DS2120
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GENERAL DESCRIPTION
The DS2120 Ultra3 LVD SCSI terminator 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 high-
voltage 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.
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.
