SCSI pinout

• Ask a question
• Comment
• Edit
• Submit New

The SCSI standard can be divided into SCSI1, SCSI2, SCSI wide and so on. SCSI advantages includes flexible device attachment (up to 7-15 devices per SCSI bus), support for almost any peripheral type (disk, tape, CDROM, scanner etc), greater speed. All SCSI commands can overlap with commands on other devices. SCSI is widely used in servers, but due to it cost less used in end-user computers (ATA devices are cheaper and offer comparable perfomance).

Parallel SCSI interface revisions

SCSI-1

The original standard that was adopted in 1986 by ANSI. SCSI-1 features an 8-bit parallel bus (with parity), running asynchronously at 3.5 MB/s, or 5 MB/s in synchronous mode, and a maximum bus cable length of 6 meters. A rarely seen variation on the original standard included a high-voltage differential (HVD) implementation whose maximum cable length was 25 meters.

SCSI-2

SCSI-2 was introduced in 1994 and gave rise to the Fast SCSI and Wide SCSI variants.  Fast SCSI doubled the maximum transfer rate to 10 MB/s and Wide SCSI doubled the bus width to 16 bits on top of that to reach a maximum transfer rate of 20 MB/s.  However, these improvements came at the cost of reducing the maximum cable length to three meters.  SCSI-2 also specified a 32-bit version of Wide SCSI, which used two 16-bit cables per bus. 

SCSI-3

The first parallel SCSI devices that exceeded the SCSI-2 capabilities were simply designated SCSI-3. These devices, also known as Ultra SCSI or Fast-20 SCSI, were introduced in 1996. The bus speed doubled again to 20 MB/s for narrow (8 bit) systems and 40 MB/s for wide (16-bit). The maximum cable length stayed at 3 meters but single-ended Ultra SCSI developed an undeserved reputation for extreme sensitivity to cable length and condition (faulty cables, connectors or terminators were often to blame for instability problems). Unlike previous SCSI standards, SCSI-3 (Fast-20 speed) requires active termination.

Ultra-2 LVD SCSI

This standard was introduced c. 1997 and featured a low-voltage differential (LVD) bus. For this reason ultra-2 is sometimes referred to as LVD SCSI. LVD's greater resistance to noise allowed a maximum bus cable length of 12 meters. At the same time, the data transfer rate was increased to 80 MB/s. Mixing earlier single-ended devices (SE) and Ultra-2 devices on the same bus is possible but connecting only a single SE device forces the whole bus to single-ended mode with all its limitations, including transfer speed. Ultra-2 SCSI actually had a relatively short lifespan, as it was soon superseded by Ultra-3 (Ultra-160) SCSI. Fast-Wide SCSI is actually SCSI-2, however many cable manufacturers refer to the 68-pin plugs as SCSI-3.

Ultra-3 (Ultra-160) SCSI

Also known as Ultra-160 SCSI and introduced toward the end of 1999, this version was basically an improvement on the ultra-2 standard, in that the transfer rate was doubled once more to 160 MB/s by the use of double transition clocking. Ultra-160 SCSI offered new features like cyclic redundancy check (CRC), an error correcting process, and domain validation, a way to negotiate maximum performance for each device on the chain. Ultra3 features the same base frequency as Ultra2 (40MHz) but transmits 2 bytes per data clock thus doubling the total throughput. This is the same technique used in SDRAM, DDR and RAMBUS Memory where one data word (8 or 16 Bits) is transmitted at the raising edge and one at the falling edge.

Ultra-320

This is the Ultra-160 standard with the data transfer rate doubled to 320 MB/s. The latest working draft for this standard is dated 2002 year. Nearly all SCSI hard drives being manufactured at the end of 2003 were Ultra-320 devices. Ultra-160 doubles the base frequency of Ultra2 (80MHz) and like U3 transmits 2 bytes per data clock thus doubling the total throughput vs U3. Ultra4 also features AAF (Adjustable Active Filter), QAS (Quick Arbitration and Selection). This enables these fast speeds at these long cable lengths and decreases the overall latencies due to the protocol overhead.

Ultra-640

Ultra-640 (otherwise known as Fast-320) was developed in 2003. It doubles the interface speed yet again, this time to 640 MB/s. Ultra-640 pushes the limits of LVD signaling; the speed limits cable lengths drastically, making it impractical for more than one or two devices. Because of this, manufacturers have skipped over Ultra640 and are developing for Serial Attached SCSI instead.

Most SCSI 1-4, 160/m Ultra320 Devices can be mixed on the same bus, however, the slowest device/controller determines the bus speed during negatiating and in some cases also during data transfers!
You can not mix single ended and High Voltage (HV) differential! But you can mix most LVD (Low Voltage Differential) and single ended devices. Fast-Wide SCSI-2 is also sometimes called SCSI-3 (in particular when referring to the 68 HP connector).

