Component Snapshot At-a-Glance

• Model: A4H124-24FX, Foxboro OEM part marking P0973JN
• Alt. P/N: Cross replacement A2H124-24FX(P0973BJ), alternate OEM part P0973BL
• Product Series: Enterasys SecureStack A4, factory preconfigured firmware for Foxboro I/A Series DCS control network
• Hardware Type: 19-inch 1U rackmount Layer2+ managed stackable fast fiber industrial switch
• Key Feature: 24 fixed MT-RJ multimode 100BASE-FX ports + 4 gigabit SFP uplink slots, preloaded DCS QoS prioritization rules
• Primary Field Use: Edge termination for remote Foxboro FBM/FCP controllers via multimode fiber inside refinery, power plant DCS rack rooms

Hard-Numbers: Technical Specifications

• Protocol Support: 802.1Q VLAN, 802.1p QoS, MSTP, SNMPv2c, RMON, port ACL, stacking protocol, Modbus TCP pass-through
• Port Count: 24×Fixed MT-RJ 100BASE-FX multimode fiber; 4×SFP 10/100/1000Mbps uplink slots; 1×RS232 console management port
• Baud/Data Rate: Fixed 100Mbps optical on base fiber ports; SFP auto-negotiate 10/100/1000Mbps copper/fiber uplink
• Operating Temperature: 0°C ~ +50°C continuous cabinet operation; -40°C ~ +70°C non-operational storage
• Isolation Rating: 1500VAC surge isolation between fiber transceiver bank and main ASIC PCB for MCC cabinet EMI suppression
• Power Draw: Max 65.7W full load; universal AC 100–240VAC 50/60Hz, compatible with external RPS redundant backup power supply
• Switch Fabric: 12.8Gbps standalone switching capacity, stacked aggregate up to 102.4Gbps backplane throughput
• MAC Table Depth: 16,384 MAC address entries stored on onboard flash memory
• Stack Limit: Up to 8 same-series A4 switches chained via gigabit RJ45/SFP uplink cables into single logical stack domain
• Form Factor: Standard 1U EIA rack chassis, 442mm(W)×44mm(H)×368mm(D)

The Real-World Problem It Solves

Long copper CAT5 cabling near high-current motor control centers picks up heavy EMI noise, triggering random CRC faults and spontaneous Foxboro field controller offline trips during unit startup. Scattered standalone fiber media converters waste valuable rack U-space and expand site spare inventory overhead unnecessarily.

Where you’ll typically find it:

• Refinery CDU/FCC remote I/O rack room collecting fiber drops from field-mounted FBM analog/digital modules
• Fossil power plant turbine auxiliary DCS cabinet aggregating boiler feedwater controller fiber communications
• Chemical plant outdoor process shelter with fiber runs exceeding 100m copper distance limitation

Integrated fixed MT-RJ fiber ports eliminate standalone media converters and block industrial electrical interference from disrupting critical interlock DCS traffic.

Hardware Architecture & Under-the-Hood Logic

This unit uses split-isolation PCB layout; front-panel optical transceiver board is electrically decoupled from main switch CPU/ASIC section to stop field-side cabinet surge spikes frying core control logic. It sits on DCS edge tier between remote field hardware and core central aggregation switches.

1. Inbound 100Mbps optical signals from field Foxboro controllers feed directly into fixed MT-RJ multimode transceivers on front port PCB.
2. Transceivers convert light pulses into standard electrical Ethernet frames before passing payload to central switching ASIC.
3. Pre-programmed factory QoS tags all proprietary DCS control and Modbus TCP packets to highest forwarding queue ahead of non-critical historian background traffic.
4. ASIC either switches local inter-controller traffic across base fiber ports or forwards uplink data out available SFP slots toward plant core backbone switches.
5. Stack protocol syncs running-config and firmware revisions across all chained A4 switches to function as unified single switching plane.
6. Non-volatile onboard flash retains startup-config and custom DCS VLAN assignments through full AC power loss cycles.

Field Service Pitfalls: What Rookies Get Wrong

Mixing single-mode LX SFPs into multimode-only uplink baysNew field techs install single-mode transceivers in SFP uplink slots designed for multimode SX; mismatched wavelength causes constant uplink flapping and intermittent stack splitting mid-running process.

• Quick Fix: Swap all incompatible SFPs for 850nm SX multimode Mini-GBIC matching Foxboro OEM design specs only.

Adding spare unit into live stack without firmware version matchingTechnicians plug replacement P0973JN into active stack without checking build revision; firmware mismatch locks new switch offline, all front panel port LEDs stay dark and breaks stack election process.

• Field Rule: Isolate spare switch offline, upgrade/downgrade firmware to match existing stack master build before running stacking cabling.

Poor MT-RJ fiber termination causing vibration-induced intermittent link lossInstallers leave oversized stripped fiber core inside MT-RJ jack; cabinet fan vibration shifts fiber alignment leading to sporadic DCS PV dropouts and unstable control loops.

• Quick Fix: Re-terminate faulty fiber ends per MT-RJ crimp spec, verify optical power output with handheld fiber power meter before returning to live DCS service.

Commercial Availability & Pricing Note

Please note: The listed price is for reference only and is not binding. Final pricing and terms are subject to negotiation based on current market conditions and availability.