![]() ![]() As the number of nodes in a network increases, the time between consecutive readings of each device will increase. Throughout computer networking, this ‘packet’ is the term for the collection of address, configuration, and actual data bits that are sent between devices. The data rate on a DH network is relatively slow compared to modern networks, but this is countered with shorter data packets. This is a system in which, after one node finishes with its data, it hands priority off to the next highest node in line. All other versions, DH+, DHII, and DH485, use token passing. This allows critical nodes to get onto the network more quickly than others. DH uses a system called Floating Master to send out data to the other nodes. Each node must have its own number so that it can be differentiated from other devices. Each networked device is called an individual ‘node’. Let's start by going over how the networks communicate. I have always been a proponent of ethernet networks, but I cannot discount some distinct advantages of the DH+ network. In nearly every industrial facility built before 2000 that I have maintained in my career, I have seen a DH+ network somewhere inside. More in-depth information, including network wiring instructions, can be found in the Allen-Bradley Data Highway Installation Manual. ![]() Of all the Data Highways, the one you are likely to see most is the DH+ network. DH485 uses a cable that supports the RS485 communication protocol. While both types of cable are used for data transmission, coaxial cable offers better noise protection at long distances, while twinaxial is better suited for short-range high-frequency applications. DHII utilizes coaxial cable, while DH and DH+ utilize twinaxial cable. ![]() The main difference between DH and DH+ is the communication method. DH and DH+ are similar because they use the same type of physical media and construction principles. There are several different kinds of Data Highway networks: Data Highway (DH), Data Highway Plus (DH+), Data Highway II (DHII), and Data Highway-485 (DH485). Many different types of devices can be connected across a Data Highway network. Let's dig into DH networks and learn the basic rundown of their operation.įigure 1. There is a broad range of controllers and hardware devices that can be integrated into a DH network, including PLC5 Processor, PLC3 Processor, Windows PC, ControlLogix PLC, PanelView HMI. Data Highway was developed by Allen Bradley in the late 1970s as a proprietary network bus used to connect PLCs, remote IO systems, PCs, and HMls. ![]()
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