dallas semiconductor DS2109 service manual

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DS2109

DS2109

Plug and Play SCSI Terminator

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

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 exter­nal device is connected to the system. The DS2109 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
DS2109 will automatically be isolated from the SCSI
bus thereby maintaining proper system termination.

PIN ASSIGNMENT

1

PDI

2

R1

3

R2

4

R3

5

R4

6

R5
HS–GND
HS–GND

PDO

TPWR

DS2109, DS2109S 28–PIN SOIC (300 MIL)

7
8
9

R6

10

R7

11

R8

12

R9

13
14

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 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 DS2109 contains an
output port that can control the power down pin of addi­tional terminators (DS21S07A) for Wide SCSI applica­tions.

GND

28

R18

27

R17

26

R16

25

R15

24

R14

23

TCS

22

HS–GND

21

PDE

20

R13

19

R12

18

R11

17

R10

16

V

15

REF

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DS2109

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]

PnP SCSI Specification
PnP ISA Specification
PnP BIOS Specification
PnP Option ROM Specification
Available from: Plug and Play forum on
CompuServe (Go plugplay).

DS2109 APPLICATION ENVIRONMENT Figure 1

PERIPHERAL
SUBSYSTEM

EXTERNAL

DS2109 AUTOMATIC

EXIT–POINT TERMINATOR

HOST SCSI

PROTOCOL CHIP

TERMINATOR

(TWO DS21S07A)

50–POSITION
HIGH DENSITY
CONNECTORS

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 mother­board, the DS2109 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 DS21S07A.

SCSI EXTERNAL

CABLE

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SCSI

PERIPHERAL

DEVICES

SCSI INTERNAL

CABLES

INTERNAL

TERMINATOR

(TWO DS21S07A)

DS2109

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-

DS2109 BLOCK DIAGRAM Figure 2

PDI

PDE

PDO

BUS GROUND

SENSING

CIRCUITRY

DISABLE DISABLE

POWER

DOWN

BUFFER

erly terminated without the DS2109, the power down
buffer isolates the resistors from the SCSI bus and dis­ables 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 con­nected to the pin of a DS21S07A SCSI terminator for
Wide SCSI configurations, Figure 8.

BUS CURRENT

SENSING

CIRCUITRY

TCS

BANDGAP

REFERENCE

110 OHMS

+

–

110 OHMS

110 OHMS

R14

R18

R1

V

REF

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