Description
The ABB PM856 is a processor unit for the ABB AC 800M automation platform. It is a 32-bit controller designed to execute control applications and connect directly to ABB S800 I/O through the ModuleBus. ABB documentation identifies PM856 as an AC 800M PLC unit, while PM856A is the later replacement version with increased memory.
The PM856 provides Ethernet control-network connectivity, serial interfaces, and support for CEX-Bus communication modules. It is suitable for distributed process-control applications where the controller communicates with S800 I/O and other AC 800M system components.
🔧 Main Function
The PM856 executes the PLC/control application and manages communication with connected I/O and system modules. It supports up to 12 single CEX-Bus modules and can connect to up to 96 S800 I/O modules through ModuleBus in the documented configuration.
For procurement, note that the original PM856 is commonly identified as 3BSE018104R1, while PM856A / 3BSE066490R1 is the later 16 MB version. These should not be treated as identical hardware without checking the existing system revision and software configuration.
🏭 Typical Applications
• Process control systems
• DCS / PLC automation
• ABB System 800xA
• S800 I/O control
• Chemical and petrochemical plants
• Power generation
• Water and wastewater systems
• Industrial manufacturing
• Distributed control cabinets
📋 Product Datasheet
| Parameter | Specification |
|---|---|
| Manufacturer | ABB |
| Model | PM856 |
| Part Number | 3BSE018104R1 |
| Product Family | AC 800M |
| Product Type | Processor Unit / PLC Controller |
| CPU | MPC860 |
| CPU Frequency | 48 MHz |
| Memory | 8 MB SDRAM |
| Firmware Storage | 2 MB Flash PROM |
| Performance | Approx. 0.46 ms / 1000 Boolean operations |
| Power Supply | 24 VDC |
| Supply Range | 19.2–30 VDC |
| Typical Current | 180 mA |
| Maximum Current | 300 mA |
| Ethernet Ports | 2 × RJ45 |
| Ethernet Speed | 10 Mbit/s |
| Serial Ports | 2 × RS-232C |
| CEX-Bus Modules | Up to 12 single modules |
| ModuleBus I/O | Up to 96 I/O modules |
| ModuleBus Scan Rate | 0–100 ms |
| Operating Temperature | 0 to +55°C |
| Application | AC 800M Process Control |
The processor, memory, interfaces, power supply, and I/O figures above are based on ABB’s AC 800M documentation.
⚡ Key Features
• AC 800M processor/controller architecture
• 32-bit MPC860 processor
• Direct S800 I/O ModuleBus connection
• Dual Ethernet control-network ports
• Two RS-232C interfaces
• Support for CEX-Bus communication modules
• Real-time clock
• Built-in diagnostic LEDs
• Compact rail-mounted construction
• Suitable for System 800xA environments
🔌 Communication & Configuration
The PM856 provides two Ethernet ports for the AC 800M Control Network. The documented Control Network protocols include MMS and IAC. It also provides two RS-232C interfaces, with COM3 supporting selectable baud rates and COM4 providing a dedicated 9,600-baud interface.
A typical configuration is:
PM856 → CEX-Bus / Communication Modules → S800 I/O → Field Devices
The controller is configured and programmed within the ABB AC 800M engineering environment. The exact controller firmware and System 800xA/Control Builder version should be checked before replacing an existing CPU.
🛠️ Installation & Commissioning
- Verify the complete part number 3BSE018104R1.
- Check the controller hardware revision.
- Verify the 24 VDC supply.
- Inspect the DIN-rail/baseplate connection.
- Confirm Ethernet and CEX-Bus connections.
- Verify S800 ModuleBus configuration.
- Check controller diagnostics and LEDs.
- Confirm the correct application program and firmware.
- Establish Control Network communication.
- Test connected I/O before returning the process to normal operation.
⚠️ Compatibility Notice
The ABB PM856 and PM856A belong to the same AC 800M controller family but are different processor revisions. ABB documentation specifically describes PM856A as the replacement for PM856 and notes its 16 MB total RAM, compared with 8 MB for PM856.
For a replacement order, match the processor model, article number, hardware revision, firmware, baseplate, and System 800xA/Control Builder version rather than purchasing solely on the “PM856” name.
📦 Product Identification
Brand: ABB
Model: PM856
Part Number: 3BSE018104R1
Series: AC 800M
Product Type: Processor Unit / PLC Controller
CPU: MPC860
Application: Process Control / DCS / PLC
I/O Platform: S800 I/O
Engineering Environment: ABB AC 800M / System 800xA
