DS21T09 Datasheet by Analog Devices Inc./Maxim Integrated

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FEATURES

 Fully compliant with SCSI-1, Fast SCSI and

Ultra SCSI

 Compatible with Plug and Play SCSI Profile

 Functional drop in replacement for the

DS2109

 Complementary part to DS21T07 for wide

SCSI

 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

DESCRIPTION

The DS21T09 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 external device is connected to the system. The DS21T09 satisfies 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 DS21T09 will automatically be isolated from the SCSI bus thereby

maintaining proper system termination.

The DS21T09 integrates a low drop-out regulator, 18 precisely switched 110Ω=termination resistors, and

bus termination sensors into a 28-pin, 300-mil SOIC package. Active termination provides: greater

immunity to voltage drops on the TERMPWR (TERMination PoWeR) line, enhanced high-level noise

immunity, intrinsic TERMPWR decoupling, and very low quiescent current consumption. The DS21T09

contains an output port that can control the power-down pin of additional terminators (DS21T07) for

Wide SCSI applications.

REFERENCE DOCUMENTS

SCSI-2 (X3.131-1994)

SCSI-3 Parallel Interface (X3T10/855D)

Available from: Global Engineering Documents

15 Inverness Way East

Englewood, CO 80112-5704

Phone: (800) 854-7179, (303) 792-2181

Fax: (303) 792-2192

DS21T09

Plug and Play SCSI Terminato

www.dalsemi.com

R17

R16

R15

TCS

HS-GND

PDE

R13

R12

R11

R10

REF

R14

HS-GND

PDO

TPWR

DS21T09S 28-Pin SOIC

(

300-mil

)

PDI

GND

R18

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DS21T09

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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 DS21T09 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 motherboard, the DS21T09 can automatically sense the

termination 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 DS21T07.

DS21T09 APPLICATION ENVIRONMENT Figure 1

Bus GROUND SENSING cmcurmv DISABLE masts man DOWN BUFFER BUS CURRENT SENSING cumurmv TCS Ru RI! R1

DS21T09

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The DS21T09 consists of three major functional blocks, Figure 2:

• Voltage reference, terminating resistors, and isolation switches

• Bus current sensing circuitry

• Bus ground sensing circuitry

When the Bus Ground Sensing Circuitry or Bus Current Sensing Circuitry determines that the SCSI bus

is properly terminated without the DS21T09, the power-down buffer isolates the resistors from the SCSI

bus and disables the power amp, thereby placing the DS21T09 in a low power mode (the bus sensing

circuitry always stays active). The PDO (Power-Down Output) pin can be connected to the pin of a

DS21T07 SCSI terminator for Wide SCSI configurations, Figure 8.

DS21T09 BLOCK DIAGRAM Figure 2

DS21T09

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DETAILED PIN DESCRIPTION Table 1

PIN SYMBOL FCN DESCRIPTION

1PDI IPower-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…R18

TSignal Termination. 110! 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.

9PDO OPower-Down Output. Connect to DS21T07 for Wide SCSI

applications; 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 IPower-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! termination. Connect to SCSI bus signal

lines. 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 25 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 110Ω=value over the entire voltage range and Vref will move less than

60 mv. To maintain the specified regulation, a 4.7 µF capacitor is required between the VREF pin and

ground. A high frequency capacitor (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 <3 pF; R14 is slightly higher due to the bus current sensing circuitry.

The DS21T09 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

recommended between TPWR and ground. It should be placed as close as possible to the DS21T09. The

DS21T09 should be placed as close as possible to the connector to minimize signal and power trace

length, thereby resulting in less input capacitance and reflections which can degrade the bus signals.

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DS21T09

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BUS GROUND SENSING

If internal and external SCSI devices are connected to the SCSI bus, the DS21T09 will disconnect from

the SCSI bus according to the circuit in Figure 3. To utilize this automatic disconnect method of bus

sensing, configure the DS21T09 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 connector.

BUS GROUND SENSING CIRCUIT Figure 3

BUS GROUND DETECT CONFIGURATION Figure 4

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DS21T09

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BUS CURRENT SENSING

The DS21T09 has the capability to use current sensing to determine if the SCSI bus is over- or under-

terminated. A 1Ω=resistor in series with the SCSI bus (preferable the RST line) is used to monitor the bus

current when that line pulls low (Active or “asserted” state). The resistor is between pad R14 and TCS on

the die. Based on the current measured, the DS21T09 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 circuit. The voltage across the 1Ω=sense resistor is

differentially 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 voltage. 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 DS21T09 will be isolated from the SCSI bus. It is recommended that the

signal on TCS be asserted for at least 25 µsec to allow the signal (and comparator outputs) 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 additional circuitry required for bus sensing.

The DS21T09 will be isolated from the SCSI bus as shown in Table 2.

BUS CURRENT SENSING CIRCUITRY Figure 5

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DS21T09

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BUS CURRENT SENSE CONFIGURATION Figure 6

BUS CURRENT SENSE TIMING DIAGRAM Figure 7

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DS21T09

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DISCONNECT MODES Table 2

PDI PDE IBUS > 32 mA? DS21T09 ISOLATED

FROM SCSI BUS?

0 0 No Isolated

0 1 No Connected

1 0 No Connected

1 1 No Connected

00 Yes Isolated

01 Yes Isolated

10 Yes Isolated

11 Yes Isolated

NOTE: “1” denotes pin left open circuited.

WIDE SCSI CONFIGURATION Figure 8

ITP IPD TERV 14 11H 11L

DS21T09

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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 affect reliability.

The Dallas Semiconductor DS21T09 is built to the highest quality standards and manufactured for long

term reliability. All Dallas Semiconductor devices are made using the same quality materials and

manufacturing methods. However, the DS21T09 is not exposed to environmental stresses, such as burn-

in, that some industrial applications require. For specific reliability information on this product, please

contact the factory in Dallas at (972) 371-4448.

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

2, 4

2, 5

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

Termination Resistance RTERM 108 110 112 "2, 3

Die Thermal Shutdown TSD150 °C 2

Power-Down

Termination Capacitance

CPD

C14

8pF

2, 3, 6, 7

2, 3, 6, 7, 8

Input Leakage High IIH -1.0 µA 2, 12

Input Leakage Low IIL 1.0 µA 2, 9, 12

Output Current IO4mA10

Bus Current Sense

Trip 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 2.93 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 IL900 mA 2

Sink Current ISINK 400 mA 2

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DS21T09

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NOTES:

1. PDI , PDE , TCS

2. 4.00V < TERMPWR < 5.50V.

3. 0.0V < signal lines < TERMPWR.

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)

The chamfer on the body is optional. If it is not

present, a terminal 1 identifier must be

positioned so that ½ or more of its area is

contained in the hatched zone.

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

ΘΘ

Θ0° 8°

DS21T09 Datasheet by Analog Devices Inc./Maxim Integrated

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