How to Improve Communication on Allen-Bradley Legacy PLC-5s and SLC 5/04s

Programmable Logic Controllers (PLCs) are widely used in industrial automation for controlling machinery and processes. The PLC-5 and SLC 5/04, both manufactured by Allen-Bradley, are common in legacy systems. Efficient communication between these PLCs is crucial for seamless industrial operations.

Understanding PLC-5 and SLC 5/04 Communication Protocols

Both PLC-5 and SLC 5/04 support various communication protocols:

• Data Highway Plus (DH+): The primary protocol for communication between these PLCs.

• RS-232 (DF1 Protocol): A serial communication method useful for diagnostics and programming.

• Ethernet/IP (for some PLC-5 models): A modern option when bridging with newer devices.

• Remote I/O (RIO): Used for communicating with distributed I/O modules.

Since both PLCs support DH+, this is often the preferred method for high-speed and reliable data exchange.

Optimizing Data Highway Plus (DH+) Communication

Network Configuration

Proper configuration of DH+ is essential to ensure efficient communication. Consider the following:

Baud Rate Matching: Ensure both PLC-5 and SLC 5/04 operate at the same baud rate (57.6K, 115.2K, or 230.4K).

Unique Node Addresses: Assign unique node addresses to each device to prevent conflicts.

Proper Termination: Use 150-ohm termination resistors at both ends of the DH+ network.

Cabling Best Practices: Use Belden 9463 blue hose cable for DH+ networks and maintain proper grounding to minimize noise.

Message (MSG) Instruction Optimization

The MSG instruction is used to transfer data between PLC-5 and SLC 5/04.

Configure MSG Blocks Efficiently:

• Use Read and Write messages as needed.

• Avoid excessive message traffic to reduce network congestion.

• Set correct timeout and retries to handle communication failures.

Use Integer Files Instead of Floating-Point Data:

• Floating-point data transmission requires additional overhead.

• Convert floating-point to integer values when possible.

Improving RS-232 (DF1) Communication

While DH+ is preferred, RS-232 (DF1) communication is useful for diagnostics and limited data exchange.

Cable and Hardware Considerations

• Use a null modem cable when connecting directly between PLCs.

• Ensure the RS-232 port settings (baud rate, parity, stop bits) match on both PLCs.

Using the MSG Instruction Over DF1

• Minimize message frequency to prevent serial communication delays.

• Implement error handling to manage communication timeouts.

• Consider using a dedicated serial module (1747-KE module for SLC 5/04) if higher reliability is required.

Enhancing Ethernet/IP Communication

Although native Ethernet/IP is not available on the SLC 5/04, bridging through an Ethernet module is possible.

Using an Ethernet-to-DH+ Bridge

• Utilize an Ethernet to DH+ bridge (such as 1785-ENET for PLC-5 or 1761-NET-ENI for SLC 5/04).

• Configure IP addresses and routing properly to ensure seamless data flow.

Benefits of Ethernet/IP Bridging

• Higher communication speed compared to DH+ or RS-232.

• Easier integration with modern SCADA and HMI systems*.

• Remote access for diagnostics and troubleshooting.

ANC-100e: Ethernet to DH+ Bridge Option

The ANC-100e Ethernet to DH+ converter is a cost-effective and efficient solution for connecting legacy DH+ networks to modern Ethernet-based systems. Designed to bridge the communication gap between PLC-5, SLC 5/04, and Ethernet-enabled devices, the ANC-100e provides seamless integration with industrial automation systems.

Benefits of ANC-100e Ethernet to DH+ Bridge: $1,195 USD
Seamless Integration with Modern Networks Enables connectivity between Ethernet-based SCADA, HMI, and control systems with legacy PLCs on DH+.
Provides an easy-to-use interface for remote monitoring and troubleshooting of legacy Allen-Bradley PLCs.

High-Speed Data Transfer

• Unlike traditional serial connections, the ANC-100e offers significantly improved communication speeds.

• Reduces latency and ensures reliable data transmission.

Plug-and-Play DH+ Configuration

• Simple setup with web-based configuration tools

• Automatic node and baud rate detection

• No need for complex programming or additional hardware modifications.

Cost-Effective Alternative to Full System Upgrades

• Avoids the need to replace legacy PLCs by allowing modern systems to communicate with them.

• Extends the lifespan of existing automation infrastructure.

Improved Network Diagnostics and Troubleshooting

• Built-in diagnostic tools help identify network issues quickly.

• Reduces downtime by providing remote access to PLCs for maintenance and troubleshooting.

Secure and Reliable Communication

• Supports encryption and authentication features for enhanced security.

• Ensures data integrity when communicating between Ethernet and DH+ networks.

By implementing the ANC-100e, industries can modernize their control systems while maintaining legacy hardware, reducing costs, and improving overall system efficiency.

Troubleshooting Common Communication Issues with PLC-5s & SLC 5/04s

Network Traffic Bottlenecks

• Use RSLinx to monitor DH+ traffic and identify high-usage nodes.

• Optimize MSG execution timing to prevent message collisions.

Electrical Noise Interference

• Proper grounding and shielding of cables reduce noise-related communication failures.

• Avoid running communication cables parallel to high-voltage lines.

Addressing Message Instruction Failures

• Check for correct destination addresses in MSG instructions.

• Increase timeout settings for networks with high latency.

• Implement error-handling logic to retry failed communication attempts.

Upgrading Legacy Systems for Improved Communication

While improving communication between PLC-5 and SLC 5/04 is possible, considering modern upgrades can enhance reliability

Migrating to CompactLogix or ControlLogix

• Allen-Bradley’s CompactLogix and ControlLogix PLCs offer enhanced communication capabilities.

• ControlNet or Ethernet/IP support enables better scalability and speed.

• Use 1756-DHRIO modules to maintain connectivity with existing DH+ networks during migration.

Implementing OPC Servers for Integration

• Use OPC servers (such as Kepware or RSLinx Gateway) to facilitate data exchange with modern systems.

• OPC servers provide easier integration with SCADA, HMIs, and IIoT platforms.

Improving communication between PLC-5 and SLC 5/04 requires a combination of network optimization, best practices, and modern integration techniques. By properly configuring DH+ or DF1, minimizing network congestion, and considering Ethernet/IP bridging, communication reliability can be significantly enhanced. Additionally, planning for future upgrades to modern PLCs ensures long-term system efficiency and scalability.