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090199

FEATURES

Fully compliant with Ultra2 SCSI

Provides Multimode Low Voltage
Differential/Single-Ended (LVD/SE)
termination for 9 signal line pairs

Auto-selection of LVD or SE termination

5% tolerance on SE and LVD termination
resistance

Low power down capacitance of 3 pF

Onboard thermal shutdown circuitry

SCSI bus hot plug compatible

Fully supports actively negated SE SCSI
signals

PIN ASSIGNMENT

DESCRIPTION

The DS2118M Ultra2 LVD/SE SCSI Terminator is both a Low Voltage Differential (LVD) and Single-
Ended (SE) terminator. The multimode operation enables the designer to implement LVD in current
products while allowing the end-user SE backward compatibility with legacy devices. If the device is
connected in an LVD-only bus, the DS2118M will use LVD termination. If any SE devices are connected
to the bus, the DS2118M will use SE termination. This is accomplished automatically inside the part by
sensing the voltage on the SCSI bus DIFFSENS line.

For the LVD termination, the DS2118M integrates two regulated supplies with nine precision resistor
strings. For the SE termination, one regulator and nine precision 110-ohm resistors are used. Three
DS2118M terminators are needed for a Wide SCSI bus.

DS2118M

Ultra2 LVD/SE SCSI Terminator

www.dalsemi.com

34
33

31
30
29
28
27
26
25
24

LVD
SE
R9N

R8N

R8P

HS GND
HS GND
HS GND

R7N
R7P
R6N

R9P

R1P

R1N

R2N

HS GND
HS GND

R3P

R3N

R4P

R4N

R2P

DS2118MB 36-Pin SSOP

VREF

TPWR
HVD

HS GND

2
3

5
6
7
8
9

10
11
12
13

15
16
17
18

22
21
20
19

R5P

R5N

ISO

GND

DIFF_CAP
DIFFSENSE
MSTR/SLV

R6P

DS2118M

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REFERENCE DOCUMENTS

Small Computer Systems Interface (SCSI-3) SCSI Parallel Interface (SPI) Project: 0855-M, 1995
Small Computer Systems Interface (SCSI-3) SCSI Parallel Interface 2 (SPI-2) Project: 1142-M, 1998

Available from:
American National Standards Institute (ANSI) Phone: (212) 642-4900
Global Engineering Documents 15 Inverness Way East; Englewood, CO 80112 Phone: (800) 854-7179

FUNCTIONAL DESCRIPTION

The DS2118M combines LVD and SE termination with DIFFSENS sourcing and detection. The LVD
termination section consists of two source/sink amplifiers (VTOP, VBOT), biasing circuitry and nine
precision resistor strings (RTOP, RMID, RBOT). The SE termination section consists of a 2.85V
source/sink regulator with 9 precision 110 Ohm resistors. The DIFFSENSE section consists of a 1.3V, 5
mA driver and a sensing circuit (Figure 1).

The DIFFSENSE circuitry decodes trinary logic. There will be one of three voltages on the SCSI control
line called DIFFSENS. Two comparators and a NAND gate determine if the voltage is below 0.6V, above
2.15V, or in between. That indicates the mode of the bus to be HVD, SE, or LVD, respectively.

The DS2118M's DIFF_CAP pin monitors the DIFFSENS line to determine the proper operating mode of
the device; this mode is indicated by the SE/LVD/HVD outputs. The DIFFSENSE pin can also drive the
SCSI DIFFSENS line (when MSTR/SLV = 1) to determine the SCSI bus operating mode. The DS2118M
switches to the termination mode that is appropriate for the bus based on the value of the DIFFSENS
voltage. These modes are:

LVD mode LVD termination is provided by a precision laser trimmed resistor string with two amplifiers.
This configuration yields a 105

=differential and 150

=common mode impedance. A fail-safe bias of 112

mV is maintained when no drivers are connected to the SCSI bus.

SE mode When the external driver for a given signal line turns off, the active terminator will pull that
signal line to 2.85 volts (quiescent state). When used with an active negation driver, the power amp can
sink 22 mA per line while keeping the voltage reference in regulation. The terminating resistors maintain
their 110

=value.

HVD Isolation Mode The DS2118M identifies that there is an HVD (high voltage differential) device on
the SCSI bus and isolates the termination pins from the bus.

When ISO is pulled high, the termination pins are isolated from the SCSI bus, Vref remains active, and
the bus mode indicators (SE/LVD/HVD) remain active. During thermal shutdown, the termination pins
are isolated from the SCSI bus, Vref becomes high impedance, and the bus mode indicators
(SE/LVD/HVD) remain active . The DIFFSENSE driver is shut down during either of these two events.
An internal pull-down resistor assures that the DS2118M will be terminating the bus if the ISO pin is left
floating.

To ensure proper operation, the TPWR pin should be connected to the SCSI bus TERMPWR line. As
with all analog circuitry, the TERMPWR and VDD lines should be bypassed locally. A 2.2

F capacitor

and a 0.01

F high frequency capacitor is recommended between TPWR and ground and placed as close

as possible to the DS2118M. The DS2118M should be placed as close as possible to the SCSI connector

DS2118M

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to minimize signal and power trace length, thereby resulting in less input capacitance and reflections
which can degrade the bus signals.

