DS2109S - Maxim Integrated Products, Inc.

DS2109

Plug and Play SCSI Terminator

DS2109

022698 1/10

FEATURES

•Fully compliant with SCSI–1, Fast SCSI and Ultra

SCSI

•Compatible with Plug and Play SCSI Profile

•Functionally compatible with DS21S07A

•Provides active termination for 18 signal lines

•2% tolerance on termination resistors and voltage

regulator

•Bus termination sensing

•Low power down capacitance of 3 pF

•Onboard thermal shutdown circuitry

PIN ASSIGNMENT

GND

R18

R17

R16

R15

R14

TCS

HS–GND

PDE

R13

R12

R11

R10

VREF

PDI

HS–GND

PDO

TPWR

DS2109, DS2109S 28–PIN SOIC (300 MIL)

DESCRIPTION

The DS2109 is intended for one chip Plug and Play

(PnP) SCSI termination. Plug and Play SCSI requires

the exit–point terminator on computer motherboards or

host bus adapters to automatically switch off if an exter-

nal device is connected to the system. The DS2109 sat-

isfies this requirement by offering the engineer a choice

of onboard current sensing circuitry or onboard ground

detect circuitry. If an external device is connected, the

DS2109 will automatically be isolated from the SCSI

bus thereby maintaining proper system termination.

The DS2109 integrates a low drop–out regulator, 18

precise switched 110 ohm termination resistors, and

bus termination sensors into a 28–pin 300 mil SOIC

package. Active termination provides: greater immu-

nity to voltage drops on the TERMPWR (TERMination

PoWeR) line, enhanced high–level noise immunity,

intrinsic TERMPWR decoupling, and very low quies-

cent current consumption. The DS2109 contains an

output port that can control the power down pin of addi-

tional terminators (DS21S07A) for Wide SCSI applica-

tions.

DS2109

022698 2/10

REFERENCE DOCUMENTS

SCSI–2 (X3.131–1994)

SCSI–3 Parallel Interface (X3T10/855D)

Available from: Global Engineering Documents

15 Inverness W ay East

Englewood, CO 80112–5704

Phone: (800) 854–7179,(303) 792–2181

Fax: (303) 792–2192

PnP SCSI Specification

PnP ISA Specification

PnP BIOS Specification

PnP Option ROM Specification

Available from: Plug and Play forum on

CompuServe (Go plugplay).

FUNCTIONAL DESCRIPTION

The DS2109 is designed to be a single chip termination

subsystem for use in PnP SCSI systems, Figure 1.

When embedded on a host bus adapter or mother-

board, the DS2109 can automatically sense the ter-

mination status of the SCSI bus and attach or isolate its

resistors as needed to maintain proper bus termination.

External and internal active termination can be provided

by the DS21S07A.

DS2109 APPLICATION ENVIRONMENT Figure 1

PERIPHERAL

SUBSYSTEM

EXTERNAL

TERMINATOR

(TWO DS21S07A)

DS2109 AUTOMA TIC

EXIT–POINT TERMINA T OR

HOST SCSI

PROTOCOL CHIP

SCSI

PERIPHERAL

DEVICES

INTERNAL

TERMINATOR

(TWO DS21S07A)

SCSI INTERNAL

CABLES

SCSI EXTERNAL

CABLE

50–POSITION

HIGH DENSITY

CONNECTORS

DS2109

022698 3/10

The DS2109 consists of 3 major functional blocks,

Figure 2:

•Voltage reference, terminating resistors, and isolation

switches

•Bus current sensing circuitry

•Bus ground sensing circuity

When the Bus Ground Sensing Circuitry or Bus Current

Sensing Circuitry determine that the SCSI bus is prop-

erly terminated without the DS2109, the power down

buffer isolates the resistors from the SCSI bus and dis-

ables the power amp, thereby placing the DS2109 in a

low power mode (the bus sensing circuitry always stays

active). The PDO (Power Down Output) pin can be con-

nected to the pin of a DS21S07A SCSI terminator for

Wide SCSI configurations, Figure 8.

DS2109 BLOCK DIAGRAM Figure 2

VREF

–

TCS

R14

R18

110 OHMS

BANDGAP

REFERENCE

POWER

DOWN

BUFFER

BUS CURRENT

SENSING

CIRCUITRY

BUS GROUND

SENSING

CIRCUITRY

PDI

PDE

PDO

DISABLE DISABLE

DS2109

022698 4/10

DETAILED PIN DESCRIPTION Table 1

PIN SYMBOL FCN DESCRIPTION

1 PDI I Power Down Internal. For Bus Ground Sensing Circuit connect to pin 22

of internal SCSI connector, see Figure 4.

2–6,

10–13,

16–19,

24–27

R1...R13

R15...18 TSignal Termination. 110 ohm termination. Connect to SCSI bus signal

lines. For Bus Current Sensing Circuit connect to data, parity, and control

lines, except SCSI–RST line, see Figure 6.

