The AO845A is a high-performance, highly reliable analog output module within the ABB Ability™ System 800xA series, designed for process signal output requirements in industrial automation control systems. This module supports both single and redundant configurations, providing 8 channels of 4–20 mA current output. It features excellent electrical isolation, advanced diagnostic functions, and HART communication support, making it suitable for industries such as petrochemicals, power, water treatment, and pharmaceuticals where high signal stability and system reliability are critical.
The AO845A not only offers high-precision output and strong anti-interference capabilities but also supports modular installation and various wiring methods, allowing flexible adaptation to different field installation environments and control architectures. The module incorporates multiple diagnostic mechanisms to monitor module status and channel output anomalies in real-time, ensuring the system can safely switch or enter a preset state upon failure, significantly enhancing system availability and maintenance efficiency.
2. Key Features and Benefits
| Feature | Description |
| Channel Configuration | 8 independent 4–20 mA current output channels, supporting unipolar output. |
| Redundancy Support | Configurable for redundant operation to improve system reliability. |
| Electrical Isolation | 8 channels isolated as a group from system ground, with an isolation voltage of 500 V AC. |
| Short-Circuit Protection | Outputs are short-circuit proof to ZP (0 V) or +24 V, with current limiting protection. |
| HART Communication | Supports HART protocol pass-through communication for integration with smart instruments. |
| High-Precision Output | 12-bit resolution, maximum error ≤ 0.1%, temperature drift ≤ 50 ppm/°C. |
| Self-Diagnostics | Includes module-level and channel-level self-diagnostics with real-time error reporting. |
| Output Set as Predetermined (OSP) | Outputs can be set to predefined safe values upon fault detection, with selectable delay times of 256, 512, or 1024 ms. |
| Flexible Installation | Compatible with various Mounting Termination Units (MTUs) for different mechanical structures. |
| Wide Operating Temperature | Operating temperature: 0 to +55°C; Storage temperature: -40 to +70°C. |
3. Operating Principle and Diagnostics
The AO845A module incorporates high-performance digital-to-analog conversion circuits and output driver circuits, with each channel capable of independent current output. The module communicates with the main controller via the ModuleBus, receiving output commands and providing real-time status feedback.
1. Output Control and Protection
Output Range: 4–20 mA, supports ±15% overrange.
Short-Circuit Protection: Outputs withstand continuous short-circuits to ZP or +24 V.
Filtering Function: Output filtering can be enabled to smooth signals, recommended for channels used with HART communication.
2. Self-Diagnostics Mechanism
The module performs cyclical self-tests. Diagnostics include:
| Diagnostic Type | Trigger Condition | Reporting Method |
| External Channel Error | Process power supply voltage too low, or output current less than setpoint with setpoint >1 mA (open circuit). | Reported only for active channels. |
| Internal Channel Error | Output circuit cannot deliver the correct current value. | Channel status reported. |
| Module Error | Output transistor fault, short circuit, checksum error, internal power supply error, status link error, watchdog timeout, incorrect OSP behavior, etc. | Module status LED and bus alarm. |
| Redundant Pair Control | In a redundant pair, the master module commands the faulty module into an error state via the ModuleBus. | System automatically switches to the standby module. |
3. Output Set as Predetermined (OSP) Function
Upon detection of a module or communication fault, output channels can be configured to enter a predefined safe state (e.g., hold last value, output minimum, output maximum). The delay time can be selected as 256 ms, 512 ms, or 1024 ms.
4. Installation and Wiring
The AO845A module must be used with a compatible Mounting Termination Unit (MTU), supporting various installation methods:
| MTU Model | Application Type | Mounting Style | Connection Method |
| TU810 / TU814 | Compact MTU | Single module mounting | Screw terminals |
| TU812 | Compact MTU (with D-Sub 25-pin connector) | Single module mounting | D-Sub 25-pin plug |
| TU830 / TU833 | Extended MTU | Single module mounting | Screw terminals |
| TU842 / TU843 / TU852 | Redundant MTU | Horizontal or vertical DIN rail mounting | Screw terminals |
Wiring Guidelines
Use shielded cables for process signal connections, especially for channels supporting HART communication.
Power Supply Connections: L1+ (+24 V DC), L1- (0 V / ZP).
Each channel includes a "+ Output" terminal and a "Return (ZP)" terminal.
Refer to Figures 75–79 in the technical manual for detailed wiring diagrams.
5. Status Indication and Monitoring
The module front panel features four LED indicators:
| LED | Color | Status Description |
| F (Fault) | Red | Module or channel critical error. |
| R (Run) | Green | Module operating normally. |
| W (Warning) | Yellow | Module warning or non-critical anomaly. |
| O (OSP) | Yellow | Output Set as Predetermined (OSP) state is active. |
Users can monitor the module's health status in real-time via system software or the front panel LEDs, enabling quick fault localization and maintenance.
6. Environmental Conditions and Certifications
| Item | Description |
| Operating Temperature | 0 to +55°C (≤40°C for vertical mounting in compact MTU) |
| Storage Temperature | -40 to +70°C |
| Relative Humidity | 5% to 95%, non-condensing |
| Protection Class | IP20 (IEC 60529) |
| Pollution Degree | Degree 2 (IEC 60664-1) |
| Corrosion Protection | ISA-S71.04: G3 |
| Vibration/Shock Resistance | Compliant with IEC/EN 61131-2 |
| Electromagnetic Compatibility (EMC) | EN 61000-6-4 (Emission), EN 61000-6-2 (Immunity) |
| Safety Certifications | EN 61010-1, UL 61010-1, EN 61010-2-201, UL 61010-2-201 |
| Hazardous Location Certifications | C1 Div2 cULus, C1 Zone2 cULus, ATEX Zone 2 |
| Marine Certifications | BV, DNV, LR |
| Environmental Compliance | RoHS (DIRECTIVE/2011/65/EU), WEEE (DIRECTIVE/2012/19/EU) |
7. System Compatibility
Control Systems: ABB Ability™ System 800xA, Symphony Plus.
Communication Bus: ModuleBus.
Compatible MTUs: TU810, TU812, TU814, TU830, TU833, TU842, TU843, TU852.
Keying Code Setting: DB (for AO845A version).
8. Typical Application Scenarios
Process Control Valve Actuation: Output 4–20 mA signal to drive control valves.
Variable Frequency Drive (VFD) Speed Reference: Provide analog speed setpoint signals to drives.
Instrument Calibration & Simulation: Serve as a signal source for instrument calibration.
Redundant Control Systems: Achieve bumpless transfer in critical processes.
HART Smart Device Integration: Support remote parameter configuration and monitoring.
