dallas semiconductor DS2118M service manual

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DS2118M

Ultra2 LVD/SE SCSI Terminato

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

R1P

R1N
R2P
R2N

HS GND
HS GND
HS GND

R3P
R3N

R4P
R4N

R5P
R5N

ISO

GND

1

NC

2
3

NC

4
5
6
7
8
9
10
11
12
13
14
15
16
17
18

DS2118MB 36-Pin SSOP

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

DESCRIPTION

The DS2118M Ultra2 LVD/SE SCSI Terminator is both a Low Voltage Differential (LVD) and Single­Ended (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 connect ed
to the bus, the DS2118M will use SE termination. This is accomplished automatically inside the part b y
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: [phone]
Global Engineering Documents 15 Inverness Way East; Englewood, CO 80112 Phone: [phone]

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 di fferential) 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 frequenc y capacitor is recommended between TPW R and ground and placed as close
as possible to the DS2118M. The DS2118M should be placed as close as possible to the SCSI conne ctor

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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 recomm ended) 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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