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DS2105S

SCSI Terminator

Dallas Semiconductor -> Maxim Integrated 

DS2105

FUNCTIONAL DESCRIPTION

The DS2105 consists of a bandgap reference, buffer

amplifier, and nine termination resistors (Figure 1). The

bandgap reference circuit produces a precise 2.55V

level which is fed to a buffer amplifier. The buffer pro-

duces a 2.85V level and is capable of sourcing at least

24 mA into each of the termination resistors when the

signal line is low (active). When the driver for a given

signal line turns off, the terminator will pull the signal line

to 2.85V (quiescent state). To handle actively negated

SCSI signals, the buffer can sink 200 mA, and VREF will

move less than 60 mV. When all lines settle in the quies-

cent state, the regulator will consume about 5 mA.

When the DS2105 is put into power–down mode by

bringing PD low, the power–down circuitry will turn off

the transistors on each signal line. This will isolate the

DS2105 from the signal lines and effectively remove it

from the circuit. The power–down pin (PD) has an inter-

nal 50KΩ pull–up resistor. To place the DS2105 into an

active state, the PD pin should be left open circuited.

To ensure proper operation, both the TERMPWR1 and

TERMPWR2 pins must be connected to the SCSI bus

TERMPWR line and both the VREF1 and VREF2 pins

must be tied together externally. Each DS2105 requires

a 4.7 µF capacitor connected between the VREF pins

and ground. Figure 2 details a typical SCSI bus configu-

ration. In an 8–bit wide SCSI bus arrangement (“A”

Cable), two DS2105’s would be needed at each end of

the SCSI cable in order to terminate the 18 active signal

lines. In a 16–bit wide SCSI bus arrangement (“P”

Cable), three DS2105’s would be needed at each end of

the SCSI cable in order to terminate the 27 active signal

lines.

DS2105 BLOCK DIAGRAM Figure 1

PD

TERMPWR1

TERMPWR2

50K ohm

POWER

DOWN

CIRCUITRY

BANDGAP

REFERENCE

GND

110 ohms

R1

110 ohms

R2

110 ohms

R3

110 ohms

R4

110 ohms

R5

110 ohms

R6

110 ohms

R7

110 ohms

R8

110 ohms

R9

VREF1

VREF2

022698 2/7

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