ML6518
As outlined in Annex G of the ANSI SCSI-3 Parallel
Interface Specification (X3T9.2/855D), “The SCSI bus
termination shall be external to the device being inserted
or removed.” In other words, any terminator connected to
a device being hot inserted/removed should be inactive
(accomplished by grounding the
DISCNKT
pin in the case
of the ML6518). If the terminator being inserted/removed
were in the active state, at some point in time the bus
would be terminated by either 1 or 3 terminators. In either
case, data integrity on the bus will be compromised.
Figure 2 gives an application diagram showing a typical
SCSI bus configuration. To ensure proper operation, the
TERMPWR pin must be connected to the SCSI
TERMPOWER line. Each ML6518 requires parallel 0.1µF
and 10µF capacitors connected between the V
REF
and
GND pins and the TERMPOWER line needs a 10µF
bypass capacitor at one node in the system.
In an 8-bit wide SCSI bus arrangement (“A” Cable),
a single ML6518 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 one ML6518 and one
ML6599, while 32-bit wide SCSI bus would require two
ML6518s and one ML6599.
In a typical SCSI subsystem, the open collector driver in
the SCSI transceiver pulls low when asserted. The
termination resistance serves as the pull-up when negated.
Figure 2 also shows 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 (V
STEP
). With 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 MOSFET
characteristic are trimmed to ensure that the I
O
is as close
as possible to the SCSI maximum current specification.
V
STEP
is defined as:
V
STEP
= V
OL
+ (I
O
¥
Z
O
)
where V
OL
is the driver output low voltage, I
O
is the
current from the receiving terminator, and Z
O
is the
characteristic impedance of the cable.
This is a very important characteristic that the terminator
helps to overcome by increasing the noise margin and
boosting the step as high as possible.
TERMPWR LINE
ML6518
TERMPWR
V
REF
0.1µF
10µF
GND
L18
10µF
ML6518
TERMPWR
V
REF
SCSI CABLE
DISCNKT
DISCNKT
L1 L2
0.1µF
10µF
GND
...
L2 L1
. . .
L18
SCSI XCVR
SCSI XCVR
...
SCSI XCVR
V
REG
V
REG
V
STEP
V
OL
t
D
t
D
LINE ASSERTED LINE NEGATED
Figure 2. Application Diagram Showing Typical SCSI Bus Configuration with the ML6518
6
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MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
