DS2118M
Ultra2 LVD/SE SCSI Terminator
www.dalsemi.com
FEATURES
Fully compliant with Ultra2 SCSI
Provides Multimode Low Voltage
Differential/Single-Ended (LVD/SE)
termination for 9 signal line pairs
Auto-selection of LVD or SE termination
5% tolerance on SE and LVD termination
resistance
Low power down capacitance of 3 pF
Onboard thermal shutdown circuitry
SCSI bus hot plug compatible
Fully supports actively negated SE SCSI
signals
PIN ASSIGNMENT
VREF
NC
NC
R1P
R1N
R2P
R2N
HS GND
HS GND
HS GND
R3P
R3N
R4P
R4N
R5P
R5N
ISO
GND
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
TPWR
HVD
LVD
SE
R9N
R9P
R8N
R8P
HS GND
HS GND
HS GND
R7N
R7P
R6N
R6P
DIFF_CAP
DIFFSENSE
MSTR/SLV
DS2118MB 36-Pin SSOP
DESCRIPTION
The DS2118M Ultra2 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 DS2118M will use LVD termination. If any SE devices are connected
to the bus, the DS2118M 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 DS2118M integrates two regulated supplies with nine precision resistor
strings. For the SE termination, one regulator and nine precision 110-ohm resistors are used. Three
DS2118M terminators are needed for a Wide SCSI bus.
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DS2118M
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
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 DS2118M combines LVD and SE termination with DIFFSENS sourcing and detection. The LVD
termination section consists of two source/sink amplifiers (VTOP, VBOT), biasing circuitry and nine
precision resistor strings (RTOP, RMID, RBOT). The SE termination section consists of a 2.85V
source/sink regulator with 9 precision 110 Ohm resistors. The DIFFSENSE section consists of a 1.3V, 5
mA driver and a sensing circuit (Figure 1).
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 HVD, SE, or LVD, respectively.
The DS2118M’s DIFF_CAP pin monitors the DIFFSENS line to determine the proper operating mode of
the device; this mode is indicated by the SE/LVD/HVD outputs. The DIFFSENSE pin can also drive the
SCSI DIFFSENS line (when MSTR/SLV = 1) to determine the SCSI bus operating mode. The DS2118M
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 amplifiers.
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
When the external driver for a given signal line turns off, the active terminator will pull that
signal line to 2.85 volts (quiescent state). When used with an active negation driver, the power amp can
sink 22 mA per line while keeping the voltage reference in regulation. The terminating resistors maintain
their 110Ω=value.
HVD Isolation Mode
The DS2118M identifies that there is an HVD (high voltage differential) 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, Vref remains active, and
the bus mode indicators (SE/LVD/HVD) remain active. During thermal shutdown, the termination pins
are isolated from the SCSI bus, Vref becomes high impedance, and the bus mode indicators
(SE/LVD/HVD) remain active . The DIFFSENSE driver is shut down during either of these two events.
An internal pull-down resistor assures that the DS2118M will be terminating the bus if the ISO pin is left
floating.
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 is recommended between TPWR and ground and placed as close
as possible to the DS2118M. The DS2118M should be placed as close as possible to the SCSI connector
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DS2118M
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 DS2118M. 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.
NOTE:
DIFFSENS – Refers to the SCSI bus signal.
DIFFSENSE – Refers to the DS2118M pin name and internal circuitry capable of driving the
DIFFSENS line.
DIFF_CAP - Refers to the DS2118M pin name and internal circuitry relating to monitoring the
DIFFSENS line.
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