Beckhoff Implementation

Modbus RTU for Beckhoff

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

Beckhoff TwinCAT 3 implements Modbus RTU via EL6xxx EtherCAT serial terminals or embedded PC COM ports. Tc2_ModbusRTU library provides comprehensive master/slave 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.

Beckhoff Hardware Requirements

Compatible Communication Modules

EL6021 (1x RS-232)

EL6022 (2x RS-232)

EL6001 (1x RS-232/RS-485 switchable)

KL6021 (BUS Terminal RS-232)

KL6041 (BUS Terminal RS-485)

CX Embedded PC COM1/COM2

Wiring Details

EL6001 RS-485: Pin 3 (TxD+/A), Pin 6 (TxD-/B), Pin 9 (GND). DIP SW1=ON for RS-485 mode.

Termination

EL6001 built-in 120Ω termination. DIP SW2=ON enables, SW2=OFF disables. Enable at bus ends only.

Beckhoff Software & Programming

Function Blocks & Instructions

FB_MBReadRegs (Tc2_ModbusRTU)

FB_MBWriteRegs

FB_MBReadCoils

FB_MBWriteCoils

FB_ModbusRTUMasterV2 (with queue)

FB_ModbusRTUSlave

Beckhoff Code Example

fbModbusRead(
  UnitID := 1,
  Quantity := 10,
  MBAddr := 0,  // 0-indexed
  cbLength := SIZEOF(arrReadData),
  pDestAddr := ADR(arrReadData),
  Execute := TRUE,
  => Busy => bBusy,
  => Error => bError,
  => ErrorId => nErrID
);

Beckhoff-Specific Implementation Notes

EL6xxx terminals have independent mailbox communication via CoE (CANopen over EtherCAT). Configure baudrate via CoE index 0x8000:01. Use FB_ModbusRTUMasterV2 for request queuing. Supports byte swapping functions for endianness conversion.

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

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