4116R-R2R-503 datasheet(52/62 Pages) BOURNS

52 / 62 page

Specifications are subject to change without notice.

328

configuration (Fig. 1) for the single-ended implementation of the

SCSI bus, or a three-resistor terminator configuration (Fig. 2) for

the differential-line version of the SCSI bus.

In the single-ended configuration, the SCSI bus is defined for

lengths up to 6 meters, while the differential-line version pro-

vides for better commonmode noise immunity over cable

lengths up to 25 meters.

The signal assignments on the single-ended SCSI bus include

8 data lines, 1 parity line, and 9 control lines. Each of these 18

lines must be terminated, and it is convenient to do this using a

resistor network which contains all of the required resistors. An

additional 32 lines are ground or power lines which do not

require termination (there are 50 lines total in the cable). In a

similar fashion, the differential configuration of the SCSI bus

uses 18 pairs of lines, each requiring termination.

As of 1989, an extended version of the SCSI standard has

been in develop-ment by ANSI, called the SCSI-2 bus. This new

standard allows for 16-bit to 32-bit wide data transfers, while

also allowing a higher bit transfer rate.

Two cables are defined in the SCSI-2 bus, termed Cable A and

Cable B, where Cable B is optional (“wide SCSI” option). Cable

A is no different than the single cable used in the original SCSI

bus, and therefore uses the same number and types of resistive

terminators (i.e., dual terminators for single-ended and triple ter-

minators for differential).

Cable B, however, is a 68-line cable, of which 29 lines (single-

ended) or 29 line-pairs (differential) require termination.

Application Guidelines

The principles of transmission-line theory apply to SCSI termi-

nators, and therefore for proper operation their placement must

be restricted to the ends of the bus and nowhere else. This

implies that the terminators should be placed as close to the

SCSI devices as possible. It is permissible to place the termina-

tor inside the SCSI device, but only if that device is located at

the end of the bus.

For disk drive applications, SCSI terminators must be present

on the host adapter card and at the disk drive end as well. Many

disk drive manufacturers have opted to design in removable

SCSI terminators into their units to account for the possibility

that their unit may not be the one at the end of the cable. For

these manufacturers, the combination of a resistor network in a

through-hole version plus a matching socket represents the only

(and expensive) alternative.

A final consideration is the cable itself. Since the terminators

are comprised of 220 ohm and 330 ohm resistors (single-ended),

the cable ideally should have a characteristic impedance which

matches the Thevenin equivalent of this resistor combination,

that is, 132 ohms. In the differential case, a characteristic imped-

ance of 122 ohms would be ideal.

In addition, it is inadvisable to mix different, unmatched cables

in the same bus. Such a practice will result in undesirable signal

reflections which may compromise the integrity of the data transfer.

Bourns supplies a number of resistor network models designed

for both SCSI and SCSI-2 termination .

Cable A (SCSI and SCSI-2) Single-ended

330

Ω

220

Ω

+ 5V

GROUND

+ SIGNAL

FIGURE 1.

330

Ω

150

Ω

GROUND

- SIGNAL

FIGURE 2.

330

Ω

+ 5V

+ SIGNAL

330

Ω

220

Ω

Dual

Terminators

132

Ω

NO.

PACKAGE

REQ’D.

BOURNS P/N

DIP

4120R-3-221/331

CSIP*

4611X-104-221/331

CSIP*

4608X-104-221/331

MSIP*

4308R-104-221/331

Differential:

NO.

PACKAGE

REQ’D.

BOURNS P/N

4120R-820-1

DIP

4120R-820-2

DIP

4116R-8-2

CSIP*

4614M-8-2

MSIP

4310M-820-2

4120P-830-2

4420P-820-1

4420P-820-2

4420P-830-2

Cable B (SCSI-2 Only) Single-ended

NO.

PACKAGE

REQ’D.

BOURNS P/N

DIP

4118R-3-221/331

CSIP*

4612X-104-221/331

CSIP*

4610X-104-221/331

MSIP*

4310R-104-221/331

NO.

PACKAGE

REQ’D.

BOURNS P/N

DIP

4118R-820-2

DIP

4116R-8-2

CSIP*

4614M-820-2

MSIP*

4310M-820-2

*MEDIUM PROFILE (.250” SEATED HEIGHT) AND HIGH PROFILE (.350” SEATED

HEIGHT) ARE AVAILABLE BY PLACING THE LETTER “M” OR “H,” RESPECTIVELY, IN

THE FIFTH POSITION OF THE PART NUMBER.

SCSI Applications

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