ABB PM856 | AC 800M Controller

Description

🔹 Product Introduction

The ABB PM856 is a processor unit in the ABB AC 800M controller family. It is a 32-bit industrial controller designed for PLC and process-control applications and can communicate directly with ABB S800 I/O through the ModuleBus. ABB documentation identifies PM856 as a single-board computer with support for up to 12 single CEX-bus modules.

The PM856 is intended for distributed control architectures where the controller handles application logic, I/O processing, communication, diagnostics, and control tasks. It is a legacy member of the AC 800M platform, while PM856A is the later replacement variant with increased memory.

 

📋 Product Datasheet

*Country of origin should be confirmed from the actual product label and shipment documentation.

ABB specifies the PM856 with 16 MB memory, a 24 MHz MPC860 processor, 24 V DC supply, 180 mA typical current consumption, and 2 Ethernet interfaces. The published dimensions are 119 × 186 × 135 mm with a weight of approximately 1.1 kg including the base.

 

⚙️ Main Features

🔹 AC 800M Processor Unit
PM856 functions as the central processor for an AC 800M control system.

🔹 S800 I/O Integration
The controller connects directly to S800 I/O through the ModuleBus.

🔹 CEX Bus Expansion
PM856 supports up to 12 single CEX-bus communication modules, allowing additional communication interfaces to be added to the controller.

🔹 Dual Ethernet Interfaces
The PM856 provides two Ethernet channels with RJ45 interfaces for AC 800M control-network applications.

🔹 Compact Industrial Construction
The processor, power supply electronics, memory, diagnostic LEDs, real-time clock, and CompactFlash interface are integrated into the controller assembly.

 

🛠️ Operating Procedure & How to Use

1. Confirm the Controller

Before replacement or installation, verify:

• PM856 model number
• Hardware revision
• Base/termination unit
• Firmware version
• Existing Control Builder project
• ModuleBus configuration
• CEX-bus modules
• Ethernet network configuration

2. Install the Controller

Mount the PM856 on the appropriate AC 800M base/termination assembly and secure the controller correctly.

The termination/base assembly provides the external connections for power, control network, service port, CEX-Bus, and ModuleBus.

3. Connect 24 V DC

Connect the approved 24 V DC control power to the base unit.

The specified operating range is 19.2–30 V DC.

4. Connect the Control Network

Connect the Ethernet control-network cables to the appropriate RJ45 ports.

5. Connect S800 I/O

Verify the electrical or optical ModuleBus arrangement and make sure all connected I/O clusters match the project configuration.

6. Configure the Controller

Use ABB Control Builder to load or verify:

• Controller configuration
• Application program
• I/O configuration
• Network settings
• Communication interfaces
• Task configuration

7. Perform Functional Testing

After startup, verify controller diagnostics, communication, I/O status, application execution, and process signals.

 

⚡ Power-On Sequence

1. 🔒 Place the controlled process in a safe condition.
2. 🔍 Inspect the PM856 and base assembly.
3. 🔍 Verify 24 V DC polarity and voltage.
4. 🔍 Check Ethernet connections.
5. 🔍 Check ModuleBus connections.
6. 🔍 Verify CEX-bus modules.
7. ⚡ Apply 24 V DC power.
8. 🟢 Check controller status LEDs.
9. 🖥️ Connect with the engineering workstation if required.
10. 🌐 Verify control-network communication.
11. 🔄 Check S800 I/O communication.
12. 🧠 Confirm application execution.
13. ⚙️ Verify control loops and interlocks.
14. ✅ Return the process to normal operation.

 

🔌 Power-Off Sequence

1. 🛑 Stop or place the controlled process in the approved safe state.
2. 🔒 Put the controller/application into the required maintenance condition.
3. 💾 Save or back up the current controller configuration.
4. 📴 Stop the application where required.
5. ⚡ Isolate the 24 V DC supply.
6. 🔍 Confirm the controller is de-energized.
7. 🔧 Disconnect cables only after isolation.
8. 🛡️ Use ESD protection when removing the PM856.

