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ML6509CS

Active SCSI Terminator

Micro Linear Corporation 

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

intermediate level defined as VSTEP. With the higher SCSI

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

MOSFETs characteristic is trimmed to ensure that the IO is

as close as possible to the SCSI max current specification.

VSTEP is defined as follows :

where

VSTEP = VOL + (IO × ZO)

VOL is the Driver output low voltage,

IO is current from receiving terminator

ZO is characteristic impedance of cable.

V

285V

2.7V

2.5V

ML6509

IDEAL

ML6509

BOULAY

220/330

0.2V

TERMINATOR (SOURCE)

DRIVER (SINK)

I

20mA

24mA

40mA

48mA

Figure 3. V-1 Characteristics of Various

SCSI Termination Schemes

0.1µF

10µF

ML6509

TERMPWR LINE

VREF

TERMPWR

DISCNKT

GND

LPWR

L9 . . . L2 L1

10µF

SCSI CABLE

ML6509

TERMPWR

DISCNKT

VREF

LPWR

GND

L1 L2 . . . L9

0.1µF

10µF

SCSI XCVR

SCSI XCVR

SCSI XCVR

VREG

VREG

VSTEP

VOL

LINE

LINE

ASSERTED NEGATED

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

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