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