ABB AO845A Redundant Module

Description

Product Introduction & Engineering Value

Single points of failure in mission-critical output loops are every reliability engineer’s nightmare. The ABB AO845A solves this directly by offering seamless 1:1 hardware redundancy for analog output loops within the S800 I/O architecture. Paired on a redundant termination unit, two AO845A modules operate side-by-side, continually cross-checking loop current and taking over within milliseconds if one card suffers a backplane fault or internal failure.

Beyond high availability, this 8-channel module acts as a transparent bridge for smart field instrumentation. Integrated HART passthrough allows maintenance software to pull diagnostic telemetry, adjust positioner calibration, or check device health directly from the control room without disturbing the primary 4–20 mA command signal.

Technical Specifications

• Number of Channels: 8 unipolar output channels (individually current-limited)
• Output Signal Range: 4..20 mA (supports HART digital communication overlay)
• Resolution: 12-bit D/A conversion
• Redundancy Configuration: Supported (requires paired module setup on TU842 or TU843 MTUs)
• Maximum External Load: 750 Ω (at 24 VDC external loop power)
• Current Loop Accuracy: ±0.1% of full scale
• ModuleBus Power Draw: 50 mA @ +5 VDC (0 mA @ +24 VDC)
• External Power Supply: 24 VDC nominal (19.2 to 30 VDC operating range)
• Power Dissipation: 3.1 W maximum at full rated output load
• Environmental Rating: ISA-S71.04 Class G3 conformal coating for harsh chemical atmospheres
• Matching MTU Hardware: TU842 (Redundant), TU843 (Redundant), TU810 (Non-redundant)
• Mechanical Keying Position: AH

AO845A

AO845A

Field Application & The “Trench” Experience

During a planned overhaul of an ethylene cracking furnace, an automation team needed to upgrade a critical fuel gas control system to fault-tolerant architecture without running new field cabling. The existing non-redundant output cards couldn’t support true hot-standby redundancy, and losing a single fuel control valve during operation meant an automatic trip costing upwards of $200k in downtime.

By swapping to a dual AO845A configuration on a TU842 redundant base, the plant achieved full I/O fault tolerance on all 8 valve positioner loops. During commissioning tests, technicians pulled the primary AO845A card while the furnace simulator ran at 65% firing rate. The secondary card took control instantly without dropping loop current or triggering a single glitch on the valve positioner feedback.

Specific Application Scenarios

• Main Steam Pressure Regulating Valves: Dual-card redundant output driving smart electro-pneumatic positioners in combined-cycle power generation.
• Offshore Platform Emergency Shutdown (ESD) Bypass Controls: 4–20 mA current loops in offshore marine environments taking full advantage of the G3 conformal coating.
• Refinery Catalytic Hydrotreater Dosing: Monitoring HART field instrument diagnostics on control valves while maintaining unbroken 24/7 output loop control.

Transparency SOP: QA & Testing

1. Visual & Structural Audit: Every unit undergoes microscopic terminal rail check, housing inspection, and verification of the AH mechanical keying pin.
2. Backplane & Redundancy Handshake Test: The module is inserted into an AC 800M test setup on a TU842 base paired with a known-good master module. We verify proper communication sync, LED status (F, R, W, O), and seamless master-to-standby failover under active load.
3. 8-Channel Calibration Sweep: Using a Fluke multi-channel precision meter, every channel is commanded across 4.0 mA, 8.0 mA, 12.0 mA, 16.0 mA, and 20.0 mA setpoints. Output current must land within ±0.1% tolerance across 250 Ω and 750 Ω loads.
4. HART Telemetry Pass-Through Test: An AMS Device Manager test rig connects to each channel to verify bidirectional HART communication, checking secondary variable reads and device tag polling through the hardware buffer.
5. ESD Cleaning & Thermal Conditioning: Modules clear an anti-static wash, undergo a thermal burn-in run, and are sealed in heavy-duty ESD packaging before entering active inventory.

The Veteran’s Tech Trap Guide

⚠️ TRAP 1: The “HART Master Disable” Software Setting

If you drop an into a slot previously occupied by a standard non-HART card (like the AO810), HART passthrough might not work automatically. You must open Control Builder, navigate to the module hardware tree, and explicitly enable HART communication per channel. Otherwise, it operates purely as a standard analog card.

PRO TIP: Redundant Termination Unit Compatibility

Don’t attempt to install an redundant pair on standard single-channel MTUs like the TU810 if you expect redundancy. True dual-module failover requires the TU842 or TU843 terminal bases. On a TU810, the card will only operate as a standalone, non-redundant unit.

⚠️ TRAP 2: Swapping AO840 for

While the older AO840 and the newer look identical on the rack, their internal firmware structures differ. If you are replacing an older AO840 in an existing 800xA system, verify your System 800xA version and hardware library support the definition before inserting the card, or the controller may flag an “invalid module type” error.

Dynamic FAQ

Q: Can I use a single in non-redundant mode until our plant purchases the second module?

A: Yes. The functions completely fine as a standalone, single-channel output module on standard termination units (like the TU810 or TU812). When you’re ready to upgrade to full redundancy, you can migrate the card to a TU842/TU843 base and add the second module without modifying field wiring.

Q: How does HART pass-through behave during a module failover on a redundant pair?

A: During a hardware failover from the primary to secondary card, the primary 4-20 mA analog control signal remains smooth and uninterrupted. However, ongoing HART digital bursts (such as a device parameter read) will briefly pause and resume once the secondary card establishes active HART master status (typically within 1-2 seconds).

Q: Is the backwards compatible with older ABB S800 I/O installations?

A: Yes, provided your Control Builder / System 800xA software has the corresponding device object library installed. It serves as the direct upgraded replacement for the legacy AO840 module.

Q: What warranty comes with your New Surplus units?

A: All units ship with our comprehensive 1-Year Advanced Replacement Warranty. If a hardware defect occurs within 12 months, we cross-ship a replacement module immediately from our stocked inventory.

Q: What happens to the outputs if both modules in a redundant pair lose ModuleBus communication?

A: The outputs will shift to their configured Output Set on Error (OSP) state. Depending on how you configure the hardware properties in Control Builder, each channel will either hold its last valid value, drop to 0 mA, or move to a specific preset safety current (e.g., 4.0 mA).