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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.

IDEAL

ML6599

BOULAY

220/330

2.85V

2.7V

2.5V

0.2V

TERMINATOR (SOURCE)

DRIVER (SINK)

20mA

40mA

24mA

48mA

V

I

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 100

µA 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 1

µA 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

Schottky diode is used as the shorting bypass device. After

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