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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 VREF
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 110=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 1 of 10 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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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. 110 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 VREF
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 F capacitor; see Figures 4 and 6. Reference Voltage. 2.85-volt reference; must be decoupled with a 4.7 F 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 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 110=value over the entire voltage range and Vref will move less than 60 mv. To maintain the specified regulation, a 4.7 F capacitor is required between the VREF pin and ground. A high frequency capacitor (0.1 F 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 R1R13 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 F 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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DS21T09
BUS CURRENT SENSING
The DS21T09 has the capability to use current sensing to determine if the SCSI bus is over- or underterminated. A 1=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. The configuration for this automatic isolation technique is shown in Figure 6. Figure 5 shows a simplified diagram of the sensing circuit. The voltage across the 1=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 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 DS21T09 will be isolated from the SCSI bus. It is recommended that the signal on TCS be asserted for at least 25 sec 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
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DS21T09
BUS CURRENT SENSE CONFIGURATION Figure 6
BUS CURRENT SENSE TIMING DIAGRAM Figure 7
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DS21T09
DISCONNECT MODES Table 2
PDI PDE
IBUS > 32 mA?
0 0 No 0 1 No 1 0 No 1 1 No 0 0 Yes 0 1 Yes 1 0 Yes 1 1 Yes NOTE: "1" denotes pin left open circuited.
DS21T09 ISOLATED FROM SCSI BUS? Isolated Connected Connected Connected Isolated Isolated Isolated Isolated
WIDE SCSI CONFIGURATION Figure 8
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DS21T09
ABSOLUTE MAXIMUM RATINGS*
Voltage on Any Pin Relative to Ground Operating Temperature Storage Temperature Soldering Temperature * -1.0V to +7.0V 0C to 70C -55C to +125C 260C for 10 seconds
This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operation sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability.
The Dallas Semiconductor DS21T09 is built to the highest quality standards and manufactured for long term reliability. All Dallas Semiconductor devices are made using the same quality materials and manufacturing methods. However, the DS21T09 is not exposed to environmental stresses, such as burnin, that some industrial applications require. For specific reliability information on this product, please contact the factory in Dallas at (972) 371-4448.
RECOMMENDED OPERATING CONDITIONS
PARAMETER TERMPWR Voltage Logic 1 Logic 0 SYMBOL VTP VIH VIL MIN 4.00 2.0 -0.3 TYP MAX 5.25 VTP + 0.3 +0.8
(0C to 70C)
UNITS V V V NOTES 1 1
DC CHARACTERISTICS
PARAMETER TERMPWR Current Power-Down Current Termination Resistance Die Thermal Shutdown Power-Down Termination Capacitance Input Leakage High Input Leakage Low Output Current Bus Current Sense Trip Point SYMBOL ITP ITP IPD RTERM TSD CPD C14 IIH IIL IO IBCST MIN TYP 10 1 110 150 8 MAX 500 14 2 112
(0C to 70C)
UNITS mA mA mA C pF pF A A mA mA NOTES 2, 4 2, 5 2, 3, 6 2, 3 2 2, 3, 6, 7 2, 3, 6, 7, 8 2, 12 2, 9, 12 10 11
108
-1.0 1.0 4 32
REGULATOR CHARACTERISTICS
PARAMETER Output Voltage Drop Out Voltage Line Regulation Load Regulation Current Limit Sink Current SYMBOL VREF VDROP LIREG LOREG IL ISINK MIN 2.79 TYP 2.85 0.75 1.0 1.3 900 400 MAX 2.93 1.0 2.0 3.0
(0C to 70C)
UNITS V V % % mA mA NOTES 2, 3 4, 7 2, 5 2, 3 2 2
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DS21T09
NOTES:
1. PDI , PDE , TCS 2. 4.00V < TERMPWR < 5.50V. 3. 0.0V < signal lines < TERMPWR. 4. All signal lines = 0.0V. 5. All signal lines open. 6. Power down enabled. 7. Guaranteed by design; not production tested. 8. C14 slightly higher capacitance due to sensing circuitry. 9. Excluding PDI , PDE pins. 10. PDO output pin. 11. IBCST > 32 mA - disable termination. 12. Excluding PDE , PDI , R14, and TCS pins.
28-PIN SOIC (300-MIL)
PKG DIM A IN. MM A1 IN. MM A2 IN. MM b IN. MM C IN. MM D IN. MM e IN. MM E1 IN. MM H IN. MM L IN. MM 28-PIN MIN MAX 0.094 0.105 2.39 2.67 0.004 0.012 0.102 0.30 0.089 0.095 2.26 2.41 0.013 0.020 0.33 0.51 0.009 0.013 0.229 0.33 0.698 0.712 17.73 18.08 0.050 BSC 1.27 BSC 0.290 0.300 7.37 7.62 0.398 0.416 10.11 10.57 0.016 0.040 0.40 1.02 0 8
The chamfer on the body is optional. If it is not present, a terminal 1 identifier must be positioned so that 1/2 or more of its area is contained in the hatched zone.
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