Bristol FB300 RTU: Hardware and Architecture Overview

Quick Answer

The Bristol FB300 uses a modular backplane architecture with a central CPU, plug-in analog and digital I/O cards, and communication expansion modules. It is designed for harsh oilfield environments with wide-range DC power input, extended temperature ratings, and conformal-coated circuit boards.

CPU Module

The FB300 processor board is the heart of the system. It contains:

• Microprocessor and memory: Executes the ACCOL or OpenBSI control program, manages I/O scanning, and handles communication protocol stacks.
• Onboard serial ports: Typically two RS-232 ports and one RS-485 port for direct SCADA communication, local programming access, and instrument polling.
• Ethernet port: Later-model FB300 CPUs include a 10/100 Ethernet port for DNP3/TCP, Modbus/TCP, and remote OpenBSI connections.
• Non-volatile memory: Battery-backed SRAM or flash storage for configuration, historical logs, and event buffers that survive power cycles.
• Real-time clock: Battery-backed RTC for timestamping events and historical data — critical for SCADA event ordering and regulatory compliance.

Analog Input Cards

The FB300 supports multiple analog input card models:

• 8-channel and 16-channel cards: Accept standard industrial signals — 4–20 mA current loops, 1–5 V voltage inputs, and thermocouple inputs (Type J, K, T).
• Resolution: 12-bit cards provide 4096 counts across the input range; 16-bit cards provide 65,536 counts for higher-precision applications like custody transfer metering.
• Input protection: Built-in transient voltage suppression protects against lightning-induced surges common at remote wellsites.
• Channel isolation: Some card models offer channel-to- channel isolation for noisy environments where ground loops between instruments could corrupt readings.

Digital I/O Cards

Digital I/O cards provide discrete input and output channels:

• Digital inputs: Accept dry contact (switch closure) or wetted (voltage-present) signals from limit switches, valve position indicators, motor run contacts, and safety interlocks.
• Digital outputs: Drive control relays for pump start/ stop, valve open/close, and alarm horn activation. Output types include relay contact (for motor starters) and solid-state (for fast-switching applications).
• Pulse/counter inputs: Dedicated high-speed counter inputs for turbine flow meters and gas measurement instruments that output frequency or pulse signals.

Communication Cards

Beyond the CPU's onboard ports, expansion communication cards add:

• Additional RS-232/RS-485 serial ports for multi-drop Modbus instrument networks.
• Ethernet expansion for sites requiring multiple IP connections (e.g., separate SCADA and engineering networks).
• Radio modem interfaces for licensed and unlicensed radio telemetry links.

Power Supply

The FB300 power supply accepts wide-range DC input, typically 10–30 VDC, making it compatible with:

• Solar/battery systems: 12 V or 24 V solar panels with battery banks — the standard power source at remote wellsites without grid power.
• Instrument power: The same 24 VDC supply often powers both the RTU and field transmitters, simplifying wiring.
• UPS-backed systems: At powered facilities like compressor stations, a UPS ensures the FB300 continues operating and communicating during brief power interruptions.

Backplane Architecture

The FB300 backplane provides a passive bus connecting the CPU and I/O cards. This modular design means:

• I/O cards can be added or removed without affecting other channels.
• A failed card can be hot-swapped in the field (with the unit powered down) without returning the entire RTU to the shop.
• Different I/O mixes can be configured for different applications using the same base chassis — a tank battery might use mostly analog inputs, while a compressor station uses more digital I/O.

Environmental Ratings

The FB300 is rated for the conditions found in oilfield environments:

• Operating temperature: -40°C to +70°C (-40°F to +158°F) — covering West Texas summers and North Dakota winters.
• Humidity: 5–95% non-condensing.
• Vibration and shock: Rated for the mechanical stress of wellsite and pipeline installations.
• Conformal coating: Circuit boards are coated to resist moisture, dust, and corrosive gases (H₂S) found near wellheads.