What is DeviceNet?
DeviceNet is a low-cost communications link for connecting industrial devices as inductive sensors, photoelectric, limit switches, push buttons, barcode readers, indicator lights, operator interfaces, motor controllers, etc., to a network, avoiding costly and complex wiring.
DeviceNet brings LAN technology down to the lowest level of the manufacturing enterprise –theshop floor.
Direct connectivity provides better communication between devices and a diagnosis to device level that would be impossible in the case of wired input/output interfaces. For another part, allows easy exchange of products, including similar components provided by different manufacturers.
DeviceNet Topology:
DeviceNet supports a “trunk-line / drop-line” topology, which means that the nodes can be connected directly to the main line or by short branches (with maximum lengths of 6 m)
Each DeviceNet network allows them to connect up to 64 nodes where a node is used by the master “scanner” and node 63 is reserved as the default node, with 62 nodes available for the devices. However, most industrial controllers allow connecting to multiple DeviceNet networks, with which the number of interconnected nodes can be expanded.
Thick cables are used for the main line since they allow a longer distribution length of the network.
Working of DeviceNet:
It is based on the seven-layer ISO standard for communications networks (OSI model).
It uses the CAN protocol, which is a message-oriented protocol, for the link layer and CIP (Common Industrial Protocol), which is an object-oriented protocol, for higher-level layers.
Through the CIP protocol, you can send Explicit messages (or information, which usually require a response from another unit but are not time critical) or Implicit (or control, are critical information packages used to transmit data in real time). Once the connection is made, the CAN identifier directs the information to the corresponding node. The functions that the CIP protocol fulfils in the different layers are:
- Application layer: It presents the user with a list of the nodes, with their objects and at the same time the class, the services, the attributes and the different reactions of the latter.
- Presentation layer: It is responsible for “translating” the data entered by the user into data that can be understood by the lower layer.
- Session layer: Keeps track of open connections and is responsible for routing the data. The MAC ID of the node in which the object is located, the number of class and instance and the required service are placed, so that the lower layer (transport layer) can perform the routing.
The CIP object model works in producer-consumer mode, but network devices can be configured to operate in a master-slave architecture or a distributed control architecture using point-to-point communication.
The types of messages that are defined are:
- Polling: The master interrogates each node one by one to receive or send an update of the state of the node.
- Strobing: A node requests through simultaneous diffusion the update to all the nodes, which respond in a
- predetermined order by a list.
- Cyclic sending: The devices are configured to send messages on a regular basis according to the programmed time.
- Change of state: Nodes only send messages when their status changes.
- Fragmented messages: It is used for the case where it is necessary to send more than 8 bytes of data in a single shipment. In this case, sen will send messages of up to 8 bytes where the fragmented data travels to later be joined again.
