DS21T09S

Manufacturer Part NumberDS21T09S
DescriptionPlug and Play SCSI Terminator
ManufacturerDallas Semiconducotr
DS21T09S datasheet
 


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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
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 110 =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
Plug and Play SCSI Terminator
PIN ASSIGNMENT
PDI
1
2
R1
3
R2
4
R3
5
R4
6
R5
7
HS-GND
8
HS-GND
9
PDO
10
R6
11
R7
12
R8
13
R9
14
TPWR
DS21T09S 28-Pin SOIC (300-mil)
1 of 10
DS21T09
28
GND
27
R18
26
R17
25
R16
24
R15
23
R14
22
TCS
21
HS-GND
20
PDE
19
R13
18
R12
17
R11
16
R10
15
V
REF
112099

DS21T09S Summary of contents

  • Page 1

    ... Plug and Play SCSI Terminator PIN ASSIGNMENT PDI HS-GND 8 HS-GND 9 PDO TPWR DS21T09S 28-Pin SOIC (300-mil DS21T09 28 GND 27 R18 26 R17 25 R16 24 R15 23 R14 22 TCS 21 HS-GND 20 PDE 19 R13 18 R12 17 R11 16 R10 15 ...

  • Page 2

    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 single chip termination subsystem for use in PnP ...

  • Page 3

    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 ...

  • Page 4

    DETAILED PIN DESCRIPTION Table 1 PIN SYMBOL FCN 1 I PDI 2-6, R1…R13 T 10-13, R15…R18 16-19, 24- HS-GND PDO 14 TPWR REF 20 I PDE 22 TCS I 23 ...

  • Page 5

    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 ...

  • Page 6

    BUS CURRENT SENSING The DS21T09 has the capability to use current sensing to determine if the SCSI bus is over- or under- terminated =resistor in series with the SCSI bus (preferable the RST line) is used to monitor ...

  • Page 7

    BUS CURRENT SENSE CONFIGURATION Figure 6 BUS CURRENT SENSE TIMING DIAGRAM Figure ...

  • Page 8

    DISCONNECT MODES Table 2 PDI PDE NOTE: “1” denotes pin left open circuited. WIDE SCSI CONFIGURATION Figure 8 I > 32 mA? DS21T09 ISOLATED ...

  • Page 9

    ABSOLUTE MAXIMUM RATINGS* Voltage on Any Pin Relative to Ground Operating Temperature Storage Temperature Soldering Temperature * This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the ...

  • Page 10

    NOTES TCS PDI PDE 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 ...