Rockwell Automation Implementation

Modbus RTU for Rockwell Automation

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

Allen-Bradley ControlLogix/CompactLogix use MSG instruction with 'Generic Modbus' type. Micro800 series has native MBT/MBR instructions. Serial modules: 1769-ASCII, 1763-NC01.

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.

Rockwell Automation Hardware Requirements

Compatible Communication Modules

1769-ASCII (CompactLogix, 2 serial ports)

1763-NC01 (MicroLogix ASCII)

2080-SERIALISOL (Micro800 isolated serial)

1761-NET-AIC (Protocol converter)

ProSoft MVI56-MCM (3rd party dual Modbus)

Wiring Details

1769-ASCII: Terminal 3 (TXD+), 4 (TXD-), 6 (RXD+), 7 (RXD-). Short 3-6 and 4-7 for RS-485 2-wire mode.

Termination

NO built-in termination on 1769-ASCII. External 120Ω resistor required at both bus ends.

Rockwell Automation Software & Programming

Function Blocks & Instructions

MSG instruction (ControlLogix/CompactLogix)

MBT - Modbus Transmit (Micro800)

MBR - Modbus Receive (Micro800)

RSMACC AOI library (3rd party)

ProSoft configuration via web interface

Rockwell Automation Code Example

// MSG Configuration
// Type: Generic Modbus
// Function Code: 03 (Read Holding)
// Modbus Address: 1
// Starting Reference: 40001
// Elements: 10
// Data Table: ModbusData[0]
// Path: AB_ASCII_1:0

Rockwell Automation-Specific Implementation Notes

MSG instruction is asynchronous - use DN and ER bits for state machine control. Maximum 16 concurrent MSG instructions per controller. Micro800 MBT instruction is much simpler than MSG for basic Modbus operations.

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
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...