5196106372

EA Reference Notebook

Aa (sai)

Prońounced "skuzzie", the SCSI bus is now an internationally accepted interface standard for connecting hard disk drives and other mass storage peripherals to microcomputer Systems.

SCSI was developed from, and is an enhancement of, an interface bus developed by Shugart Associates for interfacing that company's hard disk and streaming tape drives. The original bus was called the SASI (Shugart Associates system interface).

Peripherals to be connected to the SCSI bus must have their own inbuilt intelligent controllers, which perform local control of all "primitive" device functions and only need to communicate with the main processor as a logical subsystem sending and receiving data blocks.

SCSI is a local area interface, designed to transfer data at up to 4 megabits per second over distances of up to 6m with unbalanced wiring, or up to 25m with balanced wiring techniques. Up to eight devices may be connected to the bus, including the Computer (or computers).

The SCSI bus uses 50-way fiat ribbon or twisted-wire cable, and ID (insulation displacement) type connectors having two rows of 25 pins with 0.1" spacing betwecn both rows and pins (Fig.l).

All devices are connected to the bus in daisy-chain fashion, with all signals common to all devices and all signal lines actively terminated at each end (Fig 2).

Normally there is a single "Talker" or

Initiator device, which initiates the data transactions on the bus. The rest of the devices are "Listeners" or Targets, which respond to commands from the Initiator. Usually the Computer is the Initiator, and the peripheral devices are the Targets (Fig.3). However there can be multiple Initiators on the bus if the SCSI bus's arbitration option is implemented.

As shown in Fig.l the SCSI interface provides eight parallel data lines DB(0-7) with a ninth (odd) parity bit DB(P). There are also nine status and control lines, and one power supply linę for the active terminations. All other lines are earthed, except in balanced Systems where all data and control lines have matching return lines.

Negative logie polarity is used throughout, with buslines driven by open collector or tristate driver outputs. Logic true or "1" corresponds to voltages from 0 to 0.4V, while logie false or "0" corresponds to voltages from 2.5 to 5.25V.

Information transfers on the SCSI bus are generally asynchronous, although synchronous transfer is an optional enhancement. Data is transferred byte by byte, with REQ/ACK handshaking. However an extended command set capability allows up to 65,535 data blocks to be transferred in response to a single command. The logical addressing capability is up to 32 bits, allowing data blocks of up to 2³² bytes in length.

Each of the up to eight devices connected to the SCSI bus is assigned a fixed identification or "address" codę (0-7), designated directly by the bits of address bytes (Fig.4). The codes are predetermined in terms of arbitration priority for bus control; the device with c<xle 7 has the highest priority, and that with codę 0 the lowest.

Typical SCSI bus data transfers consist of three main phases, the first two selected by the Initiator and the third by the designated Target.

First the Initiator looks for a 'bus free' condition, and then makes a bid to capture control of the bus. This is called the arbitration phase. If no device with higher priority bids, the Initiator gains bus control and enters the sęlection phase, flagging up the Target device with which it desires to communicate.

The third, or information transfer phase is entered when the selected Target device responds, and indicates the type of transfer it is prepared to engage in. These include Data In or Data Out, Command Request, Status acknowledge, Message In or Message Out.

Many mass storage devices and peripherals are now being provided with an inbuilt controller and SCSI interface. Similarly many smali computers are being provided with either a built-in SCSI bus port, or can be provided with one via a so-called "host interface controller". These are now available as plug-in adapter cards, madę for a variety of internal system bus standards.

- Jim Rowe

SIGNAL

(Ot

osin

00<?t

oe m 00 HI 00 (SI oo m

00(7) 00(01 GPOUNO G**0/H0 G«CXAO *11 nyPw G«OAO 0*0*0 AT*

0*0*0

Kr

*CK

*st

MSG

IS

MtO

40

PIN NUMHIR 2 4 •

a

•o

•2

14

U

10

20

22

24

24

20

20

12

14

*

M

40

42

»SV (nominał)

:ont*olc*

Sł&NAL

GffOONO

OONHOUf*

WPCKtEPl

CONTOOuE

Hole: Thh pin Is retenred tor provldlng optlonal terminator power (plus 5 volt%).

Notę: Alt odd pins eicepf pin 25 shall be connected to ground. Pin 25 should be lett open but may be connected to ground.

The minus Indicates actrve Iow.

Fig.1: SCSI Interface connector and signals. The *Tęmpwr pin (26) provides the +5V for active bus terminations. All pins not identified are normally grounded, except pin 25. All signal pins use negative logie.

Fig.2: The active terminations used for all signal lines, in the typical unbalanced case.

IBOLcCO

Fig.3: A typical SCSI bus configuration, with a single Initiator and multiple Target devices.

OB47) [0616) 10615) j0BUl|08(3)| 0B(?)| DB(1) \ 0610) |

I Cesi io«* Lscsi io.i

^LSCSI 10.2 ^LSCS1 ID. 3

^lscsj io-t

^LSCS! ID.S ^LSCSI ID*6 ^LSCSI 10*7

Fig.4: Each device on the SCSI bus is assigned a fixed address, which have a fiied order of bus priority.