Interfaces pinouts:

• SCSI External Centronics 50 (Differential) connector
• SCSI External Centronics 50 (Single-ended) connector
• SCSI External D-Sub (Future Domain) connector Seems to be available on some Future Domain SCSI-controllers only.
• SCSI External D-Sub (PC/Amiga/Apple Macintosh) connector
• SCSI External IBM Burndy connector Special invention by IBM for use on IBM RS/6000 and IBM PS/2
• SCSI Internal (2.5 inch)
• SCSI Internal (Differential) connector
• SCSI Internal (Single-ended) connector SCSI=Small Computer System Interface. Based on an original design by Shugart Associates. SCSI was ratified in 1986.
• SCSI-II External Hi D-Sub (Differential) connector
• SCSI-II External Hi D-Sub (Single-ended) connector
• SCSI-III External Hi D-Sub (Differential) connector SCA=Single connector Attachment
• SCSI-III External Hi D-Sub (Differential) connector
• SCSI-III External Hi D-Sub (Single-ended) connector SCA=Single connector Attachment
• SCSI-III External Hi D-Sub (Single-ended) connector

The words narrow stand for 8 bits and wide is 16 bit. The majority of the millions of SCSI devices in use today use single-ended transceivers (Asymmetrical, ground referenced). Differential (symmetrical +-) devices, which provide for greater bus lengths, are typically used in applications which can properly bear the added expense. A transceiver technology, Low-Voltage Differential (LVD), will combine the best features of single-ended and high-powered differential transceivers. LVD will also enable higher speeds. To ease the migration to LVD, most new SCSI devices will support Universal transceivers which include both single-ended and LVD transceivers.

SCSI terms:

Host adapter:
The card that connect your computer to the SCSI-bus. Usually called SCSI-controller by marketing droids.

Terminators (passive).
A group of resistors on the physical ends of a single ended SCSI-bus (and only on these ends) that dampens reflected signals from the ends of the bus.
Each terminated signal is connected by:
* 220 Ohm to +5 volt (TERMPWR)
* 330 Ohm to ground.
The 18 signals that are terminated are: I/O, Req, C/D, Sel, Msg, Rst, Ack, Bsy, Atn, DB(p), DB(7) ... DB(0).

Terminators (active).
Rather than passive terminators that use TERMPWR which may not be exactly +5v, active terminators use a voltage regulator.

Single ended:
Normal electrical signals. Uses open collector to the SCSI bus, [usually] survives wrong cable insertion. DIFFSENSE signal is used to detect
connection of wrong type devices. The max. length for SCSI-1 is a 6 meter cable with stubs of max 10cm allowed to connect a device to the main-cable. Most devices are single ended.

Differential:
Uses two wires to drive one signal. Max. cable length of 25 meters. Electrically incompatible with single ended devices! SCSI-1 and upwards. Normal SCSI is also called Single-ended SCSI. For each signal that needs to be sent across the bus, there exists a wire to carry it. With differential SCSI, for each signal that needs to be sent acrossthe bus, there exists a pair of wires to carry it. The first in this pair carries the same type of signal the single-ended SCSI carries.The second in this pair, however, carries its logical inversion. The receiver takes the difference of the pair (thus the name differential), which makes it less susceptible to noise and allows for greater cable length.

Apple kludge:
The single ended 50 pins cable has been reduced to 25 pins by tying most grounds together. DB25 connector (like a parallel port). Often used as the external SCSI connector.

Asynchronyous SCSI:
A way of sending data over the SCSI-bus. The initiator sends a command or data over the bus and then waits until it receives a reply (e.g. an ACKnowledge). All commands are send asynchronously over the 8 bit part of the SCSI-bus.

Synchronous SCSI.
Rather then waiting for an ACK, devices that both support synchronous SCSI can send multiple bytes over the bus. This improves throughput, especially if you use long cables. (The time that a signal travels from one end of the cable to the other end of the cable IS relevant.)

RAID:
A Redundant Array of Independant Disks is a set of drives connected to a special dual ported SCSI adapter that allows certain types of access
optimization. A RAID 0 array stripes the data accross multiple drives to decrease data latency. A RAID 1 array mirrors the data on multiple drives
for increased data integrity. A RAID 5 array contains extra drives that are used to apply ECC data correction and provide high reliability.

Wide SCSI:
Uses an extra cable (or 68 pin P cable) to send the data 16 or 32 bits wide. This allows for double or quadruple speed over the SCSI-bus.
Note that no *single* drive reaches these speeds, but groups of several drives can.

Cables are:

(SCSI3 Refers to Wide 68pin Connectors rather than Ultra SCSI)