NINE-CHANNEL, MULTIMODE ULTRA2 SCSI TERMINATOR HOLDS RESISTANCE TO ±5% IN EITHER LOW-VOLTAGE DIFFERENTIAL (LVD) OR SINGLE-ENDED (SE) MODE
DALLAS, TEXAS-June 9, 1998-A new multimode SCSI terminator that holds termination resistance more closely to the 110-ohm spec than comparable devices is available from Dallas Semiconductor. Fully compliant with emerging Ultra2 SCSI specifications, the DS2118M is a multimode, low-voltage differential/single-ended (LVD/SE) Ultra2 SCSI terminator that provides active termination for nine signal line pairs.
Because Dallas laser-trims resistor ladder structures within each device, the DS2118M can hold the SCSI-specified 110-ohm termination resistance to ±5 percent. The specifications of comparable devices show a tolerance of ±8 percent.
SCSI terminators are popular devices that are used in a variety of applications: high-end PCs, workstations and file servers, SCSI host bus adapters, RAID systems, SCSI peripherals such as CD-ROM players, high-speed data communications systems, terminated cable assemblies and terminated connectors.
SCSI Interface Basics
SCSI, the Small Computer Systems Interface, is the name of a family of industry-standard parallel interfaces for connecting peripheral devices to computers. Several variations of the SCSI standard have been developed including SCSI-1, Fast SCSI, Plug and Play SCSI, Ultra SCSI and Ultra2 SCSI.
Termination is required at the two physical ends of every SCSI bus. Legacy devices designed under SCSI-1, Fast SCSI and Ultra SCSI are single-ended (SE) designs. These systems use one wire per channel because they compare signal levels to ground. This approach assumes the ground level is the same throughout a system, but it often is not.
Newer products designed under the low voltage differential (LVD) Ultra2 SCSI standard use two wires per channel. This approach can be used to design products that transfer data at faster speeds with less noise. LVD Ultra2 SCSI products also can be connected over longer distances than SE designs.
Multimode Terminator Has Mode-Sensing Circuitry
"As a multimode SCSI terminator, the DS2118M contains circuitry that determines if a bus requires SE or LVD termination," said Charles Tashbook, product manager. "The chip automatically selects the proper mode.
"Multimode operation means a designer can order and stock one device, implementing LVD SCSI in current products and having the backwards-compatibility needed to continue production of legacy SE devices," Tashbook said.
If the DS2118M is connected to an LVD-only bus, the DS2118M will use LVD termination. If any SE devices are connected to the bus, the DS2118M will use SE termination. If a high-voltage differential (HVD) device is connected to the bus, the DS2118M will isolate the termination from the bus. These adjustments are accomplished automatically inside the device by sensing the voltage on the SCSI bus DIFFSENS line.
The DS2118M has a low power-down capacitance of 3 pF and on-chip thermal shutdown circuitry. Packaged in a 36-pin SSOP, the device costs $5.05 in 10,000-piece quantities. Tape-and-reel product also is available.
Other Dallas Semiconductor Bus Termination Products and Design Aids
Dallas Semiconductor's Bus Termination product family consists of integrated terminator solutions for single-ended SCSI, differential SCSI and BTL systems. SCSI products support the SCSI-1, Fast SCSI, Ultra SCSI, Ultra2 SCSI and Plug and Play SCSI standards.
Four SCSI-related application notes are available from the company's Web site at www.maxim-ic.com
- Application Note 70: DS2107A SCSI Bus Waveforms
- Application Note 71: DS2107A Thermal Considerations
- Application Note 72: Power Down Capacitance
- Application Note 73: Active Negation
Dallas Semiconductor designs, manufactures and markets CMOS chips and chip-based subsystems. In its 14-year history, the company has sold its products to more than 14,000 customers worldwide. Major markets include Original Equipment Manufacturers (OEMs) in personal computers, telecommunications, office equipment, instrumentation, factory automation, medical equipment and mainframe computers.
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