2108.PDF

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DS2108
Differential SCSI Switchable Terminator
www.dalsemi.com
FEATURES
Fully compliant with SCSI, SCSI-2 and SCSI-
3 standards
Conforms to EIA RS-485 standard
Provides differential termination for 9 pairs of
signal lines
Operates with SCSI signal voltages of -7 to
+12 Volts
Laser-trimmed 330 and 150-ohm termination
resistors have ±5% tolerance over full
temperature range
Switchable power-down mode
Low power down capacitance of 6 pF
24-pin plastic SOIC (DS2108S)
PIN ASSIGNMENT
TPWR
R1N
R2N
R3N
R4N
R5N
R6N
R7N
R8N
R9N
TPWR
NC
1
2
3
4
5
6
7
8
9
10
11
12
24
23
22
21
20
19
18
17
16
15
14
13
PD
GND
R1P
R2P
R3P
R4P
R5P
R6P
R7P
R8P
R9P
GND
DS2108S 24-Pin SOIC (300-mil)
DESCRIPTION
The DS2108 SCSI Terminator has been specifically designed for High Voltage Differential (HVD) SCSI
systems requiring switchable termination. The DS2108 integrates eighteen 330-ohm and nine 150-ohm
precise switched termination resistors into a monolithic IC. The surface mount SOIC package saves board
space over conventional resistor SIPs. The termination resistors can be isolated from the SCSI bus under
software or hardware control. While in the powered down mode, the DS2108 isolates the nine terminator
blocks from the bus while adding only 6 pF capacitance to each signal line of the SCSI bus.
FUNCTIONAL DESCRIPTION
The DS2108 consists of power-down circuitry, eighteen 330-ohm and nine 150-ohm termination resistors
(Figure 1). The DS2108 can be removed from the circuit by bringing the power down pin (PD) low. The
power down capacitance on the terminating resistors is 6 pF, well below the SCSI-3 allotment of 25 pF.
The DS2108 supports SCSI signal voltages of -7 to +12 volts when powered on or off. When all lines
settle into the quiescent state (no signal transitions), 56 mA is typically consumed. Only 1 mA is
typically consumed in the powered down mode.
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112099
DS2108
DS2108 BLOCK DIAGRAM
Figure 1
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DS2108
TYPICAL DIFFERENTIAL SCSI BUS CONFIGURATION
Figure 2
NOTES:
1. Two DS2108s required per 8–bit SCSI device and three DS2108s per 16–bit SCSI device.
2. Mid–bus termination effectively removed by grounding
PD
pin.
3. Termination power to be provided as specified in SCSI–3 Parallel Interface (SPI) document.
4. Local TERMPWR bypassing is recommended by the SPI document with values shown. The bypass
capacitors should be located as close as possible to the DS2108s. Only one pair of capacitors is
required per SCSI device (may be shared between DS2108s on same device).
PIN DESCRIPTION
Table 1
PIN
1, 11
DESCRIPTION
Termination Power.
Should be connected to the SCSI TERMPWR line.
Bypass with a 2.2 µF cap in parallel with a 0.1 µF cap as shown in Figure
2.
R1N…R9N
Signal Termination Negative.
Connect to –SIGNAL of SCSI bus.
NC
GND
R1P…R9P
No Connect.
Do not connect any signal to this pin.
Ground.
Signal ground; 0.0V.
Signal Termination Positive.
Connect to +SIGNAL of SCSI bus.
SYMBOL
TPWR
2, 3, 4, 5, 6,
7, 8, 9, 10
12
13, 23
14, 15, 16,
17, 18, 19,
20, 21, 22
24
PD
Power Down.
When tied low, the DS2108 enters a power-down mode.
Contains an internal 50K pull-up. Strap low to deactivate the DS2108,
leave open circuited to activate the DS2108.
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DS2108
ABSOLUTE MAXIMUM RATINGS*
Voltage on Any Pin Relative to Ground
Operating Temperature
Storage Temperature
Soldering Temperature
-7.0V to +12.0V
0°C to 70°C
-55°C to +125°C
260°C for 10 seconds
* This is a stress rating only and functional operation of the device at these or any other conditions above
those indicated in the operation sections of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods of time may affect reliability.
RECOMMENDED OPERATING CONDITIONS
PARAMETER
TERMPWR Voltage
PD
Active
PD
Inactive
SYM
V
TP
V
PDA
V
PDI
MIN
4.00
-0.3
2.00
TYP
MAX
5.25
0.8
V
TP
+
0.3
(0°C to 70°C)
UNITS
V
V
V
NOTES
DC CHARACTERISTICS
PARAMETER
TERMPWR Current
Power-down Current
Command Mode
Voltage
Termination Resistance
Termination Resistance
Termination Resistance
Power-down
Termination
Capacitance
SYMBOL
I
TP
I
PD
V
em
R
TTN
R
TNP
R
TPG
C
PD
MIN
TYP
56
1
MAX
65
2
12.0
346.5
157.5
346.5
6
UNITS
mA
mA
V
ohms
ohms
ohms
pF
NOTES
1, 6
1, 5
7, 8
2
3
4
1, 5, 7
7.0
313.5
142.5
313.5
330
150
330
5
NOTES:
1. 4.00V < TERMPWR < 5.25V
2. 330
Ω=resistor
between TERMPWR and -SIGNAL.
3. 150
Ω=resistor
between -SIGNAL and +SIGNAL.
4. 330
Ω=resistor
between +SIGNAL and GDN.
5.
PD
= 0.0V.
6. Signal pins left open.
7. Guaranteed by design; not production tested.
8. Signal lines RxN and RxP only.
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DS2108
24-PIN SOIC (300-MIL)
The chamfer on the body is optional. If it is not present, a terminal 1 identifier must be positioned so that
�½ or more of its area is contained in the hatched zone.
PKG
DIM
A IN.
MM
A1 IN.
MM
A2 IN.
MM
b IN.
MM
C IN.
MM
D IN.
MM
e IN.
MM
E1 IN.
MM
H IN.
MM
L IN.
MM
Θ
24-PIN
MIN
MAX
0.094 0.105
2.38
2.68
0.004 0.012
0.102
0.30
0.089 0.095
2.26
2.41
0.013 0.020
0.33
0.51
0.009 0.013
0.229
0.33
0.598 0.612
15.19 15.54
0.050 BSC
1.27 BSC
0.290 0.300
7.37
7.62
0.398 0.416
10.11 10.57
0.016 0.040
0.40
1.02
0°
8°
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