DS2118MB

Overview

The DS2118MB is a SCSI Ultra3 LVD/SE terminator manufactured by Analog Devices Inc./Maxim Integrated. It provides multimode low-voltage differential and single-ended termination for nine signal line pairs within a 36-pin SSOP package. The device operates at 5V with an ambient temperature range of 0°C to +70°C. Key electrical parameters include SE termination resistance of 110Ω (typical) and LVD differential termination resistance of 110Ω (typical). It features a low power-down capacitance of 3pF, on-board thermal shutdown circuitry, and supports SCSI bus hot-plug compatibility.

Feature Summary

• Multimode operation enabling automatic selection between LVD and SE termination based on bus voltage sensing.
• Integrated DIFFSENSE circuitry that decodes trinary logic to identify HVD, SE, or LVD bus modes.
• On-board thermal shutdown circuitry that isolates termination pins during overheating events.
• Supports actively negated SE SCSI signals with active pull-up capabilities.

Applications

• SCSI Parallel Interface (SPI-2, SPI-3, SPI-4) compliant systems
• Ultra160 and Ultra320 SCSI bus implementations
• Legacy SE SCSI backward compatibility integration

Procurement Notes

Verify the specific ordering suffix required for the desired packaging format. The standard 'MB' suffix indicates tube packaging, while '/T&R' denotes tape and reel configuration. Confirm lead-free status if RoHS compliance is mandatory, as indicated by the '+' suffix in related part numbers. Ensure the component meets the required operating temperature range for the target application environment before finalizing procurement.

Selection Notes

Select this component when designing SCSI controllers requiring simultaneous support for legacy single-ended devices and modern low-voltage differential signaling. The device eliminates the need for external mode-switching logic by automatically detecting the bus state via the DIFFSENS line. Ensure adequate decoupling capacitors are placed near the VREF and TPWR pins to maintain regulation stability during high-speed data transfers.

Part Context

The DS2118M belongs to the Maxim Integrated family of SCSI interface components designed for high-reliability data storage systems. It serves as a critical bridge between older single-ended SCSI standards and newer Ultra320 specifications. The device simplifies board layout by integrating precision resistor strings and current sources directly into the termination block, reducing component count and potential signal integrity issues associated with discrete termination networks.

Replacement Considerations

Public confirmed substitute part numbers: None identified in verified documentation.

FAQ

How does the DS2118MB determine whether to use LVD or SE termination?

The device uses internal DIFFSENSE circuitry to monitor the voltage on the SCSI DIFFSENS line. If the voltage indicates an LVD bus, it applies LVD termination; if SE devices are present, it switches to SE termination automatically.

What is the function of the ISO pin on the DS2118MB?

When pulled high, the ISO pin isolates the termination pins from the SCSI bus. This allows the bus mode indicators to remain active while disconnecting the terminator, which is useful during hot-plug operations or thermal shutdown events.

What happens during thermal shutdown?

During thermal shutdown, the termination pins are isolated from the SCSI bus, the VREF output becomes high impedance, and the DIFFSENSE driver is shut down to protect the device and the bus.