dallas semiconductor DS2121A service manual

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DS2121A

Ultra3 LVD/SE SCSI Terminato

FEATURES

PIN ASSIGNMENT

 Fully compliant with Ultra2, Ultra3, and

Ultra 160 LVD-only SCSI

 Provides Low Voltage Differential (LVD)

termination for 9 signal line pairs

 5% tolerance on SE and LVD termination

resistance

 Low power-down capacitance of 3 pF
 Onboard thermal shutdown circuitry
 SCSI bus hot plug compatible

VREF

RIP

RIN
R2P
R2N

HS GND

R3P

R3N

R4P
R4N
R5P
R5N

ISO

GND

1
2
3

4
5
6
7
8
9
10
11
12
13
14

DS2121AE 28-Pin TSSOP

28
27
26
25
24
23
22
21
20
19
18
17
16
15

TPWR

TPWR
R9N
R9P
R8N
R8P
HS GND
R7N
R7P
R6N
R6P
DIFF_CAP
DIFFSENSE
MSTR/SLV

DESCRIPTION

The DS2121A LVD SCSI Terminator is a Low Voltage Differential (LVD) terminator. If the device is
connected in an LVD-only bus, the DS2121A will use LVD termination. If any SE devices are connected
to the bus, the DS2121A will disconnect from the bus. This is accomplished automatically inside the part
by sensing the voltage on the SCSI bus DIFFSENS line.

For the LVD termination, the DS2121A integrates two current sources with nine precision resistor
strings. Three DS2121A terminators are needed for a wide SCSI bus.

REFERENCE DOCUMENTS

SCSI Parallel Interface 2 (SPI-2) {X310/1142D}
SCSI-3 Parallel Interface (SPI) {X3T10/855D}
SCSI-3 Fast-20 {X3T10/1071D}
SCSI-2 {X3.131-1994}

Available from:

Global Engineering Documents
15 Inverness Way East
Englewood, CO 80112-5704
Phone: [phone]
Fax: [phone]

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DS2121A

FUNCTIONAL DESCRIPTION

The DS2119 combines LVD and SE termination with DIFFSENS sourcing and detection.

A bandgap reference is fed into two amplifiers, which creates a 1.25V If the SCSI bus is in LVD mode,
then the 1.25V reference will be used. The DIFFSENSE circuitry decodes trinary logic. Th ere will be on e
of three voltages on the SCSI control line called DIFFSENS. Two comparators and a NAND gate
determine if the voltage is below 0.6V, above 2.15V, or in between. That indicates the mode o f the bus to
be SE, HVD, or LVD, respectively.

The DS2121A’s DIFF_CAP pin monitors the DIFFSENS line to determine the proper operating mode of
the device. The DIFFSENSE pin can also drive the SCSI DIFFSENS line (when MSTR/SLV = 1) to
determine the SCSI bus operating mode. The DS2121A switches to the termination mode that is
appropriate for the bus based on the value of the DIFFSENS voltage. These modes are:

LVD mode LVD termination is provided by a precision laser trimmed resistor string with two current
sources. This configuration yields a 105Ω=differential and 150Ω=common mode impedance. A fail-safe
bias of 112 mV is maintained when no drivers are connected to the SCSI bus.

SE mode The DS2121A identifies that there is a SE (single-ended) device on the SCS I bus and isolates
the termination pins from the bus.

HVD Isolation Mode The DS2121A identifies that there is an HVD (high voltage differential) device on
the SCSI bus and isolates the termination pins from the bus.

When ISO = 1, the termination pins are isolated from the SCSI bus, Vref remains active. During thermal
shutdown, the termination pins are isolated from the SCSI bus and Vref becomes high impedance. The
DIFFSENSE driver is shut down during either of these two events.

To ensure proper operation, the TPWR pin should be connected to the SCSI bus TERMPWR line. As
with all analog circuitry, the TERMPWR and VDD lines should be bypassed locally. A 2.2 µF capacitor
and a 0.01 µF high frequency capacitor are recommended between TPWR and ground and placed as close
as possible to the DS2121A. The DS2121A should be placed as close as possible to the SCSI connector to
minimize signal and power trace length, thereby resulting in less input capacitance and reflections which
can degrade the bus signals.

To maintain the specified regulation, a 4.7 µF capacitor is required between the Vref pin (VREF) and
ground of each DS2121A. A high frequency cap (0.1 µF ceram ic recommended) can also be placed on
the Vref pin in applications that use fast rise/fall time drivers. A typical SCSI bus configuration is shown
in Figure 2.

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DS2121A

DIFFSENS noise filtering The DS2121A incorporates a digital filter to remove high frequency
transients on the DIFFSENS control line, thereby eliminating erroneous switching between modes. This
filter eliminates the need for the external capacitor and resistor, which heretofore performed this function.

NOTE:

DIFFSENS - Refers to the SCSI bus signal.
DIFFSENSE - Refers to the Dallas Semiconductor pin name and internal circuitry relating the
DIFFSENS driver.

DS2121A BLOCK DIAGRAM Figure 1

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