SCSI Cables and specs for normal people



SCSI Standards & Cables for the "normal"* person.

SCSI Terms and the basics!

For super geeks, here's the real SCSI "specs" link Hey you figure it out!

Read a Serbo-Croatian translation of this page!

SCSI Pronunciation:

SCSI is pronounced "scuzzy" as in, "that computer geek sure was scuzzy looking, like he/she hadn't bathed or hit the laundry for weeks!" Hey to be a true SCSI geek puts you in the "upper echelon" of computer geeks, so if bathing is not "time efficient" during your indoctrination into SCSI, so be it. Mo money later! SCSI Guru status can only be achieve after posting help messages continually for at least a year in the usenet newsgroup "comp.periphs.scsi". A good way to browse about and find answers to your SCSI questions is by using Google Groups:

Google groups

SCSI History and Future:

SCSI stands for "small computer systems interface" or some such nonsense. Apple and some Server manufacturers made it what it is. It is and always has managed to be a better interface for hard drives and CDROM drives than IDE and its various versions, even the new ATA66/100/133 (Ultra DMA66/100/133) even though "IDE" keeps improving and keeping itself pretty close in performance in many ways. SCSI has also managed to support a number of other devices besides hard drives and CDROM drives, unlike IDE. SCSI supports scanners, and other peripherals (those that support it) and over the years has been THE best computer peripheral interface where speed is concerned. Nowadays firewire (IEEE-1394) and now USB and USB 2, are attempting to wipe it out as a peripheral interface, and they will probably be successful. As a Server and high performance interface for storage (hard drives, RAID, etc) , SCSI is still way ahead of these newcomers and is likely to continue to be for some time.

SCSI Basics:

SCSI is a "parallel" interface.** That means it sends an entire "chunk" (byte) of information at a time, rather than sending things one "bit" at a time. This can give it great speed, but also tends to cause problems with the length of the cables involved. It also means a cable will have a lot of wires in it rather than a few, which basically causes the whole length problem thingy with SCSI. The length thingy may be a problem with SCSI, but with "IDE" the length problem is REALLY bad. Every IDE spec tends to be 18 inches for a maximum cable length. *** SCSI has way longer lengths like 12 meters nowadays. Thats like 36 feet for us American non metric types! IDE is also very limited as far as the number of devices per "channel". Two devices (hard drives, CDROM's) and that's it! Every "modern" IDE controller has two channels, so that's four devices, maximum. SCSI goes way past that. Depending on the SCSI type you can have 7, 15, or even more devices connected. This can be greatly reduced by which type of SCSI you have, which types of devices you have connected and also their cable lengths.

Here's a link to the SCSI cable length guide. It should keep you confused for a bit.

The main thing to remember now, is there are basically two types of SCSI out there for the average SCSI user. "Single ended" and "Low Voltage Differential" (LVD). Everything before Ultra2 and Ultra 160 SCSI (not Ultra SCSI-2, or Ultra SCSI-3, they were before Ultra2) is "Single ended, or "SE". These devices have slower speeds, and can possibly slow down a "LVD" (Ultra2/Ultra160) device when connected together with them. They can also severely limit the cable lengths allowed when connected with LVD (Ulra2/Ultra160) devices. Got it? I didn't think so... OK, so say you have a new groovy Ultra160 or even Ultra320 SCSI controller card connected to some amazingly fast Seagate Cheetah 15K (15000 RPM for you complete non geeks, that's a real fast freaking hard drive) drives and you want to hook up a scanner or a zip drive or maybe some CD burner to the same cable or the external connector. BAM! You could be slowing down the hard drives "throughput" (slowing down its data transfer rate). Some SCSI cards do have the various connectors "segmented" or isolated from each other so that there will be no problem as long as you don't connect these slower devices to the same cable. RTFM! (read the "friendly" manual?).

Another very misunderstood problem is the "High Byte Termination" problem.

High-byte termination:

When a wide SCSI bus or device must connect to a narrow SCSI bus or device, care should be taken to assure the proper termination of the high data byte. What's the high data byte? Well, "wide SCSI" means 16 bit or 2 "bytes" (8 bits make a byte) SCSI. Narrow SCSI is 1 byte (8 bits) SCSI. Normally wide SCSI devices will work just fine using only the first byte, and therefore can be used on a narrow bus (cable). The problem is that if the second or "high byte" just, sort of, disappears, then it will not be terminated properly and all sorts of problems can occur. Also, whereas the wide drives of the past (non LVD) were perfectly happy to be directly connected to a narrow bus with no termination at the drive itself, it seems that most LVD (Ultra2 or Ultra160) drives require this high byte to be terminated at the drive

More Important SCSI Information:

SCSI connectors page Pictures of the connectors and what types of SCSI they are used for. Also SCSI length information, and various SCSI specifications.
SCSI connections FAQ: Diagrams of howto connect various SCSI drives and peripherals.

* If you were really normal you'd be watching TV and forgetting you ever heard of this "SCSI" thing!

**There are new forms of "serial SCSI" but we won't get into that. Buy a book.

SCSI Geek specs:

SCSI Terms & Terminology-- the actual specs:


SCSI-1 is a complete document with all the physical and protocol Layers, it is obsolete. 8 bit SCSI, Single-ended open collector drivers with Asynchronous transfer for all commands and data transfers. Bipolar technology for the drivers and receivers, bus DC loads a major consideration, passive termination. The connectors are the low density 50 pin internal and external connectors, the external low density connector is also known as the Centronics connector, in Cable terms the SCSI-1 Connector.


