DS2109 ,Plug and Play SCSI TerminatorFUNCTIONAL DESCRIPTIONSCSI–2 (X3.131–1994) The DS2109 is designed to be a single chip terminationSC ..
DS2117M ,Ultra2 LVD/SE SCSI TerminatorFUNCTIONAL DESCRIPTIONThe DS2117 combines LVD and SE termination with DIFFSENSE sourcing and detect ..
DS2118M ,Ultra3 LVD/SE SCSI TerminatorFEATURES PIN CONFIGURATION Fully Compliant with SCSI SPI-2, SPI-3, SPI-4, Ultra160, and Ultra32 ..
DS2118MB ,Ultra2 LVD/SE SCSI TerminatorFEATURES PIN CONFIGURATION Fully Compliant with SCSI SPI-2, SPI-3, SPI-4, Ultra160, and Ultra32 ..
DS2118M-B ,Ultra2 LVD/SE SCSI TerminatorFEATURES PIN CONFIGURATION Fully Compliant with SCSI SPI-2, SPI-3, SPI-4, Ultra160, and Ultra32 ..
DS2119M ,Ultra3 LVD/SE SCSI TerminatorFUNCTIONAL DESCRIPTION The DS2119 combines LVD and SE termination with DIFFSENS sourcing and detect ..
DS2109
Plug and Play SCSI Terminator
DS2109Plug and Play SCSI Terminator
DS2109
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FEATURESFully compliant with SCSI–1, Fast SCSI and Ultra
SCSICompatible with Plug and Play SCSI ProfileFunctionally compatible with DS21S07AProvides active termination for 18 signal lines2% tolerance on termination resistors and voltage
regulatorBus termination sensingLow power down capacitance of 3 pFOnboard thermal shutdown circuitry
PIN ASSIGNMENTGND
R18
R17
R16
R15
R14
TCS
HS–GND
PDE
R13
R12
R11
R10
VREF
PDI
HS–GND
HS–GND
PDO
TPWR
DS2109, DS2109S 28–PIN SOIC (300 MIL)
DESCRIPTIONThe 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.
DS2109
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REFERENCE DOCUMENTSSCSI–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 DESCRIPTIONThe 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
DS2109
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The DS2109 consists of 3 major functional blocks,
Figure 2:Voltage reference, terminating resistors, and isolation
switchesBus current sensing circuitryBus 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 2VREF
TCS
R14
R18
PDI
PDE
PDO
DS2109
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DETAILED PIN DESCRIPTION Table 1
ACTIVE TERMINATIONThe 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 VREF pin and ground. A high frequency cap (0.1 μF
ceramic recommended) can also be placed on the VREF
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.
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.
BUS GROUND SENSINGIf 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.
DS2109
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BUS GROUND SENSING CIRCUIT Figure 3PDE
PDI
DISABLE
BUS GROUND DETECT CONFIGURATION Figure 4
DS2109
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BUS CURRENT SENSINGThe DS2109 has the capability to use current sensing to
determine if the SCSI bus is over– or under–terminated.
A series 1 ohm resistor between pads R14 and TCS is
inserted into the SCSI bus (preferably the –RST line)
and used to monitor the current when that line pulls low
(active, or “asserted” state). Based on the current mea-
sured, the DS2109 will disconnect or connect from the
SCSI bus. The configuration for this automatic isolation
technique is shown in Figure 6.
Figure 5 shows a simplified diagram of the sensing cir-
cuit. The voltage across the 1 ohm sense resistor is dif-
ferentially amplified and converted into a single–ended
voltage with respect to ground. This is fed into a bank of
comparators and measured against a reference volt-
age. The circuit takes a measurement each time TCS is
driven below a 0.8 volts threshold and the outputs are
latched on the rising edge of TCS. If IBUS is greater than
32 mA, the DS2109 will be isolated from the SCSI bus. It
is recommended that the signal on TCS be asserted for
at least 25 msec to allow the signal (and comparator out-
puts) to settle into a known state. A timing diagram of the
sensing and latching operation is shown in Figure 7.
It is preferred that the –RST line be used for monitoring
the bus termination status because –RST is only
asserted during power up or during a major change in
bus configuration. Note that R14 will have a higher input
capacitance than the other lines because of the addi-
tional circuitry required for bus sensing.
The DS2109 will be isolated from the SCSI bus as
shown in Table 2.
BUS CURRENT SENSING CIRCUITRY Figure 5REF
TCSR14
1 OHM
110 OHM
TO SCSI CONTROLLERFROM SCSI BUS (–RST)
