4608X-102-102 Datasheet, PDF (52/62 Pages) Bourns Electronic Solutions – Resistor Networks Product Selection Guide

SCSI Applications
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.
+ 5V
330Ω
+ 5V
+ SIGNAL
220Ω
+ SIGNAL
150Ω
- SIGNAL
GROUND
330Ω
GROUND
330Ω
FIGURE 1.
FIGURE 2.
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
328
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
Dual
Terminators
220Ω
5V
330Ω
132Ω
3V
PACKAGE
DIP
CSIP*
CSIP*
MSIP*
NO.
REQ’D.
1
2
3
3
BOURNS P/N
4120R-3-221/331
4611X-104-221/331
4608X-104-221/331
4308R-104-221/331
Differential:
PACKAGE
DIP
DIP
CSIP*
MSIP
NO.
REQ’D.
2
3
3
5
BOURNS P/N
4120R-820-1
4120R-820-2
4116R-8-2
4614M-8-2
4310M-820-2
4120P-830-2
4420P-820-1
4420P-820-2
4420P-830-2
Cable B (SCSI-2 Only) Single-ended
PACKAGE
DIP
CSIP*
CSIP*
MSIP*
NO.
REQ’D.
2
3
4
4
BOURNS P/N
4118R-3-221/331
4612X-104-221/331
4610X-104-221/331
4310R-104-221/331
Differential:
PACKAGE
DIP
DIP
CSIP*
MSIP*
NO.
REQ’D.
4
5
5
8
BOURNS P/N
4118R-820-2
4116R-8-2
4614M-820-2
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.
Specifications are subject to change without notice.