The PM851A Controller is a compact and high-reliability CPU unit within the ABB AC 800M control family, designed for demanding industrial automation and process control applications.
It integrates processing, communication, and diagnostic functions in a modular design, enabling step-by-step system expansion and easy maintenance. The unit ensures stable real-time performance for distributed control systems and medium-to-large automation architectures.

2. Hardware Description

CPU and Memory

The CPU board of the PM851A contains the microprocessor, RAM memory, real-time clock, and CompactFlash interface, providing reliable and fast data processing and storage.
An INIT push button enables safe initialization and configuration, while status LEDs provide visual indication for power, communication, and fault diagnostics.

Communication Interfaces

The base plate (TP830) of the PM851A controller is equipped with multiple communication ports for flexible system connectivity:

Ethernet Ports (CN1, CN2): Two RJ45 connectors are available for Control Network communication.

Note: For PM851/PM851A, only one Ethernet port (CN1) is active; redundant Ethernet configuration is not supported.

Serial Ports (COM3, COM4):

COM3: RS-232C interface with modem control signals.

COM4: Electrically isolated serial port used for connection with configuration tools.

Each base plate includes a unique Ethernet hardware address, ensuring unambiguous identification of the CPU within the network. The Ethernet address is printed on the identification label attached to the TP830 base plate.

Mounting and Installation

The controller employs ABB’s slide-and-lock DIN rail mechanism, allowing quick attachment and removal without tools.
Compact design and IP20 protection class enable installation in standard control cabinets without additional enclosures.

3. Features and Benefits

Feature	Description / Benefit
High reliability	Proven hardware design with simple fault diagnosis procedures reduces downtime and enhances operational stability.
Modular design	Supports step-by-step expansion with additional communication and I/O modules for scalable system architecture.
Ease of maintenance	Slide-and-lock mounting and accessible interfaces simplify replacement and service operations.
Integrated communication	Supports Ethernet and PROFIBUS DP connectivity for flexible integration with distributed control systems.
Configuration environment	Fully supported by ABB 800xA Control Builder, providing a unified engineering and programming platform.
Industrial compliance	Full EMC certification and adherence to industrial communication standards ensure dependable performance in harsh environments.
CompactFlash interface	Enables reliable program and data storage for long-term operation.

4. Technical Highlights

Parameter	Specification
Processor	Industrial-grade CPU with integrated real-time clock
Memory	RAM memory with CompactFlash storage interface
Ethernet Ports	2 × RJ45 (CN1 active, CN2 reserved)
Serial Ports	COM3 (RS-232C), COM4 (Isolated serial, for configuration)
Mounting	DIN rail, slide-and-lock mechanism
Protection Class	IP20
Configuration Software	800xA Control Builder
Standards / Certification	EMC certified; compliant with Ethernet, PROFIBUS DP standards
Identification	Unique Ethernet MAC address per base plate