DS21T09 ,Plug and Play SCSI TerminatorFEATURES PIN ASSIGNMENT Fully compliant with SCSI-1, Fast SCSI andUltra SCSI PDI 1 28 GND2 27 R18R ..
DS2223 ,256朾it EconoRAMDS2223/DS2224DS2223/DS2224EconoRAM
DS2223 ,256朾it EconoRAMDS2223/DS2224DS2223/DS2224EconoRAM
DS2223Z ,256朾it EconoRAMDS2223/DS2224DS2223/DS2224EconoRAM
DS2224 ,32朾it serial number (ROM), 224朾it SRAMDS2223/DS2224DS2223/DS2224EconoRAM
DS2227-100 ,Flexible NV SRAM StikFEATURES PIN ASSIGNMENT Flexibly organized as 128k x 32, 256k x 16,or 512k x 8 bits Data retentio ..
E53NA50 ,NABSOLUTE MAXIMUM RATINGSSymbol Parameter Value UnitV Drain-source Voltage (V =0) 500 VDS GSV 500 VD ..
DS21T09
Plug and Play SCSI Terminator
FEATURESFully compliant with SCSI-1, Fast SCSI and
Ultra SCSICompatible with Plug and Play SCSI ProfileFunctional drop in replacement for the
DS2109Complementary part to DS21T07 for wide
SCSIProvides 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 ASSIGNMENT
DESCRIPTIONThe DS21T09 is intended for one-chip Plug and Play (PnP) SCSI termination. Plug and Play SCSIrequires 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 DS21T09contains an output port that can control the power-down pin of additional terminators (DS21T07) for
Wide SCSI applications.
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
DS21T09R17
R16
R15
TCS
HS-GND
PDE
R13
R12
R11
R10
VREF
R14R3
HS-GND
HS-GND
PDO
TPWR
PDI
GND
R18
DS21T09
PnP SCSI Specification
PnP ISA Specification
PnP BIOS SpecificationPnP Option ROM Specification
Available from: Plug and Play forum on
CompuServe (Go plugplay).
FUNCTIONAL DESCRIPTIONThe 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 thetermination 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
DS21T09
The DS21T09 consists of three major functional blocks, Figure 2:Voltage reference, terminating resistors, and isolation switchesBus current sensing circuitryBus 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
DS21T09
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 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 negationdriver, 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 Vref will move less than
60 mv. To maintain the specified regulation, a 4.7 μF capacitor is required between the VREF pin and
ground. A high frequency capacitor (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 <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 tracelength, thereby resulting in less input capacitance and reflections which can degrade the bus signals.
DS21T09
BUS GROUND SENSINGIf 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
DS21T09
BUS CURRENT SENSINGThe DS21T09 has the capability to use current sensing to determine if the SCSI bus is over- or under-
terminated. A 1Ω=resistor in series with the SCSI bus (preferable the RST line) is used to monitor the bus
current when that line pulls low (Active or “asserted” state). The resistor is between pad R14 and TCS on
the die. Based on the current measured, the DS21T09 will disconnect or connect from the SCSI bus. Theconfiguration for this automatic isolation technique is shown in Figure 6.
Figure 5 shows a simplified diagram of the sensing circuit. The voltage across the 1Ω=sense resistor is
differentially 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 voltage. The circuit takes a measurement eachtime 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 DS21T09 will be isolated from the SCSI bus. It is recommended that the
signal on TCS be asserted for at least 25 μsec to allow the signal (and comparator outputs) 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 additional circuitry required for bus sensing.
The DS21T09 will be isolated from the SCSI bus as shown in Table 2.
BUS CURRENT SENSING CIRCUITRY Figure 5
