ultra3/Wide
160

9. Internal SCSI Cables
Internal SCSI cables (ribbon cables) usually come with a SCSI device when you buy it. However they often only support a few devices.
They come in either 50 pin (standard) or 69 pin (wide).
To accommodate more devices you must order special or custom made cables.
You may also purchase the materials and tools to make your own cables.
Cable length nor cable quality are usually not a big problem for internally mounted SCSI devices.

10. External SCSI Cables Cable Connector “A” Centronics 50-pin
High Density 50-pin
“P”
High Density 68-pin
Hot-Swap Cage
80-pin
Apple & some others
DB-25 (same as printer connectors)

11. External SCSI Cables
Purchase high quality cables that match or exceed your specific requirements.
Observe overall cable length requirement for your equipment.
Avoid using the DB-25 SCSI cable. They have poor transmission quality and are susceptible to damage by connecting to the wrong connector.

12. Passive SCSI Bus Termination
Each end of the SCSI bus must be terminated.
The host adapter is usually preset to terminate one end of the SCSI bus. (or designed to automatically provide termination if needed)
The other end of the bus may use a terminating resistor – used on standard SCSI external devices – it plugs into the open SCSI bus connector of the last device in the bus.

13. Active SCSI Bus Termination
Newer and faster SCSI devices require active termination circuitry to ensure that the SCSI signals are terminated at the proper voltage levels.
Whenever Fast or Wide SCSI devices are used the SCSI bus must be terminate with the appropriate type of active termination circuitry.
Active terminator, Forced Perfect Termination (FPT), and LVD Termination may be set by setting a switch or jumper on the last device on the bus.
Special terminators are required for SCSI devices that do not have built in active termination circuitry.

14. SCSI
4/21/2017
Plug-and-Play SCSI
SCSI host adapters, devices, and operating systems that comply with the SCSI Configuration AutoMatically (or AutoMagically) (SCAM) standard can be Plug-and-Play.
SCAM compliant host adapters and device can dynamically assign SCSI ID numbers at startup and automatically terminate the SCSI bus.
Richard Goldman

15. SCSI BIOS
The SCSI host adapter BIOS can load into system memory and controls the operation of the SCSI bus.
Most system BIOS's can access a SCSI drive, execute the load program in the MBR, and load the SCSI device drivers stored on the hard drive into memory.
If an IDE drive is present in the system the system BIOS will usually want to boot from it.

16. Other SCSI Setup Options
Start On Command - Starts devices in order by SCSI number, starting with 15. (reduces load on power supply) - set to ON for internal devices
SCSI Parity - Provides error checking - set to ON*
Termination Power - Supplies power to active termination devices (normally supplied by the host adapter - OK to have more than one supply on) - set to ON
Synchronous Negotiations - Allows devices to select best type data transfer mode. - Enable*
* Some very old devices may need to be off or disabled.

17. Why use SCSI? Expandability:
Can operate up to 15 devices on one SCSI host adapter. 4 host adapters will operate 60 devices per PC.
Allows computers (such as older Macintoshes) that do not have expansion slots to add additional peripheral devices.
Used for data servers, internet servers, application servers, etc. that need to access large amounts of data.
Required for RAID. (Redundant Array of Inexpensive Disks) A method used on servers to insure that data is not lost in the event of a hard drive failure.
(IDE RAID may be in limited applications)

18. Why use SCSI?
Speed
Currently high end SCSI devices have a max transfer rate of 160 MB per Second for Ultra-3/Wide vs. a max transfer rate of 100 MB per second for the IDE ATA-5.
Slower hard drive speeds will limit data transfer rate despite higher SCSI bus speed.
SCSI devices can operate simultaneously in a multitasking mode and support command queuing.
Used for workstations that must be able to read and or write large amounts of data very quickly.
Computer Aided Design (CAD) workstations
Graphics/Animation workstations
Video Production Workstations
SCSI devices operation may all be operating at the same time on the SCSI channel. (This will result in slower individual devise transfer rates if several devices are using the SCSI bus at the same time.)

19. Why use SCSI? Flexibility Reliability
Allows more types of devices to be connected to the computer.
Hard Drives, CD-ROM’s, Scanners, Printers, Tape Drives, Etc.
External SCSI devices can easily be moved form computer to computer.
Reliability
Many server are configured with two or more SCSI host adapter in a RAID. If one of the host adapters should fail in this type of configuration the server can continue to operate without interruption.

20. Why Not To Use SCSI? Expensive More Work Cost/benefit
Requires the purchase of a host adapter card.
Requires special cables and termination devices.
The price of SCSI devices are more expensive than other devices.
More Work
Usually requires special drivers and additional steps to setup SCSI devices.
Usually requires additional knowledge/skill to setup SCSI devices.
Cost/benefit
System performance improvement may not even be noticeable in many ordinary uses.