Analog Devices Inc./Maxim Integrated DS21T05Z+

Overview

DS21T05Z+ from Analog Devices (formerly Dallas Semiconductor) is a monolithic SCSI active terminator IC for Fast and Ultra SCSI single-ended buses. It integrates nine laser-trimmed 110-Ω termination resistors, a precision 2.85V reference regulator with ≤1.0V dropout, thermal shutdown, and power-down control. It operates from 4.0–5.5V TERMPWR and delivers ≥24mA per line in active-low state, used in disk drive backplanes and RAID controller terminations.

For engineers reviewing the DS21T05Z+ datasheet, DS21T05Z+ pinout, DS21T05Z+ application, or DS21T05Z+ equivalent, key selection criteria include 110-Ω ±2.5% termination tolerance, dual TERMPWR/VREF inputs, 16-pin narrow SOIC package, hot-plug compatibility, and 0°C to 70°C industrial temperature range.

Technical Context

The DS21T05Z+ implements a bandgap-based voltage reference (2.55V) buffered to 2.85V output, enabling precise active termination across nine SCSI signal lines. Its regulator supports ≥24mA sourcing per line when signals are low and sinks up to 200mA total during actively negated transitions, with <60mV VREF shift.

Power-down mode is controlled by an open-drain PD pin with internal 50kΩ pull-up; pulling PD low isolates all termination resistors from the bus. Dual TERMPWR and VREF pins require external decoupling (4.7µF at VREF, 2.2–4.7µF at each TERMPWR) and must be tied together per layout guidelines.

Key Specifications

Pinout & Package

DS21T05Z+ is housed in a 16-pin plastic narrow SOIC (150-mil width) package, compliant with JEDEC MS-012AC. Pin pitch is 1.27 mm, body width 3.81–4.01 mm, and height 1.24–1.57 mm.

Key Features

Applications

Equivalent & Alternatives

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

Compared with DS21T05Z+, DS21T07+ offers higher drive strength and tighter resistor matching for mission-critical SCSI infrastructure, while MAX4893EASA+ targets LVD SCSI and lacks thermal protection - making DS21T05Z+ the optimal choice for cost-sensitive, thermally constrained single-ended Ultra SCSI deployments.

Availability

DS21T05Z+ is available at Aetrix Electronics and suitable for RAID controller termination, disk drive backplane integration, SCSI tape library interface, and legacy workstation expansion requiring stable component supply and long-term obsolescence management.

Supply support for DS21T05Z+ 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

Analog Devices acquired Dallas Semiconductor in 2011 and maintains its legacy timing, interface, and mixed-signal product lines with full datasheet, reliability, and cross-reference support.

The DS21T05Z+ belongs to Dallas Semiconductor's SCSI interface family, engineered specifically for high-volume, cost-optimized active termination in single-ended Fast and Ultra SCSI systems where thermal resilience and hot-plug safety are mandatory.

FAQ

What is the function of the PD pin on the DS21T05Z+?

The PD (Power Down) pin on the DS21T05Z+ controls isolation of all nine termination resistors from the SCSI bus. When PD is pulled low (≤0.8V), internal transistors disconnect the resistors, removing DS21T05Z+ from the circuit. An internal 50kΩ pull-up keeps DS21T05Z+ active when PD is left open. This feature enables dynamic bus reconfiguration without hardware changes.

How many DS21T05Z+ devices are needed for an 8-bit SCSI bus?

An 8-bit single-ended SCSI bus requires two DS21T05Z+ devices at each end of the cable to terminate all 18 active signal lines (nine per DS21T05Z+). Each DS21T05Z+ handles R1–R9, covering half the signal set. The DS21T05Z+ does not support 16-bit Wide SCSI alone - three units per end are required for full 27-line termination.

What capacitor values are required for DS21T05Z+ operation?

The DS21T05Z+ requires a 4.7 µF tantalum capacitor between VREF1 and VREF2 (tied together) and ground, plus either a 2.2 µF or 4.7 µF capacitor on each TERMPWR pin (pins 1 and 9). A 0.1 µF high-frequency bypass capacitor may be added in parallel with the 4.7 µF VREF capacitor for fast-edge driver applications to suppress switching noise.

Is DS21T05Z+ compatible with actively negated SCSI signals?

Yes, DS21T05Z+ fully supports actively negated SCSI signals. Its buffer amplifier can sink up to 200 mA total while maintaining VREF shift under 60 mV, ensuring stable termination during assertion of signals like ACK, REQ, or BSY. This capability is explicitly verified in the DS21T05Z+ functional description and DC characteristics table.

What is the maximum operating temperature for DS21T05Z+?

The DS21T05Z+ has a specified operating temperature range of 0°C to 70°C, validated across all electrical parameters in the Recommended Operating Conditions table. Its integrated thermal shutdown activates at 150°C to prevent damage, but continuous operation above 70°C is outside specification and may cause parameter drift or premature failure.