Understanding Allen-Bradley Data Highway Networks and Their Integration with Ethernet
Allen-Bradley’s Data Highway (DH) network, developed in the late 1970s, revolutionized industrial automation by providing a proprietary network bus for connecting PLCs, remote I/O systems, PCs, and HMIs. This robust network has evolved over time, with variations such as DH, DH+, DHII, and DH485, each offering unique features and capabilities.
Data Highway Network Types:
- DH and DH+: Utilizing twinaxial cable, these networks are known for their high-frequency communication suitability over short ranges. DH+ introduced an improved communication method, making it the more prevalent choice.
- DHII: Employs coaxial cable, offering better noise protection over long distances, making it ideal for larger industrial facilities.
- DH485: Uses cable supporting the RS485 communication protocol, known for its reliability in noisy industrial environments.
Network Communication:
Each device on a DH network is referred to as a ‘node,’ and each node is assigned a unique number for identification. The Floating Master system in DH allows critical nodes to access the network more swiftly, while DH+, DHII, and DH485 use token passing to manage data transmission顺序.
Data Transmission Rate:
While the data rate on DH networks is slower compared to modern networks, the use of shorter data packets compensates for this, ensuring timely communication between devices. However, the increasing number of nodes can lead to longer time intervals between consecutive readings, which is crucial for systems requiring instant responses.
Data Highway vs. Ethernet:
DH networks are limited to a maximum of 64 devices, making them suitable for smaller-scale applications. In contrast, Ethernet networks offer greater complexity and scalability, requiring IP addresses, subnet masks, and gateway configurations for device communication.
Advantages of DH Networks:
DH networks are known for their simplicity in configuration, often manageable through software readily available on a programmer’s computer. Some devices even feature DIP switch banks or rotational switches for field identification without a computer.
Ethical Integration:
Despite the prevalence of Ethernet in modern control systems, DH networks still hold their ground in many legacy systems. To bridge the gap between modern and legacy equipment, network adapters or gateways are used to ensure seamless integration.
Conclusion:
Allen-Bradley Data Highway networks remain a staple in industrial automation, with their compatibility with modern protocols ensuring continued relevance. These networks, along with the appropriate adapters, allow for the integration of new and old systems, providing flexibility and convenience in industrial settings.
In subsequent articles, we will delve into the practical applications of these networks in the field and discuss strategies for upgrading to more integrated facility networks. Stay tuned for more insights into enhancing your industrial automation infrastructure.