Field Interface Devices & RTUs

Varec 8810 RTU, Varec 9810 miniRTU, L&J MCG 3221

Connect, convert, and consolidate tank data.

What It Is & How It Works

Remote Terminal Units (RTUs) and protocol converters sit between field instruments and host systems. They poll level gauges, temperature probes, alarm switches, and other tank-side devices, normalize data across incompatible protocols, and deliver validated measurements upstream to SCADA, terminal automation software, and inventory systems.

Field instruments generate level, temperature, alarm, and status signals using native protocols: Tankway, GPU Bi-Phase Mark, Mark/Space, FSK, or analog 4–20 mA. The RTU receives those signals through dedicated interface modules, translates them into a standard host protocol such as OPC UA or Modbus TCP, buffers locally, and publishes upstream over Ethernet or serial connections.

RTUs are typically installed in control panels or marshalling cabinets within the tank farm. Proper grounding and surge protection are essential in outdoor environments. Early decisions on host protocol simplify system design, and selecting systems with modular plug-in cards allows the installation to grow without wholesale replacement.

RTU vs. Protocol Converter

An RTU is a multi-function communications hub that aggregates data from many tanks, supports multiple protocols, and can perform local calculations such as API volume corrections. A protocol converter is a focused translation device, converting one field protocol into one host protocol so existing gauge networks communicate with modern control systems without replacing field hardware.

Choose a protocol converter when the primary need is simple one-to-one translation between an L&J Tankway gauge network and a Modbus or PLCG host, and the full aggregation capability of an RTU is not required.

When to Choose Field Interface Devices

Field interface devices are the right choice when tank-side instruments need a centralized path to host systems:

Tank Farms & Petroleum Terminals Centralized inventory control, alarm management, and custody-transfer calculations across multi-product facilities, integrated directly with Varec FuelsManager®.
Refineries & Pipeline Terminals Level, temperature, and alarm data routed into SCADA, DCS, and process safety systems under EPA SPCC, API 2350, and EU Seveso III requirements.
Legacy Modernization Projects Bringing older float & tape, servo, or Mark/Space gauge networks onto modern Ethernet infrastructure without replacing field hardware.
Military & Aviation Fuel Operations Custody-transfer and inventory management to DoD, JIG, and FAA standards across fuel farms and defense installations.
Chemical & Petrochemical Plants Certified data collection in hazardous-area environments, consolidating mixed-vendor instruments into a single host connection.
Remote & Unmanned Sites Local data collection with onboard buffering and watchdog recovery where operator intervention is infrequent.

Why It Excels

Facilities Running Multiple Gauge Types and Protocols

A single RTU communicates simultaneously with radar, servo, float & tape, alarm probes, and third-party instruments through interchangeable interface modules, eliminating the need to standardize field hardware.

Large-Scale Operations Requiring Hundreds of Tank Connections

Modular architecture scales from a handful of tanks to 400 from one unit, with plug-in cards added as operations grow.

Sites Where Host Protocol Flexibility Determines System Selection

Native OPC UA, Modbus TCP, Modbus RTU, and legacy serial support ensure compatibility with virtually any SCADA, DCS, or inventory management platform.

Installations Where Data Continuity Is Non-Negotiable

Onboard data buffering, watchdog timers, surge protection, and real-time operating systems maintain uninterrupted data flow during network interruptions.

Mixed-Vendor Tank Farms Requiring Unified Data Collection

Interfaces with instruments from Enraf, Endress+Hauser, Emerson (Rosemount), GSI, Hectronic, and Veeder-Root alongside native Varec and L&J devices.

Choosing the Right Device

Attribute	Varec 8810 RTU	Varec 9810 miniRTU	L&J MCG 3221
Primary role	Multi-protocol RTU hub for large-scale operations	Single-protocol compact RTU for small-to-medium sites	Bi-directional protocol converter for L&J gauges
Tank capacity	Up to 400 tanks	Up to 32 tanks	Up to 225 tanks (software capacity)
Field protocols	Multiple simultaneous via plug-in modules (Serial, Tankway, Mark/Space, GPU Bi-Phase, FSK, Analog, Digital I/O)	One per unit (Serial, GPU Bi-Phase, Mark/Space, or Tankway)	L&J Tankway
Host protocols	OPC UA, Modbus TCP, Modbus RTU, RS-232/485	OPC UA, Modbus TCP, Modbus RTU, RS-232/485	Modbus PLCG, Modbus CLM, Allen Bradley, custom
Ethernet	Yes, native OPC UA server	Yes, native OPC UA server	RS-232/RS-485 standard; Ethernet optional
Form factor	Full industrial chassis, 6 module slots	Compact, drop-in replacement for Varec 8300 TGI	19” rack-mount or tabletop
Configuration tool	VeRTUe (browser-based)	VeRTUe (browser-based)	Factory-configured to site requirements
Best suited for	Large terminals, refineries, multi-product tank farms, military fuel	Small-to-medium tank farms, 8300 TGI replacements	Bridging L&J Tankway gauges to Modbus or PLCG hosts

What to Consider Alongside Field Interface Devices

Consider an alternative when:

You need only local tank-side display without upstream data integration. See Ground Level Displays (L&J MCG 1350/1350M).
The installation requires a touch-panel operator interface with local alarm management rather than a headless RTU. See Touch Panel Monitors (L&J MCG 3630/7030).
The primary requirement is wireless connectivity from field instruments to a host network. See Wireless Integration (L&J MCG 5200 WirelessHART Gateway).

How Field Interface Devices Fit Into a Larger System

RTU + Varec FuelsManager® for enterprise inventory intelligence. RTUs collect and normalize tank data; FuelsManager applies strapping tables, volume corrections, custody-transfer reporting, and compliance automation across the terminal. See FuelsManager®.
RTU + L&J Servo, Radar, and Float & Tape Gauges for complete measurement architecture. Field interface devices communicate natively with the full range of Cognesense level gauges, translating tank-side measurements into host-ready data. See Servo Level Gauges, Radar Level Gauges.
RTU + L&J Level Alarm Probes for independent safety layers. Digital I/O modules integrate discrete alarm signals from MCG 1090/1095 probes into SCADA and emergency shutdown systems, satisfying API 2350 overfill prevention requirements. See Level Alarms.