DS21T06 Dallas Semiconducotr, DS21T06 Datasheet - Page 2

• Manufacturer Part Number: DS21T06
• Description: SCSI Terminator
• Manufacturer: Dallas Semiconducotr

DS21T06
FUNCTION DESCRIPTION
The DS21T06 is designed to be a single chip termination subsystem for use in Narrow SCSI systems. The
DS21T06 consists of a bandgap reference, buffer amplifier, and 18 termination resistors (Figure 1). The
voltage regulator circuitry (bandgap reference and class AB power amplifier) produces a precise laser
trimmed 2.85 volt level and is capable of sourcing 25 mA into each of the terminating resistors when the
signal line is low (active). When the external driver for a given signal line turns off, the active terminator
will pull that signal line to 2.85 volts (quiescent state). When used with an active negation driver, the
power amp can sink 22 mA per line while keeping the voltage reference in regulation; the terminating
resistors maintain their 110 ohms value over the entire voltage range. When all lines settle in the
quiescent state, the regulator will consume about 10 mA.
When the DS21T06 is put into power-down mode by bringing
low, the power-down circuitry will
PD
turn off the transistors on each signal line. This will isolate the DS21T06 from the signal lines and
) has an internal 50K =pullup resistor. To
effectively remove it from the circuit. The power-down pin (
PD
place the DS21T06 into an active state, the
pin should be tied higher left open circuited.
PD
To ensure proper operation, the TPWR pin should be connected to the SCSI bus TERMPWR line. As
with all analog circuitry, the TERMPWR lines should be bypassed locally. A 2.2 F capacitor and a 0.01
F high frequency capacitor is recommended between TPWR and ground and placed as close as possible
to the DS21T06. The DS21T06 should be placed as close as possible to the connector to minimize signal
and power trace length, thereby resulting in less input capacitance and reflections which can degrade the
bus signals.
To maintain the specified regulation, a 4.7 F capacitor is required between the VREF pin and ground of
each DS21T06. A high frequency cap (0.1 F ceramic recommended) can also be placed on the VREF
pin in applications that use fast rise/fall time drivers. Figure 2 details a typical SCSI bus configuration. In
an 8-bit data SCSI bus (Narrow SCSI), one DS21T06 is needed for proper termination of the 18 active
signal lines. One DS21T06 and one DS21T07 can be used to terminate the 27 active signal lines in a 16-
bit data SCSI bus (Wide SCSI).
DS21T06 BLOCK DIAGRAM Figure 1
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