 

📖 Product Usage Instructions

A typical PM856 control architecture can be represented as:

Engineering Station → Ethernet Control Network → PM856 → ModuleBus → S800 I/O → Field Devices

The PM856 executes the configured control application and exchanges data with connected I/O and communication interfaces.

The controller can support multiple applications and tasks. ABB’s technical data lists up to 32 applications per controller, up to 64 programs per application, and up to 32 tasks per controller for the PM856.

For S800 I/O, the PM856 supports one electrical ModuleBus cluster and up to seven optical I/O clusters in a non-redundant configuration. ABB also specifies a maximum of 96 I/O modules on the ModuleBus architecture for the controller.

 

🔍 Installation & Commissioning Checklist

Before commissioning:

• Confirm PM856 identification.

• Check base/termination unit.

• Verify 24 V DC supply.

• Check power polarity.

• Verify Ethernet connections.

• Check CEX-bus configuration.

• Verify ModuleBus configuration.

• Confirm S800 I/O modules.

• Check controller firmware.

• Verify Control Builder project.

• Download/restore the correct application.

• Check controller diagnostics.

• Test I/O communication.

• Verify HMI/SCADA communication.

• Test alarms and interlocks.

• Record the final configuration.

 

❓ Common Questions & Answers

Q: What is ABB PM856?

A: PM856 is a processor unit in the ABB AC 800M controller family. It is used for PLC and process-control applications and connects directly to S800 I/O through ModuleBus.

Q: Is PM856 a CPU?

A: Yes. It is the processor/controller unit of an AC 800M system.

Q: What processor does PM856 use?

A: The PM856 uses an MPC860 32-bit processor operating at 24 MHz.

Q: How much memory does PM856 have?

A: The PM856 has 16 MB memory. The later PM856A variant increased the total memory to 16 MB compared with the earlier PM851A architecture; ABB’s hardware selector specifies PM856A with 16 MB.

Q: What is the power supply for PM856?

A: The controller uses 24 V DC, with an operating range of 19.2–30 V DC.

Q: How much power does PM856 consume?

A: ABB specifies approximately 180 mA typical / 300 mA maximum at 24 V DC, corresponding to approximately 5.0 W typical / 8.0 W maximum power dissipation.

Q: Can PM856 communicate with S800 I/O?

A: Yes. PM856 connects directly to S800 I/O through the ModuleBus.

Q: Does PM856 support communication modules?

A: Yes. ABB specifies support for up to 12 single CEX-bus modules.

Q: Does PM856 have Ethernet?

A: Yes. The PM856 provides two Ethernet channels with RJ45 interfaces.

Q: Is PM856 redundant?

A: No. ABB’s supported-hardware documentation identifies PM856 as non-redundant. PM861 and several higher-end AC 800M processors provide redundancy options.

Q: What is the difference between PM856 and PM856A?

A: PM856A is the later replacement variant. The PM856A has 16 MB memory and is identified by ABB as the replacement for PM856.

Q: Can PM856 be replaced directly by PM856A?

A: PM856A is the designated replacement family, but the complete hardware revision, Control Builder version, firmware, application, and connected communication modules should be checked before replacement.

Q: What is the PM856K01?

A: PM856K01 is a controller kit designation containing the PM856A controller and required accessories in ABB’s product structure. ABB lists PM856AK01, article number 3BSE066490R1, in its current hardware selector.

 

🔟 Recommended ABB AC 800M Related Models

1. ABB PM851 — AC 800M Processor Unit
2. ABB PM851A — AC 800M Processor Unit
3. ABB PM856A — AC 800M Processor Unit / PM856 Replacement
4. ABB PM858 — AC 800M Processor Unit
5. ABB PM860A — AC 800M Processor Unit
6. ABB PM861A — Redundant AC 800M Controller
7. ABB PM862 — AC 800M Controller
8. ABB PM864A — High-Performance AC 800M Controller
9. ABB PM866A — AC 800M Processor Unit
10. ABB PM867 — AC 800M Processor Unit

The AC 800M family includes processor units with different memory, processing performance, redundancy, and application capabilities. ABB’s current hardware selector lists PM851A, PM856A, PM857, PM858, PM860A, PM862, PM863, PM864A, PM866A and other AC 800M controller variants.

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