DS2109
DS2109
Plug and Play SCSI Terminator
FEATURES
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
•
Fully
SCSI
compliant with SCSI–1, Fast SCSI and Ultra
•
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
DS2109, DS2109S 28–PIN SOIC (300 MIL)
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 exter-
nal device is connected to the system. The DS2109 sat-
isfies 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.
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 immu-
nity to voltage drops on the TERMPWR (TERMination
PoWeR) line, enhanced high–level noise immunity,
intrinsic TERMPWR decoupling, and very low quies-
cent current consumption. The DS2109 contains an
output port that can control the power down pin of addi-
tional terminators (DS21S07A) for Wide SCSI applica-
tions.
022698 1/10
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: (800) 854–7179,(303) 792–2181
Fax: (303) 792–2192
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 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 mother-
board, the DS2109 can automatically sense the ter-
mination 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.
DS2109 APPLICATION ENVIRONMENT
Figure 1
PERIPHERAL
SUBSYSTEM
DS2109 AUTOMATIC
EXIT–POINT TERMINATOR
EXTERNAL
TERMINATOR
(TWO DS21S07A)
50–POSITION
HIGH DENSITY
CONNECTORS
HOST SCSI
PROTOCOL CHIP
SCSI EXTERNAL
CABLE
SCSI
PERIPHERAL
DEVICES
SCSI INTERNAL
CABLES
INTERNAL
TERMINATOR
(TWO DS21S07A)
022698 2/10
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-
erly terminated without the DS2109, the power down
buffer isolates the resistors from the SCSI bus and dis-
ables 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 con-
nected to the pin of a DS21S07A SCSI terminator for
Wide SCSI configurations, Figure 8.
DS2109 BLOCK DIAGRAM
Figure 2
PDI
BUS GROUND
SENSING
CIRCUITRY
DISABLE
DISABLE
BUS CURRENT
SENSING
CIRCUITRY
TCS
PDE
PDO
POWER
DOWN
BUFFER
BANDGAP
REFERENCE
+
110 OHMS
R14
–
110 OHMS
R18
110 OHMS
R1
V
REF
022698 3/10
DS2109
DETAILED PIN DESCRIPTION
Table 1
PIN
1
2–6,
10–13,
16–19,
24–27
7, 8, 21
9
14
15
20
22
SYMBOL
PDI
R1...R13
R15...18
FCN
I
T
DESCRIPTION
Power Down Internal.
For Bus Ground Sensing Circuit connect to pin 22
of internal SCSI connector, see Figure 4.
Signal Termination.
110 ohm termination. Connect to SCSI bus signal
lines. For Bus Current Sensing Circuit connect to data, parity, and control
lines, except SCSI–RST line, see Figure 6.
Heat Sink Ground.
Internally connected to the mounting pad. Should be
either grounded or electronically isolated from other circuitry.
Power Down Output.
Connect to DS21S07A for Wide SCSI applica-
tions, 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.
110 ohm termination. Connect to SCSI bus signal
line. For Bus Current Sensing Circuit connect to SCSI controller chip, see
Figure 6.
Ground.
Signal ground; 0.0 volts.
As with all analog circuitry, the TERMPWR lines should
be bypassed locally. A 2.2
µF
capacitor is recom-
mended between TPWR and ground and placed as
close as possible to the DS2109. The DS2109 should
be placed as close as possible to the connector to mini-
mize signal and power trace length, thereby resulting in
less input capacitance and reflections which can
degrade the bus signals.
HS–GND
PDO
TPWR
V
REF
PDE
TCS
P
O
P
O
I
I
23
R14
T
28
GND
P
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 24 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. To maintain the spe-
cified regulation, a 4.7
µF
capacitor is required between
the V
REF
pin and ground. A high frequency cap (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 <4 pF; R14 is slightly higher
due to the bus current sensing circuitry.
The DS2109 can be removed from the SCSI bus by
using either of two automatic methods, Bus Ground
Sensing or Bus Current Sensing.
BUS GROUND SENSING
If internal and external SCSI devices are connected to
the SCSI bus, the DS2109 will disconnect from the SCSI
bus according to the circuit in Figure 3. To utilize this
automatic disconnect method of bus sensing, configure
the DS2109 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 con-
nector.
022698 4/10
DS2109
BUS GROUND SENSING CIRCUIT
Figure 3
PDI
DISABLE
PDE
BUS GROUND DETECT CONFIGURATION
Figure 4
INTERNAL SCSI CONNECTOR
22
26
TERMPWR
2.2
µF
DS2109
38
36
PDE
TPWR
V
REF
4.7
µF
EXTERNAL SCSI
CONNECTOR
022698 5/10
PDI
GND
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