7, 8, 21 HS–GND P Heat Sink Ground. Internally connected to the mounting pad. Should be

either grounded or electronically isolated from other circuitry.

9 PDO O Power Down Output. Connect to DS21S07A for Wide SCSI applica-

tions, see Figure 8.

14 TPWR P Termination Power . Connect to the SCSI TERMPWR line. Bypass with

a 2.2 µF capacitor, see Figures 4 and 6.

15 VREF OReference Voltage. 2.85 volt reference; must be decoupled with a 4.7 µF

capacitor, see Figures 4 and 6.

20 PDE I Power Down External. For Bus Ground Sensing Circuit, connect to pin

36 of external SCSI connector, see Figure 4.

22 TCS I Termination Current Sense. Used to sense current on the SCSI bus.

For Bus Current Sensing Circuit, connect to SCSI signal line –RST, see

Figure 6.

23 R14 T Signal Termination. 110 ohm termination. Connect to SCSI bus signal

line. For Bus Current Sensing Circuit connect to SCSI controller chip, see

Figure 6.

28 GND P Ground. Signal ground; 0.0 volts.

ACTIVE TERMINATION

The voltage regulator circuitry (bandgap reference and

class AB power amplifier) produces a precise laser–

trimmed 2.85 volt level and is capable of sourcing 24 mA

into each of the terminating resistors when the signal

line is low (active). 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 1 10Ω

value over the entire voltage range. T o maintain the spe-

cified regulation, a 4.7 µF capacitor is required between

the VREF pin and ground. A high frequency cap (0.1 µF

ceramic recommended) can also be placed on the VREF

pin in applications that use fast rise/fall time drivers. The

power down capacitance on terminating resistors

R1–R13 and R15–R18 is <4 pF; R14 is slightly higher

due to the bus current sensing circuitry.

The DS2109 can be removed from the SCSI bus by

using either of two automatic methods, Bus Ground

Sensing or Bus Current Sensing.

As with all analog circuitry , the TERMPWR lines should

be bypassed locally. A 2.2 µF capacitor is recom-

mended between TPWR and ground and placed as

close as possible to the DS2109. The DS2109 should

be placed as close as possible to the connector to mini-

mize signal and power trace length, thereby resulting in

less input capacitance and reflections which can

degrade the bus signals.

BUS GROUND SENSING

If internal and external SCSI devices are connected to

the SCSI bus, the DS2109 will disconnect from the SCSI

bus according to the circuit in Figure 3. To utilize this

automatic disconnect method of bus sensing, configure

the DS2109 as shown in Figure 4. The PDI (Power

Down Internal) pin should be connected to pin 22 of the

internal SCSI connector, and the PDE (Power Down

External) connected to pin 36 of the external SCSI con-

nector.

DS2109

022698 5/10

BUS GROUND SENSING CIRCUIT Figure 3

PDE

PDI DISABLE

BUS GROUND DETECT CONFIGURATION Figure 4

VREF

INTERNAL SCSI CONNECTOR

EXTERNAL SCSI

CONNECTOR

22 26

TERMPWR

2.2 µF

4.7 µF

DS2109

PDI

TPWR

GND

PDE

DS2109

022698 6/10

BUS CURRENT SENSING

The DS2109 has the capability to use current sensing to

determine if the SCSI bus is over– or under–terminated.

A series 1 ohm resistor between pads R14 and TCS is

inserted into the SCSI bus (preferably the –RST line)

and used to monitor the current when that line pulls low

(active, or “asserted” state). Based on the current mea-

sured, the DS2109 will disconnect or connect from the

SCSI bus. The configuration for this automatic isolation

technique is shown in Figure 6.

Figure 5 shows a simplified diagram of the sensing cir-

cuit. The voltage across the 1 ohm sense resistor is dif-

ferentially amplified and converted into a single–ended

voltage with respect to ground. This is fed into a bank of

comparators and measured against a reference volt-

age. The circuit takes a measurement each time TCS is

driven below a 0.8 volts threshold and the outputs are

latched on the rising edge of TCS. If IBUS is greater than

32 mA, the DS2109 will be isolated from the SCSI bus. It

is recommended that the signal on TCS be asserted for

at least 25 msec to allow the signal (and comparator out-

puts) to settle into a known state. A timing diagram of the

sensing and latching operation is shown in Figure 7.

It is preferred that the –RST line be used for monitoring

the bus termination status because –RST is only

asserted during power up or during a major change in

bus configuration. Note that R14 will have a higher input

capacitance than the other lines because of the addi-

tional circuitry required for bus sensing.

The DS2109 will be isolated from the SCSI bus as

shown in Table 2.

