DS2108 Datasheet, PDF (1/5 Pages) Dallas Semiconductor – Differential SCSI Switchable Terminator

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DS2108
Differential SCSI Switchable Terminator
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 1
R1N 2
R2N 3
R3N 4
R4N 5
R5N 6
R6N 7
R7N 8
R8N 9
R9N 10
TPWR 11
NC 12
24 PD
23 GND
22 R1P
21 R2P
20 R3P
19 R4P
18 R5P
17 R6P
16 R7P
15 R8P
14 R9P
13 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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