DS21T09
Plug and Play SCSI Terminator
www.dalsemi.com
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
PDI
R1
R2
R3
R4
R5
HS-GND
HS-GND
PDO
R6
R7
R8
R9
TPWR
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
GND
R18
R17
R16
R15
R14
TCS
HS-GND
PDE
R13
R12
R11
R10
V
REF
DS21T09S 28-Pin SOIC (300-mil)
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
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112099
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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DS21T09
DETAILED PIN DESCRIPTION
Table 1
PIN
1
2-6,
10-13,
16-19,
24-27
7, 8, 21
9
14
15
20
22
23
28
SYMBOL FCN
DESCRIPTION
I
Power-Down Internal.
For Bus Ground Sensing Circuit, connect to pin
PDI
22 of internal SCSI connector; see Figure 4.
R1…R13
T
Signal Termination.
110Ω termination. Connect to SCSI bus signal
R15…R18
lines. For Bus Current Sensing Circuit connect to data, parity, and
control lines, except SCSI-RST line; see Figure 6.
HS-GND
PDO
P
O
P
O
I
I
T
P
TPWR
V
REF
PDE
TCS
R14
GND
Heat Sink Ground.
Internally connected to the mounting pad. Should
be either grounded or electronically isolated from other circuitry.
Power-Down Output.
Connect to DS21T07 for Wide SCSI
applications; see Figure 8.
Termination Power.
Connect to the SCSI TERMPWR line. Bypass
with a 2.2 µF capacitor; see Figures 4 and 6.
Reference Voltage.
2.85-volt reference; must be decoupled with a 4.7
µF capacitor; see Figures 4 and 6.
Power-Down External.
For Bus Ground Sensing Circuit, connect to pin
36 of external SCSI connector; see Figure 4.
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.
Signal Termination.
110Ω termination. Connect to SCSI bus signal
lines. For Bus Current Sensing Circuit connect to SCSI controller chip;
see Figure 6.
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 V
ref
will move less than
60 mv. To maintain the specified regulation, a 4.7
µF
capacitor is required between the V
REF
pin and
ground. A high frequency capacitor (0.1
µF
ceramic recommended) can also be placed on the V
ref
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
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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