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
intermediate level defined as V
STEP
. 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 I
O
is
as close as possible to the SCSI max current specification.
V
STEP
is defined as follows :
V
STEP
=
V
OL
+
(I
O
×
Z
O
)
V
285V
2.7V
2.5V
IDEAL
ML6509
BOULAY
220/330
0.2V
I
TERMINATOR (SOURCE)
DRIVER (SINK)
20mA
40mA
24mA
48mA
where
V
OL
is the Driver output low voltage,
I
O
is current from receiving terminator
Z
O
is characteristic impedance of cable.
Figure 3. V-1 Characteristics of Various
SCSI Termination Schemes
TERMPWR LINE
ML6509
TERMPWR
VREF
0.1µF
10µF
GND
L9
10µF
ML6509
TERMPWR
VREF
DISCNKT
LPWR
SCSI CABLE
DISCNKT
LPWR
L1 L2
0.1µF
10µF
GND
...
L2 L1
...
L9
SCSI XCVR
SCSI XCVR
...
V
REG
SCSI XCVR
V
REG
V
STEP
V
OL
LINE
ASSERTED
LINE
NEGATED
Figure 2. Application Diagram Showing Typical SCSI Bus Configuration with the ML6509
...
5
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