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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
DESCRIPTION
The DS2108 SCSI Terminator has been specifically designed for Hi gh Voltage Differ ential (HVD) SCSI
systems requiring switchable termination. The DS2108 integrates eighteen 330-ohm and nine 150-ohm
precise switched termination resistors into a monolithic IC. Th e surfac e mount SO IC package saves bo ard
space over conventional resistor S IPs. The termination resistors c an be isolated from the SCS I bus under
software or hardwar e 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 circuitr y, eighteen 330-ohm and nine 150-ohm termination resistors
(Figure 1). The DS2108 can be removed f rom 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.
DS2108
Differential SCSI Switchable Terminato
r
www.dalsemi.com
1
2
3
4
5
6
7
8
9
10
11
12
21
24
23
22
20
19
18
17
16
15
14
13
PD
GND
R1P
R3P
R4P
R5P
R6P
R7P
R8P
R9P
GND
R2P
TPWR
R1N
R2N
R4N
R5N
R6N
R7N
R8N
R9N
TPWR
NC
R3N
DS2108S 24-Pin SOIC
(
300-mil
)
DS2108
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DS2108 BLOCK DIAGRAM Figure 1
DS2108
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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 SYMBOL DESCRIPTION
1, 11 TPWR 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.
2, 3, 4, 5, 6,
7, 8, 9, 10 R1N…R9N Signal Termination Negative. Connect to –SIGNAL of SCSI bus.
12 NC No Connect. Do not connect any signal to this pin.
13, 23 GND Ground. Signal ground; 0.0V.
14, 15, 16,
17, 18, 19,
20, 21, 22
R1P…R9P Signal Termination Positive. Connect to +SIGNAL of SCSI bus.
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.
DS2108
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ABSOLUTE MAXIMUM RATINGS*
Voltage on Any Pin Relative to Ground -7.0V to +12.0V
Operating Temperature 0°C to 70°C
Storage Temperat ure -55°C to +125°C
Soldering Temperature 260°C for 10 seconds
* This is a stress rating only and functional operation of the d evice 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 (0°C to 70°C)
PARAMETER SYM MIN TYP MAX UNITS NOTES
TERMPWR Voltage VTP 4.00 5.25 V
PD Active VPDA -0.3 0.8 V
PD Inactive VPDI 2.00 VTP +
0.3 V
DC CHARACTERISTI CS
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
TERMPWR Current ITP 56 65 mA 1, 6
Power-down Current IPD 1 2 mA 1, 5
Command Mode
Voltage Vem 7.0 12.0 V 7, 8
Termination Resistance RTTN 313.5 330 346.5 ohms 2
Termination Resistance RTNP 142.5 150 157.5 ohms 3
Termination Resistance RTPG 313.5 330 346.5 ohms 4
Power-down
Termination
Capacitance
CPD 5 6 pF 1, 5, 7
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.
DS2108
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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 24-PIN
DIM MIN MAX
A IN.
MM 0.094
2.38 0.105
2.68
A1 IN.
MM 0.004
0.102 0.012
0.30
A2 IN.
MM 0.089
2.26 0.095
2.41
b IN.
MM 0.013
0.33 0.020
0.51
C IN.
MM 0.009
0.229 0.013
0.33
D IN.
MM 0.598
15.19 0.612
15.54
e IN.
MM 0.050 BSC
1.27 BSC
E1 IN.
MM 0.290
7.37 0.300
7.62
H IN.
MM 0.398
10.11 0.416
10.57
L IN.
MM 0.016
0.40 0.040
1.02
Θ
ΘΘ
Θ0° 8°
