ABB Implementation

Modbus RTU for ABB

Serial communication protocol using binary encoding over RS-232/RS-485. Most widely used industrial protocol worldwide.

Type: Serial CommunicationSpeed: Up to 115.2 kbpsReleased: 1979Organization: Modbus Organization

115.2 kbps max

Bandwidth

247

Max Nodes

1200m (RS-485)

Max Distance

Yes

Real-Time

Protocol Overview

ABB AC500 PLCs use CM577-ETH or CM572-RS communication modules. Automation Builder (CODESYS-based) provides SysLibModbus library with MODBUSMASTER_RTU and MODBUSSLAVE_RTU function blocks.

When to Use Modbus RTU

Choose Modbus RTU for simple applications, legacy integrations, cost-sensitive projects, or when connecting basic I/O devices over short to medium distances.

ABB Hardware Requirements

Compatible Communication Modules

CM577-ETH (Ethernet + 2x serial)

CM572-RS (2x RS-232/485 isolated)

TB521-ETH (AC500-eCo built-in)

AI562 (S500 I/O with Modbus)

TB511-ETH (AC500-eCo V3)

Wiring Details

CM577-ETH COM1 RS-485: Terminal 1 (RxD/TxD+), 2 (RxD/TxD-), 5 (GND). Bridge JP1 for 2-wire RS-485.

Termination

Install jumper JP2 on CM577-ETH to enable 120Ω termination. Remove to disable. TB521-ETH has software-controlled termination.

ABB Software & Programming

Function Blocks & Instructions

MODBUSMASTER_RTU (SysLibModbus)

MODBUSSLAVE_RTU (SysLibModbus)

ClientRequest (IoDrvModbus)

ModbusRequest

MBus_ReadRegs wrapper

ABB Code Example

fbModbusMaster(
  config := modbusConfig,
  request := modbusRequest,
  xExecute := xExecute,
  pData := ADR(readBuffer),
  => xDone => xDone,
  => xError => xError,
  => eState => eState
);

ABB-Specific Implementation Notes

CODESYS-based platform with portable code. eState output provides detailed state machine status. Supports MODBUSFUNCTION enum for function code selection. Arrays are 0-indexed (Modbus 40001 = array[0]).

Technical Specifications

Physical Layer

Physical Layer: RS-232, RS-422, RS-485
Cable Type: RS-485 twisted pair, RS-232
Topology: Bus, Star, Tree
Power Over Cable: No

Data Layer

Data Link: Master/Slave
Encoding: Binary (RTU mode)
Error Detection: CRC-16
Frame Size: 256 bytes max

Performance

Scan Time: 10-100ms typical
Deterministic: Yes
Real-Time: Yes

Certification

Certification Required: No
License Cost: Free (open standard)
Version: Current

Advantages & Disadvantages

Advantages

✓Simple and easy to implement
✓Very low cost
✓Excellent industry support
✓Reliable in harsh environments
✓Minimal hardware requirements
✓Open standard
✓Wide device compatibility

Disadvantages

✗Limited speed (115.2 kbps max)
✗No built-in security
✗Master/slave only (no peer-to-peer)
✗Limited diagnostic capabilities
✗No standardized device profiles
✗Requires proper termination

Common Use Cases & Applications

Simple sensor and actuator networks

Building automation

Energy management systems

Water/wastewater treatment

Legacy equipment integration

Small to medium manufacturing lines

Industry Adoption

Adoption LevelVery High

Market Share30-40% of industrial protocols

Geographic PresenceWorldwide

Primary Industries

ManufacturingBuilding AutomationEnergyWater Treatment

Supported Message Types

Read Coils

Read Discrete Inputs

Read Holding Registers

Write Single Coil

Write Single Register

Write Multiple Registers

Supported PLC Brands

Siemens Allen-Bradley Mitsubishi Schneider Electric Beckhoff ABB Rockwell

Phoenix Contact

Wago

Protocol Comparisons

Modbus RTU vs PROFINET

Modbus RTU is simpler and cheaper; PROFINET is faster and more capable

View Detailed Comparison →

Quick Links

Alternative Protocols

Related Standards

IEC 61131-3
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IEC 61131 Series
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