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ML6599 Datasheet PDF : 8 Pages
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ML6599
FUNCTIONAL DESCRIPTION
SCSI terminators are used to decrease the transmission
line effects of SCSI cable. Termination must be provided
at the beginning and end of the SCSI bus to ensure that
data errors due to reflections on the bus are eliminated.
With the increasing use of higher data rates and cable
lengths in SCSI subsystems, active termination has
become necessary. Active termination also minimizes
power dissipation and can be activated or deactivated
under software control, thus eliminating the need for end
user intervention. The V-I characteristics of popular SCSI
termination schemes are shown in Figure 1. Theoretically,
the desired V-I characteristics are the Boulay type for
signal assertion (high to low) and the ideal type for signal
negation (low to high). The ML6599 with its MOSFET-
based nonlinear termination element provides the most
optimum V-I characteristics for both signal assertion and
negation.
V
2.85V
2.7V
2.5V
IDEAL
ML6599
BOULAY
220/330
0.2V
TERMINATOR (SOURCE)
DRIVER (SINK)
I
20mA
24mA
40mA
48mA
Figure 1. V-1 Characteristics of Various
SCSI Termination Schemes
The ML6599 provides active termination for 9 signal lines,
thus accommodating basic SCSI which requires 9 lines to
be terminated. When used with the ML6599, wide SCSI,
which requires 27, 36 or 45 lines to be terminated, can
also be accommodated. The ML6599 integrates an
accurate voltage reference (1V dropout voltage) and 9
MOSFET-based termination lines. A single internal resistor
is trimmed to tune the V-I characteristic of the MOSFETs.
The voltage reference circuit produces a precise 2.85V
level and is capable of sourcing 24mA into each of the
nine terminating lines when low (active). When the signal
line is negated (driver turns off), the terminator pulls the
signal line back to 2.85V. The regulator will source
200mA and sink 100mA while maintaining regulation of
2.85V.
The ML6599 SCSI terminator provides an active low
control signal (DISCNKT) which has an internal 200kΩ
pull-up resistor. The DISCNKT input isolates the ML6599
from the signal lines and effectively removes the
terminator from the SCSI bus with a disconnect mode
current of less than 100µA when pulled low. In addition,
the ML6599 provides for negative clamping of signal
transients and also supports current sink capability in
excess of 10mA per signal line to handle active negation
driver overshoot above 2.85V, a common occurrence with
SCSI transceivers.
Disconnect mode capacitance is a very critical parameter
in SCSI systems. The ML6599 provides a capacitance
contribution of only 5pF.
HOT-INSERTABILITY
“Hot” insertion of a SCSI device refers to the act of
plugging a SCSI device which is initially unpowered into
a powered SCSI bus. The SCSI device subsequently draws
power from the TERMPWR line during its startup routine
and thereafter. “Hot” removal refers to the act of removing
a powered SCSI device from a powered SCSI bus. A
device which performs both tasks with no physical
damage to itself or other devices on the bus, nor which
alters the existing state of the bus by drawing excessive
currents, is termed “hot-swappable.”
The ML6599 hot-insertable SCSI terminator typically
draws 1µA from any given output line (L1–L9) during a
hot-insertion/removal procedure, thereby protecting itself
and preserving the state of the bus. The low insertion
current is achieved by effectively shorting the gate to drain
of the output PMOS device until the 2.85V reference
(VREF) has powered up. A second PMOS in series with a
Schottky diode is used as the shorting bypass device. After
VREF reaches a sufficient level, the bypass device is turned
off and the part operates normally. 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. 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
4
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