To maintain the specified regulation, a 4.7

F capacitor is required between the Vref pin (VREF) and

ground of each DS2118M. A high frequency cap (0.1

F ceramic recommended) can also be placed on

the Vref pin in applications that use fast rise/fall time drivers. A typical SCSI bus configuration is shown
in Figure 2.

NOTE:

DIFFSENS Refers to the SCSI bus signal.
DIFFSENSE Refers to the DS2118M pin name and internal circuitry capable of driving the

DIFFSENS line.

DIFF_CAP - Refers to the DS2118M pin name and internal circuitry relating to monitoring the

DIFFSENS line.

DS2118M

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PIN DESCRIPTION Table 1

PIN

SYMBOL

DESCRIPTION

VREF

Reference Voltage. 2.85 volt reference; must be decoupled with a 4.7
ľF cap.

2, 3

No Connect. Do not connect these pins.

4-7, 11-16,

22-25, 29-32

RxP,

RxN

Signal Termination. Connect to SCSI bus signal lines.

8, 10, 26,

9, 28, 27

HS GND

Heat Sink Ground. Internally connected to the mounting pad. Should
be grounded.

ISO

Isolation. When pulled high, the DS2118M isolates it's bus pins (RxP,
FxP) from the SCSI bus.

GND

Ground. Signal ground; 0.0 volts.

MSTR/SLV Master/slave. Mode select for the non-controlling terminator. When

pulled high (MSTR), the DIFFSENSE driver is enabled.

DIFFSENSE DIFFSENSE. Output to drive the SCSI bus DIFFSENS line.

DIFF_CAP

DIFFSENSE CAPACITOR. Connect 0.1 ľF capacitor for
DIFFSENSE filter. Input to detect the type of device (differential or
single-ended) on the SCSI bus.

Single-ended. SE output of DIFFSENSE receiver; indicates SE bus
operation.

LVD

Low Voltage Differential. LVD output of DIFFSENSE receiver;
indicates LVD bus operation.

HVD

High Voltage Differential. HVD output of DIFFSENSE receiver;
indicates HVD bus operation or thermal shutdown.

TPWR

Termination Power. Connect to the SCSI TERMPWR line and
decouple with 2.2 ľF capacitor.

RECOMMENDED OPERATING CONDITIONS

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS

NOTES

Termpower Voltage

tpwr

4.0

5.50

Logic 0

-0.3

+0.8

Logic 1

2.0

tpwr

+ 0.3

Operating Temperature

amb

°C

SINGLE ENDED CHARACTERISTICS

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS

NOTES

SE Termination Resistance

Rse

104.5

110

115.5

Ohms

Vline=0-

3.0 volts

SE Voltage Reference

Vref

2.79

2.93

Volts

SE Output Current

lose

25.4

Vline =

0.2 volts

Output Capacitance

Cout

guaranteed

by design

DS2118M

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LOW VOLTAGE DIFFERENTIAL CHARACTERISTICS

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS

NOTES

Differential Mode
Termination Resistance

Rdm

100

110

Ohms

Common Mode
Termination Resistance

Rcm

110

190

Ohms

Differential Mode Bias

Vdm

100

125

All lines

open

Common Mode Bias

Vcm

1.125

1.375

DC CHARACTERISTICS

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS

NOTES

Termpower Current

tpmr

All lines

open

Input Leakage High

-1.0

ľA

Input Leakage Low

1.0

ľA

Output Current High

-1.0

OUT

= 2.4

volts;

SE/LVD/

HVD

pins only

Output Current Low

4.0

OUT

= 0.4

volts;

SE/LVD/

HVD

pins only

DIFFSENS SE
Operating Range

seor

-0.3

0.5

DIFFSENS LVD
Operating Range

lvdor

0.7

1.9

DIFFSENS HVD
Operating Range

hvdor

2.4

tpwr

+ 0.3

DIFFSENSE Driver
Output Voltage

dso

1.2

1.4

MSTR/

SLV=1;

=0-5 mA

DIFFSENSE Driver
Source Current

dsh

MSTR/

SLV=1;

dso

=0V

DIFFSENSE Driver
Sink Current

dsl

200

ľA

MSTR/

SLV=1;

dso

=2.75V

REGULATOR CHARACTERISTICS

(0°C to 70°C)

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS

NOTES

Line Regulation

REG

1.0

2.0

Load Regulation

REG

1.3

3.0

Current Limit

LIM

350

Sink Current

SINK

200

DS2118M

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DS2118M 36-PIN SSOP PACKAGE

DIM

MIN

MAX

2.44

2.64

0.12

0.29

0.43

0.23

0.32

15.20

15.54

10.11

10.52

7.40

7.60

0.80 BSC

0.25

0.71

0.51

1.02

DIMENSIONS D AND E1 INCLUDE MOLD MISMATCH, BUT DO NOT
INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD
FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.254
MM PER SIDE.

SECTION A-A DIMENSIONS APPLY TO THE FLAT SECTION OF THE
LEAD BETWEEN 0.13 TO 0.25 MM FROM THE LEAD TIP.

THE CHAMFER ON THE BODY IS OPTIONAL. IF IT IS NOT PRESENT,
A VISUAL INDEX FEATURE MUST BE LOCATED WITHIN THE
CROSS-HATCHED AREA.

DIMENSIONS ARE IN MILLIMETERS

Part Number DS2118M