5

ML6509

Thus in an 8-bit wide SCSI bus arrangement (“A” cable),

two ML6509s would be needed at each end of the SCSI

cable in order to terminate the 18 active signal lines.

16-bit wide SCSI would use three ML6509s, while 32-bit

wide SCSI bus would require five ML6509s.

In a typical SCSI subsystem, the open collector driver in

the SCSI transceiver, when asserted, pulls low and when

negated, the termination resistance serves as the pull-up.

Shown in figure 2 is a typical cable response to a pulse.

The receiving end of the cable will exhibit a single time

delay. When negated, the initial step will reach an

data rates, sampling could occur during this step portion.

In order to get the most noise margin, the step needs to be

as high as possible to prevent false triggering. For this

reason the regulator voltage and the resistor defining the

as close as possible to the SCSI max current specification.

VV

I

Z

STEP

OL

O

O

=+

×

()

where

Figure 3. V-1 Characteristics of Various

SCSI Termination Schemes

IDEAL

ML6509

BOULAY

220/330

285V

2.7V

2.5V

0.2V

TERMINATOR (SOURCE)

DRIVER (SINK)

20mA

40mA

24mA

48mA

V

I

Figure 2. Application Diagram Showing Typical SCSI Bus Configuration with the ML6509

TERMPWR LINE

ML6509

TERMPWR

VREF

GND

L9

L2 L1

DISCNKT

LPWR

0.1µF

10µF

TERMPWR

VREF

GND

L9

L1 L2

ML6509

SCSI XCVR

SCSI XCVR

SCSI XCVR

LINE

ASSERTED

LINE

NEGATED

10µF

0.1µF

10µF

. . .

. . .

SCSI CABLE

DISCNKT

LPWR