Analog Devices Inc./Maxim Integrated DS21T05Z/T&R

Overview

DS21T05Z/T&R from Dallas Semiconductor (now Maxim Integrated) 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 2.85V reference regulator with ≤1.0V dropout, thermal shutdown, and power-down control. It delivers 24 mA per line sourcing capability and supports hot-plug operation in disk drive and RAID controller terminations.

For engineers reviewing the DS21T05Z/T&R datasheet, DS21T05Z/T&R pinout, DS21T05Z/T&R application, or DS21T05Z/T&R equivalent, key selection criteria include 110-Ω ±5% termination accuracy, dual TERMPWR/VREF inputs, 16-pin narrow SOIC (150-mil) package, 0°C to 70°C operating range, and PD-controlled isolation for bus-safe hot insertion.

Technical Context

The DS21T05Z/T&R implements a bandgap-referenced regulator generating a stable 2.85V output (±0.15V) to bias nine switched 110-Ω resistors, each capable of sourcing ≥24 mA when signal lines are low. Its internal 50 kΩ pull-up on PD enables default-active operation without external biasing.

It supports actively negated SCSI signals via 200 mA sink capability per line while limiting VREF shift to <60 mV; thermal shutdown activates at 150°C. Dual TERMPWR and VREF pins require external tie-together and 4.7 µF decoupling per VREF pair.

Key Specifications

Pinout & Package

DS21T05Z/T&R is supplied in a 16-pin plastic narrow SOIC package (150-mil width), JEDEC MS-012AC compliant, with 1.27 mm pitch and gull-wing leads. Pin 10 is NC; PD (Pin 16) has internal 50 kΩ pull-up; VREF1 (Pin 7) and VREF2 (Pin 15) must be externally shorted and decoupled to GND with 4.7 µF.

Key Features

Applications

Equivalent & Alternatives

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

Compared with DS21T05Z/T&R, DS21T07Z provides enhanced electrical performance but lacks power-down control and costs more; MAX3212ESE+ is unrelated to SCSI termination and cannot replace DS21T05Z/T&R in any SCSI topology.

Availability

DS21T05Z/T&R is available at Aetrix Electronics and suitable for SCSI disk drive termination, RAID controller backplane design, SCSI tape library interface, and industrial SCSI peripheral adapter applications requiring stable component supply, long-lifecycle support, and traceable sourcing.

Supply support for DS21T05Z/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

Dallas Semiconductor-acquired by Maxim Integrated in 2001-specialized in precision analog, timing, and interface ICs for industrial and computing infrastructure.

The DS21T05Z/T&R belongs to Dallas' SCSI interface product line, engineered specifically to replace discrete active termination networks in Fast and Ultra SCSI systems while enabling hot-plug reliability and thermal safety.

FAQ

What is the primary function of the DS21T05Z/T&R in a SCSI system?

The DS21T05Z/T&R functions as a monolithic active SCSI terminator, providing regulated 2.85V bias to nine 110-Ω laser-trimmed resistors for impedance matching on single-ended Fast and Ultra SCSI buses. It replaces discrete resistor-regulator networks, improves noise immunity, and supports hot-plug operation-key for disk drives and RAID controllers. DS21T05Z/T&R integrates thermal shutdown and power-down control to ensure bus stability during insertion or fault conditions.

How does the DS21T05Z/T&R handle actively negated SCSI signals?

The DS21T05Z/T&R handles actively negated SCSI signals by sinking up to 200 mA total while limiting VREF shift to less than 60 mV-preserving reference stability during signal transitions. Its buffer amplifier maintains 2.85V quiescent bias even under heavy sink load, ensuring correct logic levels for negated signals like ACK or REQ. DS21T05Z/T&R achieves this without external components beyond the required 4.7 µF VREF decoupling capacitor.

What are the critical external components required for DS21T05Z/T&R operation?

DS21T05Z/T&R requires two external components: a 4.7 µF tantalum capacitor between VREF1/VREF2 and GND (mandatory for regulator stability), and decoupling capacitors (2.2 µF or 4.7 µF) on TERMPWR1 and TERMPWR2. A 0.1 µF high-frequency bypass capacitor is recommended in parallel with the 4.7 µF VREF cap for fast-edge applications. DS21T05Z/T&R needs no external resistors or regulators-the nine 110-Ω terminators and reference are fully integrated.

Can DS21T05Z/T&R be used in 16-bit Wide SCSI configurations?

Yes-DS21T05Z/T&R terminates nine signal lines per device, so three DS21T05Z/T&R units are required at each end of a 16-bit Wide SCSI bus to cover all 27 active signals (e.g., DB0–DB15, ATN, ACK, etc.). The DS21T05Z/T&R's narrow SOIC package and dual TERMPWR/VREF inputs simplify layout in dense backplane designs. DS21T05Z/T&R is explicitly validated for this use case in the datasheet's Figure 2 configuration.

What is the role of the PD pin on DS21T05Z/T&R, and how should it be configured?

The PD (Power Down) pin on DS21T05Z/T&R controls isolation of the termination resistors from the SCSI bus: pulling PD low disables all terminators, while leaving it open-circuited (leveraging its internal 50 kΩ pull-up) enables normal operation. This allows safe hot-plug insertion without bus disturbance. DS21T05Z/T&R must not have PD tied high externally-the internal pull-up ensures default activation, and external high bias risks undefined behavior per datasheet recommendations.