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022002
FEATURES
Fully compliant with SCSI SPI-2, SPI-3,
SPI-4, Ultra160, and Ultra320
Provides multimode low-voltage
differential/single-ended (LVD/SE)
termination for nine signal-line pairs
Auto-selection of LVD or SE termination
5% tolerance on SE and LVD termination
resistance
Low power-down capacitance of 3pF
Onboard thermal-shutdown circuitry
SCSI-bus hot-plug compatible
Fully supports actively negated SE SCSI
signals
PIN ASSIGNMENT
DESCRIPTION
The DS2119M Ultra3 LVD/SE SCSI Terminator is both a low-voltage differential (LVD) and single-
ended (SE) terminator. The multimode operation enables the designer to implement LVD in current
products while allowing the end-user SE-backward compatibility with legacy devices. If the device is
connected in an LVD-only bus, the DS2119M will use LVD termination. If any SE devices are connected
to the bus, the DS2119M will use SE termination. This is accomplished automatically inside the part by
sensing the voltage on the SCSI bus DIFFSENS line.
For the LVD termination, the DS2119M integrates two current sources with nine precision resistor
strings. For the SE termination, one regulator and nine precision 110 resistors are used. Three
DS2119M 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
DS2119M
Ultra3 LVD/SE SCSI Terminator
www.maxim-ic.com
1
2
3
4
5
6
7
8
9
10
11
12
24
27
26
25
23
22
21
20
19
18
17
16
R9N
R9P
R8N
HS GND
R7N
R7P
R6N
R6P
DIFF_CAP
DIFFSENSE
MSTR/SLV
R8P
RIN
R2P
R2N
R3P
R4P
R4N
R5P
R5N
ISO
GND
HS GND
DS2119ME 28-Pin TSSOP
13
14
28
15
VREF
RIP
TPWR
TPWR
R3N
DS2119M
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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 reference voltage and a 2.85V
reference voltage. The control logic determines which of these references will be applied to the
termination resistors. If the SCSI bus is in LVD mode, then the 1.25V reference will be used. If the SCSI
bus is in SE mode, then the 2.85V reference will be used. That same control logic will switch in/out
parallel resistors to change the total termination resistance accordingly. Finally, in SE mode, the Rp pins
will be switched to ground.
The DIFFSENSE circuitry decodes trinary logic. There will be one 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 of the bus to be SE, HVD, or LVD, respectively.
The DS2119M'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 DS2119M 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 105 differential and 150 common-mode
impedance. A fail-safe bias of 112mV is maintained when no drivers are connected to the SCSI
bus.
SE Mode: When the external driver for a given signal line turns off, the active terminator will pull
that signal line to 2.85V (quiescent state). When used with an active negation driver, the power
amp can sink 22mA per line and the reference voltage will move less than 100mV. The
terminating resistors maintain their 110
=value.
HVD Isolation Mode: The DS2119M 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, V
REF
remains active. During thermal
shutdown, the termination pins are isolated from the SCSI bus and V
REF
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 V
DD
lines should be bypassed locally. A 2.2
F capacitor
and a 0.01
F high-frequency capacitor are recommended between TPWR and ground and placed as close
as possible to the DS2119M. The DS2119M 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.
DS2119M
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To maintain the specified regulation, a 4.7
F capacitor is required between the V
REF
pin (VREF) and
ground of each DS2119M. A high frequency cap (0.1
F ceramic recommended) can also be placed on the
V
REF
pin in applications that use fast rise/fall time drivers. A typical SCSI bus configuration is shown in
Figure 2.
DIFFSENS Noise Filtering: The DS2119M 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.
NOTES:
1) DIFFSENS. Refers to the SCSI bus signal.
2) DIFFSENSE. Refers to the Dallas Semiconductor pin name and internal circuitry relating the
DIFFSENS driver.
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