The UCC5611 provides 18 lines of active termination for a SCSI (Small

Computer Systems Interface) parallel bus. The SCSI standard recom-

mends active termination at both ends of the cable segment.

The UCC5611 is ideal for high performance 3.3V SCSI systems. The key

features contributing to such low operating voltage are the 0.1V drop out

regulator and the 2.75V reference. The reduced reference voltage was

necessary to accommodate the lower termination current dictated in the

SCSI-3 specification. During disconnect the supply current is typically

only 1

µA, which makes the IC attractive for battery powered systems.

The UCC5611 is designed with an ultra low channel capacitance of 1.8pF,

which eliminates effects on signal integrity from disconnected terminators

at interim points on the bus.

The UCC5611 can be programmed for either a 110 ohm or 2.5k ohm ter-

mination. The 110 ohm termination is used for standard SCSI bus lengths

and the 2.5k ohm termination is typically used in short bus applications.

When driving the TTL compatible DISCNCT pin directly, the 110 ohm ter-

mination is connected when the DISCNCT pin is driven high, and discon-

nected when low. When the DISCNCT pin is driven through an

impedance between 80k and 150k, the 2.5k ohm termination is con-

nected when the DISCNCT pin is driven high, and disconnected when

driven low.

UCC5611

18-Line 3-5 Volt SCSI Active Terminator, Reverse Disconnect

FEATURES

DESCRIPTION

BLOCK DIAGRAM

Circuit Design Patented

•

Complies with SCSI, SCSI-2, SCSI-3

and FAST-20 (Ultra) Standards

•

2.75V to 7V Operation

•

1.8pF Channel Capacitance during

Disconnect

•

1

µA Supply Current in Disconnect

Mode

•

110 Ohm/2.5k Programmable

Termination

•

Completely Meets SCSI Hot Plugging

•−650mA Sourcing Current for

Termination

•

+400mA Sinking Current for Active

Negation Drivers

•

Trimmed Termination Current to 4%

•

Trimmed Impedance to 7%

•

Current Limit and Thermal Shutdown

Protection

continued

3/97

UDG-94131-1