Honeywell Enraf S0894921 PCB XPU-2 Board for 854 XTG Gauge

The Honeywell Enraf S0894921 is a central processing unit motherboard (PCB XPU-2) engineered for the Honeywell Enraf 854 XTG Servo Transmitting Tank Level Gauge. This processor assembly features an integrated Intrinsically Safe (IS) Output dedicated to powering and communicating with a Tank Side Indicator (TSI / 877 FDI) directly from the gauge enclosure.

Manufactured to support custody transfer bulk storage and high-accuracy marine/terminal tank gauging, the S0894921 board processes precision signals from the optical encoder, executes micro-step stepper motor drive profiles, and performs real-time calculations for product level, multi-layer oil/water interface detection, and servo density profiles. The module is supplied with onboard programmable read-only memory (EEPROM) containing core operating firmware and factory algorithms, alongside non-volatile static RAM (NOVRAM) to safeguard field calibration factors, drum corrections, strapping tables, and safety threshold alarms during power interruptions.

Model Details

• Manufacturer: Honeywell Process Solutions / Honeywell Enraf
• Model Name: XPU-2 Board Assembly (Cross Processing Unit 2)
• Manufacturer Part Number (MPN): S0894921
• Assembly Type: Primary Processor PCB with IS Drive Circuit for Remote Display (TSI)
• Host Platform: Honeywell Enraf 854 XTG Series Servo Gauges
• Memory Complement: Factory-flashed Firmware EEPROM + High-Reliability Industrial NOVRAM

Technical Specifications

Key Features

• Dedicated Intrinsically Safe TSI Link: Integrated galvanically isolated Ex [ia] barrier circuitry engineered to interface directly with Enraf Tank Side Indicators (TSI) over a 2-wire serial loop without external secondary barriers.
• Non-Volatile Configuration Retention: Equipped with dual on-board memory architectures (firmware EEPROM and field NOVRAM) ensuring zero data loss of site parameters, strapping tables, and drum calibration profiles across power downcycles.
• Conformal Coated Industrial Construction: Built with marine-grade IPC-Class 3 multilayer printed circuit trace design and protective conformal coating to resist high humidity, marine salt spray, and hydrocarbon vapor ingress.

Applications

• Crude Oil & Refined Products Bulk Storage: Shore terminal atmospheric and low-pressure floating/fixed roof tanks.
• Offshore Floating Production (FPSO / FSO): Deep-draft cargo and slop tank inventory measurement under maritime motion conditions.
• Chemical Tank Farms: Highly volatile solvents, hydrocarbons, and chemicals requiring explosion-proof isolation and local ground-level readout.
• Custody Transfer & Fiscal Metering: Certified revenue metering points requiring OIML R85 verified repeatability and accuracy down to 1 mm.
• Automated Interface & Density Profiling: Real-time top-to-bottom stratified density measurements and continuous oil/water interface monitoring.

Frequently Asked Questions

What is the primary difference between Honeywell Enraf S0894921 and S0894920?

The S0894921 includes an onboard intrinsically safe (IS) output loop circuit specifically designed to drive an external Tank Side Indicator (TSI / 877 FDI) at the bottom of the tank. The S0894920 board lacks this IS interface hardware and is intended for standalone installations communicating exclusively across the central Enraf GPU fieldbus or secondary Modbus interfaces.

Does the S0894921 board come with firmware pre-installed?

Yes. The assembly includes a factory-programmed EEPROM chip pre-flashed with Honeywell Enraf operating system microcode, as well as an industrial NOVRAM chip for configuration persistence.

Can I install an S0894921 PCB into an Enraf 854 ATG gauge?

The S0894921 is specifically designed for the blind-cover 854 XTG gauge. While the core processor can physically communicate with the servo mechanics of an 854 ATG, the 854 ATG uses an integrated front-viewing LCD display. If you require local display compatibility behind the glass window of an 854 ATG, you must use part number S0854921.

Can I configure the S0894921 in the field without opening the flameproof housing?

Yes. Parameter configuration, alarm adjustments, and level recalibrations can be performed non-intrusively through the housing’s optical window using the Honeywell Portable Enraf Terminal (PET) via infrared link, or remotely across the 2-wire fieldbus using the Engauge service software utility.