FOXBORO FBM231 Serial Communication Module

Technical Specifications:

• Number of Ports: 2 independent RS-422/RS-485 ports
• Port Type: RS-422 (full-duplex) or RS-485 (half-duplex), selectable per port
• Baud Rate: 300 to 115200 bps
• Data Bits: 7 or 8
• Parity: None, Even, Odd, Mark, Space
• Stop Bits: 1 or 2
• Protocol Support: Modbus RTU, Modbus ASCII, DF1 Master/Slave
• Max Devices per Port: 32 devices (Modbus addressing)
• Max Cable Length (RS-485): 1200 meters at 9600 bps
• Max Cable Length (RS-422): 1200 meters at 19200 bps

Functional Features:

• Dual independent ports with separate protocol configuration
• Full-duplex RS-422 for simultaneous transmit and receive
• Half-duplex RS-485 with automatic direction control (DE/RE pin)
• 15kV ESD protection on all serial lines
• Optical isolation: 500V RMS per port
• Auto-direction control for RS-485 (no external RTS/CTS required)
• Diagnostic LEDs: TX, RX, Activity, Fault per port

Performance Parameters:

• Isolation Voltage: 500V RMS between each port and backplane
• Common Mode Rejection Ratio (CMRR): Greater than 70dB
• MTBF: Greater than 150,000 hours
• Operating Temperature: 0°C to 60°C (32°F to 140°F)
• Storage Temperature: -40°C to 85°C (-40°F to 185°F)
• Relative Humidity: 5% to 95% non-condensing

Material Composition:

• Chassis: Galvanized steel with epoxy powder coating
• PCB: FR-4 glass epoxy, conformal coated
• Connectors: Terminal block, screw-type, tin-plated
• Isolation: Digital opto-isolators (6N137 or equivalent)

Structural Features:

• Form Factor: Standard I/A Series module, single-width
• Mounting: DIN rail or chassis mounting via backplane connector
• Dimensions (W x H x D): 25.4mm x 160mm x 120mm (approximate)
• Weight: Approximately 0.38 kg (0.84 lb)

Working Principle:
The FBM231 employs dedicated UART controllers with hardware-based auto-direction control for RS-485. The module buffers incoming serial data in dual-port RAM, then transfers it to the I/A Series backplane via the system I/O bus. For RS-422 full-duplex operation, separate TX and RX lines are used simultaneously. The optical isolation barrier prevents ground loops and provides galvanic isolation between the field bus and the controller backplane.

Advantages & Highlights:

• True full-duplex RS-422 eliminates collision issues in half-duplex networks
• Automatic direction control simplifies RS-485 wiring (2-wire only)
• 32 device addressing per port supports large distributed networks
• Robust isolation ensures reliable operation in electrically noisy environments

Applicable Industries:

• Oil and Gas (pipeline SCADA, wellhead monitoring)
• Chemical and Petrochemical
• Power Generation and Distribution
• Water and Wastewater
• Mining and Minerals Processing

Installation Requirements:

• Use shielded twisted-pair cable (120Ω characteristic impedance) for RS-485
• Install 120Ω termination resistor at each end of the RS-485 bus
• Maintain minimum 50mm separation from high-voltage power cables
• Ensure single-point grounding of shield at controller end only
• Verify DIP switch settings match protocol configuration

Usage Precautions:

• Do not exceed 32 devices per Modbus port without using a repeater
• Do not daisy-chain RS-485 devices; use a linear bus topology
• Terminate both ends of the bus; unterminated buses cause signal reflections
• Check polarity (A/B lines) before energizing the system