Dallas Semiconductor DS21T09S Datasheet

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DS21T09

Plug and Play SCSI Terminato

FEATURES

PIN ASSIGNMENT

 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

PDI

R1
R2
R3
R4

R5
HS-GND
HS-GND

PDO

R6
R7
R8
R9

TPWR

DS21T09S 28-Pin SOIC

1
2
3

4
5
6
7
8
9
10
11
12
13
14

28
27
26
25
24
23
22
21
20
19
18
17
16
15

300-mil

GND
R18
R17
R16
R15

R14
TCS

HS-GND
PDE
R13
R12
R11
R10
V

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: [phone]
Fax: [phone]

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DS21T09

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

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DS21T09

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

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