Dallas Semiconductor DS2108S Datasheet

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DS2108

Differential SCSI Switchable Terminato

FEATURES

PIN ASSIGNMENT

 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)

TPWR

R1N
R2N
R3N

R4N

R5N
R6N
R7N
R8N

R9N

TPWR

NC

DS2108S 24-Pin SOIC

1
2
3

4
5
6
7
8
9
10
11
12

24
23
22
21
20
19
18
17
16
15
14
13

300-mil

PD
GND
R1P
R2P
R3P
R4P
R5P
R6P
R7P
R8P
R9P
GND

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 SC SI 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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DS2108 BLOCK DIAGRAM Figure 1

DS2108

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