BUS CURRENT SENSING CIRCUITRY Figure 5

REF

DECODE

AND

LATCH

TCSR14

1 OHM

110 OHM

+–

IBUS

TO SCSI CONTROLLER FROM SCSI BUS (–RST)

DS2109

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BUS CURRENT SENSE CONFIGURATION Figure 6

VREF

INTERNAL SCSI

CONNECTOR

26 40

–RST

SCSI

CONTROLLER

EXTERNAL SCSI

CONTROLLER

TERMPWR

DS2109

TPWR

GND

R13

R15

R18

TCS

R14

2.2 µF

4.7 µF

BUS CURRENT SENSE TIMING DIAGRAM Figure 7

25 µs min

TCS

COMPARATORS

LATCH

DS2109

022698 8/10

DISCONNECT MODES Table 2

PDI PDE IBUS > 32 mA? DS2109 ISOLATED

FROM SCSI BUS?

0 0 No Isolated

0 1 No Connected

1 0 No Connected

1 1 No Connected

0 0 Yes Isolated

0 1 Yes Isolated

1 0 Yes Isolated

1 1 Yes Isolated

NOTE: “1” denotes pin left open circuited.

WIDE SCSI CONFIGURATION Figure 8

VREF2

TERMPWR2

4.7 µF

GND

R18

R17

R16

R15

R14

TCS

HS–GND

PDE

R13

R12

R11

R10

VREF

PDI

HS–GND

PDO

TPWR

TERMPWR1

VREF1

GND

DS21S07A 16–PIN SOIC (300 MIL)

2.2 µFTERMINATION POWER LINE

DS2109S 28–PIN SOIC (300 MIL) 4.7 µF

DS2109

022698 9/10

ABSOLUTE MAXIMUM RATINGS*

Voltage on Any Pin Relative to Ground –1.0V to +7.0V

Operating Temperature 0°C to 70°C

Storage Temperature –55°C to +125°C

Soldering Temperature 260°C for 10 seconds

* This is a stress rating only and functional operation of the device at these or any other conditions above those

indicated in the operation sections of this specification is not implied. Exposure to absolute maximum rating

conditions for extended periods of time may af fect reliability.

RECOMMENDED OPERATING CONDITIONS (0°C to 70°C)

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

TERMPWR Voltage VTP 4.00 5.25 V

Logic 1 VIH 2.0 VTP+0.3 V 1

Logic 0 VIL –0.3 +0.8 V 1

DC CHARACTERISTICS (0°C to 70°C)

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

TERMPWR Current ITP

ITP 10 500

14 mA

mA 2, 4

2, 5

Power Down Current IPD 1 2 mA 2, 3, 6

Termination Resistance RTERM 108 110 112 ohms 2, 3

Die Thermal Shutdown TSD150 °C 2

Power Down Termination

Capacitance CPD

C14 3

84.5 pF

pF 2, 3, 6, 7

2,3,6,7,8

Input Leakage High IIH –1.0 µA2, 12

Input Leakage Low IIL 1.0 µA2, 9, 12

Output Current IO4 mA 10

Bus Current Sense T rip Point IBCST 32 mA 11

REGULATOR CHARACTERISTICS (0°C to 70°C)

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

Output Voltage VREF 2.79 2.85 2.91 V 2, 3

Drop Out Voltage VDROP 0.75 1.0 V 4, 7

Line Regulation LIREG 1.0 2.0 % 2, 5

Load Regulation LOREG 1.3 3.0 % 2, 3

Current Limit IL700 mA 2

Sink Current ISINK 400 mA 2

The chamfer on the body is optional. If it is not present, a termi-

nal 1 identifier must be positioned so that 1/2 or more of its area

is contained in the hatched zone.

DS2109

022698 10/10

NOTES:

1. PDI, PDE, TCS

2. 4.00V < TERMPWR < 5.25V.

3. 0.0V < signal lines < 3.0V.

4. All signal lines = 0.0V.

5. All signal lines open.

6. Power down enabled.

7. Guaranteed by design; not production tested.

8. C14 slightly higher capacitance due to sensing circuitry.

9. Excluding PDI, PDE pins.

10.PDO output pin.

11. IBCST > 32 mA – disable termination.

12.Excluding PDE, PDI, R14, and TCS pins.

28–PIN SOIC (300 MIL)

PKG 28–PIN

DIM MIN MAX

A IN.

MM 0.094

2.39 0.105

2.67

A1 IN.

MM 0.004

0.102 0.012

0.30

A2 IN.

MM 0.089

2.26 0.095

2.41

b IN.

MM 0.013

0.33 0.020

0.51

C IN

MM 0.009

0.229 0.013

0.33

D IN.

MM 0.698

17.73 0.712

18.08

e IN.

MM 0.050 BSC

1.27 BSC

E1 IN.

MM 0.290

7.37 0.300

7.62

H IN

MM 0.398

10.11 0.416

10.57

L IN

MM 0.016

0.40 0.040

1.02

Θ0°8°