MAXIM DS2120 Technical data

Note:

ce may incorporate deviations from published specifications known as errata. Multiple

revisions of any device may be simultaneously available through various sales channels. For information about device

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DS2120

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Ultra3 LVD SCSI Terminator

FEATURES

§ Fully compliant with Ultra3, Ultra160,
Ultra320, and Ultra2 (LVD only) SCSI

§ Provides low-voltage differential (LVD)
termination for nine signal line pairs

§ Zero-temperature coefficient-termination
resistors

§ Auto-select of LVD termination

§ 5% tolerance on LVD termination resistance

§ Low power-down capacitance of 3pF

§ Built-in mode change filter/delay

§ On-board thermal-shutdown circuitry

§ SCSI bus hot-plug compatible

PIN ASSIGNMENT

VREF

R1P
R1N
R2P

R2N

HS GND

R3P

R3N

R4P
R4N
R5P
R5N

ISO

GND

DS2120E 28-Pin TSSOP

TPWR

TPWR
R9N
R9P
R8N
R8P

HS GND
R7N
R7P
R6N

R6P

DIFF_CAP

DIFFSEN SE

MSTR/SLV

APPLICATIONS

§ Raid Systems

§ SCSI Host Bus Ad apter Cards (HBA)

§ Servers

§ SCSI Cables

§ Network Attached Storage (NAS)

§ Storage Area Networks (SANs)

ORDERING INFORMATION

DS2120E 28-Pin TSSOP 0°C to +70°C
DS2120B 36-Pin SSOP 0°C to +70°C

VREF

R1N

R2N
HS GND
HS GND
HS GND

R3N

R4N

R5N

GND

1

R1P
R2P

R3P
R4P
R5P

ISO

DS2120B 36 -Pin SSOP

TPWR
LVD

R9N
R9P

R8N

R8P

HS GND
HS GND

HS GND
R7N
R7P
R6N
R6P
DIFF_CAP
DIFFSENSE
MSTR/SLV

DESCRIPTION

The DS2120 Ultra3 LVD SCSI terminato r is a low-voltage differential (LVD) terminator. If the device is
connected in an LVD-only bus, the DS2120 uses LVD termination. If any single-ended (SE) or high­voltage differential (HVD) devices are connected to the bus, the DS2120 disconnects from the bus. This
is accomplished inside the part automatically by sensing the voltage on the SCSI bus DIFFSENS line.

For the LVD termination, the DS2120 integrates two current sources with nine precision resistor strings.
Three DS2120 terminators are needed for a wide SCSI bus.

Some revisions of this devi

errata, click here: http://dbserv.maxim-ic.com/errata.cfm.

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REFERENCE DOCUMENTS

Small Computer Systems Interface (SCSI-3) SCSI Parallel Interface (SPI) Project: 0855-M, 1995
Small Computer Systems Interface (SCSI-3) SCSI Parallel Interface 2 (SPI-2) Project: 1142-M, 1998
Small Computer Systems Interface (SCSI-3) SCSI Parallel Interface 3 (SPI-3) Project: 1302-D, 1999
Small Computer Systems Interface (SCSI-3) SCSI Parallel Interface 4 (SPI-4) Project: 1365-D, 200x

Available from:

American National Standards Institute (ANSI) Phone: [phone]
Global Engineering Documents 15 Inverness Way East; Englewood, CO 80112 Phone: [phone]

FUNCTIONAL DESCRIPTION

The DS2120 combines LVD termination with DIFFSENSE sourcing and detection.

LVD termination is provided by a laser-trimmed resistor biased with two current sources and a common­mode voltage source, generated from a bandgap reference of 1.25V. The configuration is a y-type
terminator with a 105Ω differential and 150Ω common-mode resistance. A fail-safe bias of 112mV is
maintained when no drivers are connected to the SCSI bus. In non-LVD mode, the resistors are isolated
from the bus.

The DIFF_CAP pin of DS2120 monitors the DIFFSENS line to determine the proper operating mode of
the device. If the voltage on the DIFF_CAP is between 0.7V and 1.9V, the device enters LVD mode after
the mode-change delay. If the voltage at the DIFF_CAP later crosses one of the thresholds, the DS2120
again changes modes after the mode-change delay. The mode-change delay is the same when changing in
or out of LVD mode. A new mode change can start anytime after a previous mode change has been
detected. These modes are the following:

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 112mV is maintained when no drivers are connected to the SCSI bus.

SE Isolation Mode: The DS2120 identifies that there is a SE (single-ended) device on the SCSI bus and
isolates the termination pins from the bus.

HVD Isolation Mode: The DS2120 identifies that there is an HVD device on the SCSI bus and isolates
the termination pins from the bus.

When ISO is pulled high, the termination pins are isolated from the SCSI bus and VREF remains active.
The mode-change delay/filter is still active and the LVD pin continues to indicate the correct bus mode.
During thermal shutdown, the termination pins are isolated from the SCSI bus and VREF becomes high
impedance. The DIFFSENS driver is shut down during either of these two events. The DIFF_CAP
receiver is disabled and the LVD goes low, indicating a non-LVD condition.

To ensure proper operation, the TPWR pin should be connected to the SCSI bus TERMPWR line. As
with all analo g circuitry, the TERMPWR and VDD lines should be bypassed locally. A 2.2µF capacitor
and a 0.01µF high-frequency capacitor are recommended between TPWR and ground and placed as close
as possible to the DS2120. The DS2120 should be placed as close as possible to the SCSI connector to

minimize signal and power trace length, thereby lessening input capacitance and reflections that can
degrade the bus signals.

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To maintain the specified regulation, a 4.7µF capacitor is required between the VREF pin and ground of
each DS2120. 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. A typical SCSI bus configuration is shown in Figure 2.

DIFFSENS Noise Filtering: The DS2120 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 previously performed this function.
The external filter can be used in addition to the digital filter if the DS2120 and DS2118M or DS2119M
are to be used interchangeably.

NOTES:

1) DIFFSENS: Refers to the SCSI bus signal.

2) DIFFSENSE: Refers to the Dallas Semiconductor pin name and internal circuitry relating to
differential sensing.

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