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Automation Networks

Data Highway Plus (DH+)

Data Highway Plus (DH+) is an industrial network protocol developed by Allen-Bradley, a brand of Rockwell Automation. It was widely used in industrial automation for communication between various devices, such as programmable logic controllers (PLCs), computers, and other automation equipment. DH+ is part of the Data Highway network family, which also includes the earlier Data Highway (DH) network. Here’s an overview of DH+:

Key Features:

  1. Communication Standard: DH+ is a token-passing network, which means that a “token” is passed from one device to another in a sequential manner. Only the device holding the token can communicate on the network at that time.
  2. Network Configuration: It typically operates on a twisted-pair cable as its physical medium and uses a differential signaling method for data transmission.
  3. Speed and Distance: DH+ networks can operate at speeds up to 230.4 Kbps and can cover distances up to several thousand feet, making it suitable for large industrial facilities.
  4. Device Connectivity: The network can support multiple devices, including PLCs, remote I/O, and other Allen-Bradley equipment.


  1. Industrial Automation: DH+ was predominantly used in manufacturing and processing industries for connecting PLCs and other control equipment.
  2. Data Collection and Monitoring: Enabled data collection from PLCs for monitoring and control purposes.
  3. Program Upload/Download: Allowed for uploading and downloading of PLC programs, parameter settings, and diagnostics.

Technical Considerations:

  1. Topology: DH+ typically uses a linear or star topology, but proper termination is crucial to prevent signal reflection and network errors.
  2. Compatibility: DH+ was designed to work specifically with Allen-Bradley equipment and may require specific interfaces, like communication modules or bridge devices, for integration with non-Allen-Bradley equipment.
  3. Legacy Status: As a legacy protocol, DH+ has largely been replaced by more modern networking solutions like Ethernet/IP. However, it is still used in many existing installations.
  4. Maintenance and Upgrades: Maintaining and upgrading DH+ networks require a good understanding of the protocol and compatible equipment. In many cases, older systems are being upgraded to or integrated with newer network technologies.


Data Highway Plus (DH+) played a significant role in industrial automation networks, particularly in environments dominated by Allen-Bradley equipment. While its usage has declined with the advent of Ethernet-based networks, DH+ remains an important part of many legacy systems, requiring specific knowledge for maintenance and integration with modern technologies.

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