Over View of Device Net • Meliah Mayer • ELPT 2449 • Industrial Automation • Spring 2007
Device Net Device net was designed to replace large, complex wiring schemes with a simple trunk and branch structure, while also providing a high-level of diagnostics to each device on the network.
Characteristics of Device Net • Based on CAN (controller area network) • Maximum number of devices: 64 • Baud rate 125, 250, 500kbits/sec • Distance: 100M to 500M • Maximum message size: 8 bytes of data • Messaging formats: polling, strobe, cyclic, and COS (change of state)
Polling • The scanner asks each device to send or receive an update of its status. This requires and incoming and outgoing message for each node on the network. Polling is more precise but least time efficient way to request information from the device.
Strobing • The scanner broadcasts a request to all devices for a status update. Each device responds in turn sequentially. Node numbers can be assigned to prioritize messages.
Cyclic • Devices are configured to automatically send messages on scheduled intervals. This is sometimes called a ‘heartbeat’ and is often used in conjunction with COS messaging to indicate that the device is still functioning.
Change of State • Devices only send messages to the scanner when their status changes.This occupies the minimum time on the network, but does change the throughput and response time from deterministic to statistical.
Baud rate The rate of speed which data is being transmitted over the network 500kb 250kb 125kb default Cable length The speed of the data being transmitted affects the length of cable 100m or 328ft 250m or 820ft 500m or 1680ft Connection Speed and Cable Length
Device Net Addresses • Each device on the network must have a unique address. • An integer is assigned to each device which we call a MacID (media access identifier). • There are a maximum of 64 nodes on a device net network. • MacID 63 is the default address of new devices.
Personal Computers Operator interface stations Motor starters Barcode readers Sensors Scanner cards Push buttons PLC (programmable logic controllers) Drives (AC & DC) Limit switches Servos Block I/O Motion control devices Devices for Device Net
Devices we have installed on our network • Since we are automating a conveyor system, we need powered roller controllers to move the product down the assembly line. • We also need sensors to activate our powered roller controllers and out lift stations. • We also need tower lights to give the operators a visual indicator if the system has a problem.
Powered Roller Controller The brown wire is not used
Tower Lights Thetower light is connected using all five wires and the connections are color coded for easy installation and wiring.
Setting up a Device Net Network • Add one device at a time. • Make sure it is working correctly • Space out your address so that you can add devices later on when updating your system. • Make sure your master/slave configuration is set up correctly.
Trouble Shooting Guidelines • Make sure all Baud rates are the same on every device installed. • Make sure all MacID’s are not duplicated. • Nine times out of ten, if you are working on a device and experience a problem it is usually the device you are working on. • If all of the above is good. Recycle the power supply.