SCSI-2 is a complete document with all the Physical and protocol layers. SCSI asynchronous commands with Synchronous data transfer rates up to Fast 10 megatransfers per second or 20 megabytes per second. Bipolar technology with Open collector or active negation single ended bus transceivers, passive or active termination, and Differential (Now called HVD - High Voltage differential) bus transceivers based on EIA 485 allowed in SCSI-2. The A connectors are the 50 pin low density internal connector, low density Centronics external connector and the High density external connector. Cable terms the SCSI-2 connector is the high density 50 pin. The B connectors for wide bus was not practical because it required a second cable 68 pin High density.


SCSI-3 change the document structure, SCSI-3 is not one document with all the different layers and electrical interfaces, but a collection of documents that cover the physical layer, the basic protocol specific to that electrical interface, the primary command set layer (SPC) and the specific protocol layer. The specific protocol layer contains the Hard Disk interface Commands in the Block Commands (SBC), Steam Commands for tape drives(SSC), Controller Commands for RAID arrays (SCC), Multimedia Commands (MMC), Media Changer Commands (MCC) and enclosure services commands (SES) for example. There is an overall architectural model (SAM). Each document has its own revision level, these are normally referred to as SCSI, the -3 has been dropped.

The SCSI Parallel interface (SPI) defines the Parallel bus electrical connections and Signals. SCSI Interlock Protocol (SIP) defines the parallel command set. SIP is include in the SPI-2 and SPI-3 revisions. SPI define the P connector 68 pin High density primary cable for a wide bus in a single cable. This 68 pin High density cable is referred to as the SCSI-3 connector. SPI includes speeds up to Fast SCSI (Wide Fast SCSI is 20 MegaBytes per second), electrical interfaces are defined in CMOS terms, with the termination specified at 0.2 volts instead of 0.5 volts. The A cable is referenced to SCSI-2 but not included in SPI.

Fast-20 commonly known as Ultra SCSI is a speed doubling from the SPI document allowing Fast-20, 20 megatransfers per second or on a wide bus 40 megabytes per second. This is not complete standard, it only has the sections required for the Fast-20, all other sections use the SPI standard.

SPI-2 doubles the speed again to Fast-40 commonly know as Ultra2 SCSI, 40 megatransfers per second or on a wide bus 80 megabytes per second with a new electrical interface, low voltage differential, LVD SCSI. Single ended can not be used for speeds above Fast-20. The SPI-2 standard included SIP, the Single Connector Attachment (SCA-2) 80 pin Host swap connector and the 68 pin Very High Density Connector (VHDCI). The SCSI-2 A and SPI P connectors are include in SPI-2.

SPI-2 is a complete document and does not refer to older documents, one reference of all parallel interfaces up to Fast-40 speeds.

SPI-3 doubles the speed again to Fast-80DT commonly know as Ultra160 SCSI, 80 megatransfers per second or on a wide bus 160 megabytes per second. There are a lot of changes with SPI-3; High voltage differential (HVD SCSI) and the 32 bit data bus with the Q cable are obsolete, clocking is defined for LVD on both the rising and falling edges of the REQ/ACK clock - Double Transition (DT) defined for wide bus only, Cyclic Redundancy Check (CRC) defined, Domain Validation defined part of the domain validation will be in SPC-x, Packetized Commands and Messaging defined and Quick Arbitration Defined. **

SPI-3 is a complete document and does not refer to older documents, one reference of all parallel interfaces up to Fast-80DT speeds, but it does obsolete HVD and 32 bit data buses.

EPI (Enhanced Parallel Interface) Technical Report describes how to design SCSI systems. The cable lengths and loads are defined by the electrical parameters, allowing system designs to take advantage of faster cables and reduce loads. The technical report describes how to work with wide (68 Pin) and narrow (50 pin) devices on the same system. EPI includes documentation of Expanders, Bridging expanders, Switches and common connectors not documented in the standards.

Quick reference for cables

SCSI-1 External A cable is the 50 pin Centronics connector cable. (C50M) 85 mil (2.16 mm) spacing

SCSI-2 External A cable is the 50 pin High Density connector cable. (MiniD50M) 50 mil (1.27mm) Spacing

SCSI-3 External P cable is the 68 pin High Density connector cable. (MiniD68M) 50 mil (1.27mm) Spacing

SCSI SPI-2 very high density connector (VHDCI) external P cable is some times incorrectly called the SCSI-4 or 5 cable uses the 68 pin VHDCI connector cable. (.8mm 68M) *

SCSI-1, SCSI-2 Internal A cable is the 50 pin connector Ribbon cable. 100 mil (2.54mm) spacing

SCSI-3 Internal P cable is the 68 pin High Density ribbon Cable. 50 mil (1.27mm) Spacing

* SCSI-4 and SCSI-5 are not standards, SCSI-3 divided the standard into separate standards, each a stand alone document.

** Ultra160/m is a specific implementation of Ultra3 that includes Fast-80DT, CRC and Domain Validation. Ultra160/m was developed by a group of SCSI suppliers and customers; it is not a STA recognized term.

SCSI Information:

SCSI connector pictures

The basics of SCSI for newbies and technical information

SCSI connection FAQ

SCSI cable Length guide