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DS2120

Ultra3 LVD SCSI Terminator

Dallas Semiconductor -> Maxim Integrated 

DS2120

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: 212-642-4900

Global Engineering Documents 15 Inverness Way East; Englewood, CO 80112 Phone: 800-854-7179

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 common-

mode 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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