The DS21T09 consists of three major functional blocks, Figure 2:
DS21T09
• Voltage reference, terminating resistors, and isolation switches
• Bus current sensing circuitry
• Bus ground sensing circuitry
When the Bus Ground Sensing Circuitry or Bus Current Sensing Circuitry determines that the SCSI bus
is properly terminated without the DS21T09, the power-down buffer isolates the resistors from the SCSI
bus and disables the power amp, thereby placing the DS21T09 in a low power mode (the bus sensing
circuitry always stays active). The PDO (Power-Down Output) pin can be connected to the pin of a
DS21T07 SCSI terminator for Wide SCSI configurations, Figure 8.
DS21T09 BLOCK DIAGRAM Figure 2
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