4

ML6509

FUNCTIONAL DESCRIPTION

The SCSI terminator helps in decreasing the transmission

line effects with precise termination. Termination is

conventionally provided at the beginning and end of the

SCSI bus, however when additional peripherals are added,

the termination needs to be disabled from the current

device and enabled on the last device on the bus. Existing

termination schemes use a SIP (Single-In-Line package)

which is plugged into a socket on the PC board of the

SCSI peripheral. To remove the termination, the user

needs to pull the resistor SIP out of its socket. With the

higher levels of system integration, this is no longer a

simple task. With the increasing use of higher data rates

and cable lengths in SCSI subsystems, the need for active

termination is becoming 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 3.

Theoretically the desired V-I characteristics are the Boulay

type for signal assertion (high to low) and the Ideal type

for signal negation. The ML6509 with its MOSFET based

nonlinear termination element attempts to provide the

most optimum V-I characteristics — optimized for both

signal assertion and negation.

The ML6509 provides active termination for nine signal

lines, thus accommodating basic SCSI which requires 18

lines to be terminated and wide SCSI which requires 27,

36 or 45 lines to be terminated. The ML6509 integrates an

accurate voltage reference (1V dropout voltage) and nine

MOSFET based termination lines. A single internal resistor

is trimmed to tune the V-I characteristic of the MOSFETs as

shown in figure 1. The voltage reference circuit produces

a precise 2.85V level and is capable of sourcing at least

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 to 2.85V (quiescent

state). When all signal lines are inactive, the regulator will

source about 200mA.

The ML6509 SCSI Terminator provides two control

signals, DISCNKT & LPWR which are active low signals

and have an internal 200ký pull-up resistor. The

DISCNKT input when asserted low, isolates the ML6509

from the signal lines and effectively removes the

terminator from the SCSI bus with a disconnect mode

current of less than 100µA. The LPWR input, when

asserted low, puts the ML6509 in the low power

termination mode by providing only a 1mA drive

capability with an effective termination impedance of

2.5k

W. This is intended for power conscious portable

systems and peripheral applications where the cable

lengths are small, thus resulting is fast signal transitions

and practically no transmission line effects, while

consuming minimum power (9mA worst case if all lines

were active). At the same time, if this portable system

were connected with an external SCSI peripheral, over a

long cable, the normal terminator mode could be enabled

to ensure compliance with the SCSI standard and maintain

data integrity. In addition the ML6509 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. These

functions need to be handled with external components

in SCSI subsystems today. Thus the ML6509 helps in

eliminating a number of external components.

Disconnect mode capacitance is a very critical parameter

in SCSI systems. The ML6509 provides the lowest

capacitance contribution of maximum 5pF which is

guaranteed by production test.

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

TERMPWR line. Each ML6509 requires parallel 0.1µF and

and the TERMPWR line needs a 10µF bypass capacitor

per SCSI system.

Figure 1. Trimmed V-I Characteristic of the ML6509

NORMAL TERMINATION MODE

3.0V

2.844V

IL

22.4mA

3.00mA/div

30.0mA

0

BOULAY

(SLOPE = 110

Ω)

ML6509

IDEAL

LOWPOWER TERMINATION MODE

3.00V

2.85V

IL

1mA

0.2mA/div

2mA

0