5

ML6518

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 ML6518 with its MOSFET-

based nonlinear termination element provides the most

optimum V-I characteristics for both signal assertion and

negation.

The ML6518 provides active termination for 18 signal

lines, thus accommodating basic SCSI which requires 18

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 ML6518 integrates an

accurate voltage reference (1V dropout voltage) and 18

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

400mA and sink 150mA while maintaining regulation of

2.85V.

The ML6518 SCSI terminator provides an active low

control signal (DISCNKT) which has an internal 200ký

pull-up resistor. The DISCNKT input isolates the ML6518

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 ML6518 provides for negative clamping of signal

transients and also supports current sink capability in

excess of 8.3mA 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 ML6518 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 ML6518 hot-insertable SCSI terminator typically

draws 1µA from any given output line (L1–L18) 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 1. V-1 Characteristics of Various SCSI Termination Schemes

IDEAL

ML6518

BOULAY

220/330

2.85V

2.7V

2.5V

0.2V

TERMINATOR (SOURCE)

DRIVER (SINK)

TERMINATOR (SINK)

DRIVER (SOURCE)

20mA

40mA

24mA

48mA

12mA

24mA

7mA

14mA

8.5mA

17mA

3.7V

3.6V

3.3V

I

V