SCSI (Small Computer Systems Interface)
It’s like electronic engineers gone wild…
SCSI Formats or flavors
Historically there are at least 14 different SCSI varieties from 7 generations if tinkering.
$o why i$n’t everything $C$I?
SCSI - Weird name – cool (bus) technology
Internal SCSI Drive Chain |
External SCSI Drive Chain |
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Terminator on Cable |
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External SCSI Connector |
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SCSI Adapter |
SCSI Ribbon Cable |
Shielded SCSI Cable |
Terminator Block |
- Flexible (hard drives, scanner, CDROM, printers)
- Efficient (independent, fast, multitasking, drives are "best in class")
Rules for SCSI – Very A+
- Host
- Termination
- Numbering
- Cables
Where is my SCSI Host?
All devices talk to the host adapter rather than to the system itself. A SCSI host contains a processor that can work independent from the CPU.
- Embedded
- Expansion Card
The "Terminator"
Why you need to do it (well, sometimes…)
Termination is used to stop signals from reflecting back into the bus once it has reached the end. It is basically a resistor that absorbs the SCSI signal.
Terminate at the ends of the SCSI chain
Not in the middle!
- Electrical resistors (passive and active devices exist)
- Installed in the devices at each end of the bus
- Not installed in other devices on the bus
- With some SCSI devices you must manually insert or remove the terminating elements
What you need to do
- Termination is usually enabled by default on host adapters (Some host adapters automatically adjust their termination. Set up in SCSI BIOS)
- You don't need to change this default if your computer has only internal SCSI devices or only external SCSI devices
- You need to disable termination on your host adapter if you connect both internal and external SCSI devices to the host adapter board and the host doesn't automatically adjust its termination.
How you need to do it
- Physically remove terminating resistors from their sockets, or insert them in the sockets (for internal SCSI devices)
- Change a dip switch setting
- Remove or install a terminator plug (for external SCSI devices)
- Remove or install jumpers
- Auto-terminating resistor packs (sometimes you don’t need to do anything at all!)
Read the device documentation to find out how to change termination on your SCSI devices.
SCSI Numbering and Priority – Very A+
Each SCSI device (including the Host) gets a number
- 0 – 7 for normal / regular / narrow SCSI
- 0 – 15 for wide / ultra wide SCSI
- Give 7 to the host (that way it can talk to both normal and wide devices)
- Give 0 to the boot device
- Priority > 7 - 6 - 5 - 4 - 3 - 2 - 1 - 0 - 15 - 14 - 13 - 12 - 11 - 10 - 9 - 8
How? Very A+
- Assigned via jumpers
- May be PNP using SCAM (configured auto-magically)
- LUN (Logical Unit Numbers) enable additional numbers - 60
SCSI "Flavors" - Very A+
The following information should be studied to become familiar with the most common of these:
SCSI1 - Original 8-bit 5MB/s DTR. 50-pin "B" cable.
SCSI2 - Added "fast" and "wide". 50-pin "B" cable and 68-pin "P" cable.
SCSI3 (called Ultra)
Many speeds and types…
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Original |
Fast |
Ultra |
Ultra 2 |
Ultra 3 |
8-bit
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5MB/sec |
10MB/sec |
20MB/sec |
40MB/sec |
80MB/sec |
16bit - Wide |
10MB/sec |
20MB/sec |
40MB/sec |
80MB/sec |
160MB/sec |
Cables
Basic issues concerning cables include:
- Signals
- Connections
- Length
Signals (pay attention, this stuff can smoke your system!)
Single-ended - each bit travels on its own wire and is referenced to a common ground. Cable length was limited due to noise and skew. Historically most SCSI has been SE (Single Ended).
Differential – uses two wires per bit. The second wire is the inverse voltage of the first wire – the SCSI signal is then the difference between these lines. This allows for cables of up to 25 meters.
With regards to differential (things are getting serious now…) - Very A+
High voltage differential (HVD) - Old
- Reliable signaling in high noise environments over a long bus length (up to 25m [82ft])
- Uncommon today
- Do NOT mix HVD with ANY other signaling variety – smoke will ensue
Low voltage differential (LVD) - New
- Provides a moderately long bus length (up to 12m [39ft])
- Downward compatibility with SE hardware – i.e. no smoke
- LVD-rated equipment is required for "Ultra" (i.e. really fast) SCSI standards
- When mixed with SE, the bus runs at maximum SE speed
Signs are everywhere
Big Table of SCSI Cable Lengths - Very A+
Type |
Length Single-Ended (meters) |
Length of Differential Cable (meters) |
Regular SCSI |
6 |
25 HVD |
Fast SCSI |
3 |
25 HVD |
Ultra SCSI |
1.5 |
25 HVD |
Connections
Width |
External connector |
Internal connector |
Narrow |
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Wide |
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Single Connector Attachment (SCA-2) - includes connection pins for the power cables as well as the data wires. The current version of SCA uses 80 pins and is frequently used for high-end SCSI devices. Allows hot swapping.
Some SCSI cables can cost up to a hundred dollars apiece!
You can buy dozens of different adapters that will mate different SCSI formats.
Expanders - can work as repeaters, converters, and electrical bus isolators to expand available topologies.