Analog Devices Inc./Maxim Integrated DS21S07AS/T&R

Overview

DS21S07AS/T&R from Maxim Integrated is a monolithic SCSI active terminator IC for single-ended Fast and Ultra SCSI bus systems, integrating nine laser-trimmed 110Ω termination resistors (±2% tolerance), a low-dropout 2.85V regulator, and power-down isolation logic. It operates from 4.0V–5.5V TERMPWR, consumes 2.5mA quiescent current, and supports hot-plug operation in disk drives and RAID controllers.

For engineers reviewing the DS21S07AS/T&R datasheet, DS21S07AS/T&R pinout, DS21S07AS/T&R application, or DS21S07AS/T&R equivalent, key selection criteria include active-termination compliance with SCSI-1/Fast/Ultra standards, thermal shutdown protection, 16-pin SO package footprint, and electrical bus isolation via PD pin control.

Technical Context

The DS21S07AS/T&R implements active termination using a bandgap-referenced 2.85V regulator that sources ≥24mA per line when pulled low and sinks ≥200mA for actively negated SCSI signals, with VREF drift <60mV under load. Its internal 50kΩ pullup on PD enables default-active operation without external biasing.

It requires external 4.7µF decoupling between VREF1/VREF2 and ground, dual TERMPWR connections (TERMPWR1/TERMPWR2), and external tieing of VREF1–VREF2. Thermal shutdown activates at 150°C, and power-down mode reduces termination capacitance to 3.0–5.0pF while limiting current to 100–150µA.

Key Specifications

Pinout & Package

DS21S07AS/T&R is supplied in a 16-pin plastic SO (Small Outline) package, 300 mil width, with standard JEDEC MS-012AC outline and gull-wing leads. Pin 1 marked by notch or dot.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar SCSI active termination applications.

Compared with DS2107A, DS21S07AS/T&R delivers improved termination accuracy and built-in thermal protection, while MAX3221ESE+ serves an entirely different interface standard and cannot substitute for SCSI termination functionality.

Availability

DS21S07AS/T&R is available at Aetrix Electronics and suitable for SCSI disk drive termination, RAID controller backplanes, and legacy server storage expansion requiring stable component supply and long-term obsolescence management.

Supply support for DS21S07AS/T&R includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Maxim Integrated (now part of Analog Devices) is a U.S.-based semiconductor company specializing in analog, mixed-signal, and power management ICs for industrial, communications, and computing applications.

The DS21S07AS/T&R belongs to Maxim's legacy SCSI interface product line, designed specifically to replace discrete termination networks with integrated, standards-compliant active termination for single-ended SCSI systems.

FAQ

What is the primary function of the DS21S07AS/T&R in a SCSI system?

The DS21S07AS/T&R provides active termination for single-ended SCSI buses, replacing passive resistor networks with integrated 110Ω laser-trimmed resistors and a regulated 2.85V reference. It ensures signal integrity across Fast and Ultra SCSI implementations by maintaining precise termination voltage and enabling electrical isolation via its PD pin - critical for stable operation in disk drives and RAID controllers where TERMPWR voltage may fluctuate.

Does the DS21S07AS/T&R require external components to operate correctly?

Yes, the DS21S07AS/T&R requires external components for proper operation: a 4.7µF tantalum capacitor between VREF1 and VREF2 pins (tied together externally), plus decoupling capacitors (2.2µF or 4.7µF) on both TERMPWR1 and TERMPWR2 pins. These are mandatory per the datasheet to stabilize the internal regulator and suppress noise - omission risks reference instability, excessive ripple, or failure to meet SCSI termination specifications.

How does the power-down feature of the DS21S07AS/T&R work, and what is its purpose?

The DS21S07AS/T&R enters power-down mode when the PD pin is driven low (≤0.8V), disabling all nine termination resistors and reducing supply current to 100–150µA. An internal 50kΩ pullup resistor keeps PD high by default, so leaving it open-circuited activates the device. This feature allows dynamic bus isolation during hot-plug events or system sleep states without physically removing the DS21S07AS/T&R from the board.

Is the DS21S07AS/T&R compatible with older SCSI standards like SCSI-1?

Yes, the DS21S07AS/T&R is explicitly specified as fully compliant with SCSI-1, Fast SCSI, and Ultra SCSI standards for single-ended bus topologies. Its 110Ω termination resistance, 2.85V regulation, and noise immunity characteristics meet the electrical requirements defined in all three specifications - making it suitable for retrofitting legacy SCSI-1 systems while also supporting higher-speed Fast and Ultra modes.

What is the operating temperature range for the DS21S07AS/T&R, and where is it typically deployed?

The DS21S07AS/T&R is rated for operation from 0°C to +70°C, consistent with commercial-grade storage subsystems. It is commonly deployed inside SCSI hard drives, RAID controller cards, tape library interfaces, and server backplanes - environments where airflow and thermal management are controlled but ambient temperatures remain within this range. The integrated thermal shutdown at 150°C adds margin against localized heating in densely packed enclosures.