dallas semiconductor DS2106 service manual
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DS2106
DS2106
SCSI Terminator
FEATURES
• Fully compliant with SCSI–1, Fast SCSI and
Ultra SCSI
• Functionally compatible with DS21S07A
• Provides active termination for 18 signal lines
• 5% tolerance on termination resistors and voltage
regulator
• 5% tolerance on termination current
• Low power down capacitance of 3 pF
• Onboard thermal shutdown circuitry
• SCSI bus hot plug compatible
• Fully supports actively negated SCSI signals
PIN ASSIGNMENT
1
PD
2
R1
3
R2
4
R3
5
R4
6
R5
HS–GND
HS–GND
TPWR
7
8
9
NC
10
R6
11
R7
12
R8
13
R9
14
DS2106S 28–PIN SOIC (300 MIL)
GND
28
R18
27
R17
26
R16
25
R15
24
R14
23
NC
22
HS–GND
21
NC
20
R13
19
R12
18
R11
17
R10
16
V
15
REF
DESCRIPTION
Fast SCSI and Ultra SCSI require the use of active terminations at both ends of every cable segment in a
SCSI system with single–ended drivers and receivers.
Ultra SCSI requires support of active negation and hot
plugging. The DS2106 is fully compliant with these
standards.
The DS2106 integrates a low drop–out regulator and 18
precise switched 110 ohm termination resistors into a
28–pin 300 mil SOIC package.
Active termination provides: greater immunity to voltage drops on the TERMPWR (TERMination PoWeR)
line, enhanced high–level noise immunity, intrinsic
TERMPWR decoupling, and very low quiescent current
consumption. One DS2106 can be used to terminate
the Narrow SCSI bus, and a DS21S07A can be used
with a DS2106 for termination of a Wide SCSI bus.
022698 1/6
DS2106
REFERENCE DOCUMENTS
SCSI–2 (X3.131–1994)
SCSI–3 Parallel Interface (X3T10/855D)
SCSI–3 Fast–20 (X3T10/1071D)
FUNCTION DESCRIPTION
The DS2106 is designed to be a single chip termination
subsystem for use in Narrow SCSI systems. The
DS2106 consists of a bandgap reference, buffer amplifier, and 18 termination resistors (Figure 1). The voltage
regulator circuitry (bandgap reference and class AB
power amplifier) produces a precise laser trimmed 2.85
volt level and is capable of sourcing 24 mA into each of
the terminating resistors when the signal line is low
(active). 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 1 10 ohm value over
the entire voltage range. When all lines settle in the
quiescent state, the regulator will consume about
10 mA.
When the DS2106 is put into power–down mode by
bringing PD
the transistors on each signal line. This will isolate the
DS2106 from the signal lines and effectively remove it
from the circuit. The power–down pin (PD
nal 50KΩ pull–up resistor. To place the DS2106 into an
active state, the PD pin should be left open circuited.
low, the power–down circuitry will turn off
) has an inter-
Available from: Global Engineering Documents
15 Inverness Way East
Englewood, CO 80112–5704
Phone: (800) 854–7179,(303) 792–2181
Fax: (303) 792–2192
To ensure proper operation, the TPWR pin should be
connected to the SCSI bus TERMPWR line. As with all
analog circuitry, the TERMPWR 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
DS2106. The DS2106 should be placed as close as
possible to the connector to minimize signal and power
trace length, thereby resulting in less input capacitance
and reflections which can degrade the bus signals.
T o maintain the specified regulation, a 4.7 µF capacitor
is required between the VREF pin and ground of each
DS2106. 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. Figure 2 details a
typical SCSI bus configuration. In an 8–bit data SCSI
bus (Narrow SCSI), one DS2106 is needed for proper
termination of the 18 active signal lines. One DS2106
and one DS21S07A can be used to terminate the 27
active signal lines in a 16–bit data SCSI bus (Wide
SCSI).
DS2106 BLOCK DIAGRAM Figure 1
PD
TPWR
50K ohm
BANDGAP
REFERENCE
022698 2/6
POWER
DOWN
CIRCUIT-
RY
110 ohms
+
–
110 ohms
110 ohms
R18
R17
R1
VREF
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