Rockwell Automation Implementation

Modbus TCP/IP for Rockwell Automation

Ethernet-based variant of Modbus using TCP/IP. Combines simplicity of Modbus with speed and scalability of Ethernet.

Type: Industrial EthernetSpeed: 10/100/1000 MbpsReleased: 1999Organization: Modbus Organization

1 Gbps capable

Bandwidth

Unlimited (network dependent)

Max Nodes

100m per segment (with switches: unlimited)

Max Distance

Real-Time

Protocol Overview

ControlLogix/CompactLogix use MSG instruction with Generic Modbus over Ethernet. Modern controllers support native Modbus TCP via EtherNet/IP port without additional hardware.

When to Use Modbus TCP/IP

Choose Modbus TCP/IP for SCADA systems, plant-wide networks, remote monitoring, or when integrating industrial equipment with IT systems.

Rockwell Automation Hardware Requirements

Compatible Communication Modules

Integrated Ethernet port (1756-L7x, 1769-L3x)

1756-ENBT/1756-EN2T (ControlLogix Ethernet)

1769-L18ER (CompactLogix with dual Ethernet)

Stratix switches for managed networks

Wiring Details

Network Setup

Uses EtherNet/IP infrastructure. Port 502 for Modbus TCP. Standard Ethernet switches compatible.

Configuration

Configure IP in RSLogix/Studio 5000. Add route to Modbus device if on different subnet.

Rockwell Automation Software & Programming

Function Blocks & Instructions

MSG instruction (primary method)

ProSoft PLX-ModbusTCP (3rd party)

RSMACC TCP library

Generic Modbus configuration in MSG properties

Rockwell Automation Code Example

// MSG Properties:
// Type: Generic Modbus TCP
// Function: 03 Read Holding
// Slave Address: 1
// Starting Reference: 40001
// Elements: 10
// Target IP: 192.168.1.10
// Path: EN2T or controller Ethernet port

Rockwell Automation-Specific Implementation Notes

MSG.DN and MSG.ER bits control state machine. Error code 16#0001 = path invalid, 16#0005 = timeout. Typical 10-30ms response time. Maximum 16 concurrent MSG instructions. Byte order may need swapping (use SWAPB instruction).

Technical Specifications

Physical Layer

Physical Layer: IEEE 802.3 Ethernet
Cable Type: Cat5e/Cat6 Ethernet
Topology: Star, Tree, Ring
Power Over Cable: No

Data Layer

Data Link: TCP/IP
Encoding: Binary
Error Detection: TCP checksum
Frame Size: 1500 bytes (MTU)

Performance

Scan Time: 1-10ms typical
Deterministic: No
Real-Time: No

Certification

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

Advantages & Disadvantages

Advantages

✓High speed (up to 1 Gbps)
✓Uses standard Ethernet infrastructure
✓Unlimited nodes and distance (with switches)
✓Easy IT integration
✓Simple protocol (Modbus heritage)
✓Open standard and free
✓Remote access capability

Disadvantages

✗Not deterministic (not real-time)
✗No built-in security (requires VPN/firewall)
✗Higher latency than dedicated fieldbuses
✗Requires managed switches for large networks
✗No device profiles or configuration tools

Common Use Cases & Applications

SCADA systems

Plant-wide data collection

Building management systems

Remote monitoring

Enterprise system integration

Large distributed systems

Industry Adoption

Adoption LevelVery High

Market Share25-35% of industrial Ethernet

Geographic PresenceWorldwide

Primary Industries

Oil & GasWater/WastewaterBuilding AutomationManufacturingEnergy

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 TCP/IP vs EtherNet/IP

Modbus TCP is simpler and open; EtherNet/IP offers better diagnostics and CIP ecosystem

View Detailed Comparison →

Quick Links

Alternative Protocols

Related Standards

IEC 61131-3
International standard for programmable logic controller programming languages. Defines 5 programmin...
IEC 61131 Series
Complete series of international standards for programmable logic controllers covering hardware, sof...