The Data Highway and Data Highway plus systems are Local Area Networks (LANs). They connect programmable controllers, computers, and other devices so they can communicate and exchange data with one another. A cable system is the physical means of transmitting this data between nodes. On DH and DH+ networks, a node is a hardware interface. For the most part, the components and construction of a DH network are the same as the components and construction of a DH+ network. There are, however, some differences between the networks
DH Overview
Devices Used on DH
How Nodes Communicate on DH
A DH network uses peer-to-peer communication through a modified token-passing scheme called floating master. The master controls access to the network and can initiate messages at any time. With this arrangement, nodes bid for temporary mastership based on their need to send information. In this way, each node has equal access to become the master
Unlike a master/slave relationship, a floating master relationship does not require the current master to poll each node to grant permission to transmit information. This results in less overhead per transaction and a more efficient network. On DH, the interface modules control access to the network locally. This means if one module faults, the other modules continue to communicate on the network. Through interface modules, nodes on a DH network can communicate directly with nodes on a DH+ network. Nodes on DH networks, however, cannot communicate directly with nodes on DH II networks
A message from a node on one network to a node on another network uses message transmission time in both networks. To achieve the best network performance, configure each network so that a minimum of messages cross between them. You can do this by grouping stations that need to frequently communicate with each other on the same network.
DH+ Overview
Devices Used on DH+
How Nodes Communicate on DH+
DH+ uses token-passing protocol to allow nodes on the network to transmit messages over the cable. With token-passing protocol, only the node possessing the token can transmit messages. As long as a node possesses the token, it is the master. This is rotation of link mastership. When a node has sent all of its messages or used all of its token-hold time, it passes the token to the node with the next highest address. Token passing continues in this manner until the token is passed to the node with the lowest address. When the node with the lowest address is finished with the token, the cycle begins again. On DH+, the interface modules control access to the network locally. This means if one module faults, the other modules continue to communicate on the network.
A message from a node on one network to a node on another network uses message transmission time in both networks. To achieve the best network performance, configure each network so that a minimum of messages cross between them. You can do this by grouping stations that need to frequently communicate with each other on the same network.
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Difference between Modbus & Modbus Plus
