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These cookies will be stored in your browser only with your consent. You also have the option to opt-out of these cookies. But opting out of some of these cookies may have an effect on your browsing experience. Necessary Necessary. In this example, the user can choose from two assemblies for a simple Flow Meter. In the Connection object there are a minimum of two connection instances.
The Explicit connection describes the how explicit messages are transferred. There are a number of different ways to specify the connection path most of which require more explanation than can be provided in this document.
To change the Connection path a user can use an Explicit message to set this path directly or in many cases the vendor of the device provides a selector attribute in one of the application objects to more easily set the path. If a device only supports direct management of the Connection Path attribute the user may have to consult the device documentation or the DeviceNet specification to correctly specify the path. The multiple messages are transmitted with no special encoding or sequence number.
Service Code data associated with these messages includes:. The Router validates the Object Class. If the Explicit Message contains an invalid Class, the router rejects the message and returns an error code to the requester. If the Router validates the Object Class it passes it to the target Object for processing. The response from the target object is returned to the requester through the router. Note that this is only the network view of the device operation and may or may not be how the device is implemented.
An Explicit Message connection may support a fragmentation mechanism for messages greater than 8 bytes. Fragmented explicit Messages contain only six bytes of data. Two header bytes to manage the fragmentation are added to every message. One header byte indicates the fragmented message position First, Last, Middle in the message sequence while the second byte contains a sequence number.
The receiver Master or Slave device must transmit a message indicating acceptance of the fragmented message before the next message in the sequence is transmitted. DeviceNet devices are not required to support Explicit Message fragmentation. In fact most devices do not as Explicit Message fragmentation and the required acknowledgement handing requires a tremendous amount of device resources. A little know fact is that most DeviceNet device names an ASCII string are eight or less bytes in length to avoid support of explicit message fragmentation since longer product names would be transferred to the Master as a fragmented message sequence.
There are only two requirements for successfully placing a new device on a network. One, the baud rate of the device much match the baud rate of all other devices on the network and two, the device address must not conflict with an address of another device. The baud rate limits the length of the network. Very simply, CAN networks require every device to listen to its own bits as they are transmitted and to set the acknowledge bit in each and every message transmitted on the network.
The implementation of this latter requirement dictates the maximum network length for each baud rate:. Identifying the current baud rate of a device can be a challenge. Unless a device is configured using dipswitches and the baud rate can be discerned from the switches, there is no way to know what the baud rate might be. One of the reasons that a device may not connect is a baud rate that is different than the network baud rate. An integer address is assigned to every DeviceNet device on a network.
This integer is the device address or MacID. There are a maximum of 64 nodes on a DeviceNet network. These nodes occupy MacIDs addresses 0 to 63 and can be set using switches or using a DeviceNet configuration tool.
No two devices can occupy the same DeviceNet Address. At power on each DeviceNet device sends out a message requesting access to the network. Part of this message is the unique device serial number. If more than one devic from the same vendor all with the same vendor id power on and attempt to access the network at the identical instant the device with the lowest DeviceNet serial number is successful.
All the other devices fault. This sequence is explained in more detail in the next section. The DeviceNet protocol requires that devices transitioning from the offline to on-line state follow a very specific message sequence with specific timing. This sequence relies on the Duplicate MacID request message. This ensures that no two Duplicate MacID request messages can have the same contents. If no response is received after the second, one second delay, the device can officially transition to the online state.
There are other more subtle requirements for DeviceNet devices. For example if a device in the online state ever receives a Duplicate MacID response message it must immediately transition to the offline state. A message that is directly opposite the Duplicate MacID request message is the device shutdown message. This message broadcasts the fact that a device is transitioning from the online state to the offline or non-existent state.
This is an optional message that can be used by a device to signal a fault condition, remote command or some other reason for taking itself off the network. Another infrequently used message is the Device Heartbeat message. A device can support any one or all of the device types simultaneously. The allocation process, described later in this document, is a set of handshaking messages in which the Master device requests control of the slave and then configures the slave to transfer a particular set of data at a data rate specified by the Master.
If the slave accepts ownership it replies in the affirmative and creates the connections requested by the Master Device in the request message. A Slave may deny the allocation request from the Master Device. Slaves may deny a request if they are already allocated to another Master or if the Master device requests an unsupported connection type. For example if the Master requests a cyclic connection and the DeviceNet Slave only supports polled connections the allocation request is denied.
If the Master fails to scan at this rate, the Slave device enters a Timed-Out state and must be explicitly reactivated by the Master Device. The Produced and Consumed connection paths are the paths to the application objects where data is respectively generated or stored.
Generally these paths refer to one of the assemblies supported by the Slave device. Scanning can begin once the Slave device is fully configured. During scanning the Master device produces and consumes Slave data. Slave Devices, sometimes known as servers, are devices that receive and transmit application specific data to and from a Master device. Slave devices by definition implement the Predefined Master Slave Connection Set described in the previous section.
The Slave device can then implement whatever functionality is required of the device when its master is not in run mode. Master devices such as Configuration tools often allocate the Predefined Connection Set of a Slave device requesting only the Explicit Message request connection. These devices typically allocate the EM connection, get or set a particular attribute and then release the connection set. If these devices are currently allocated by another Master device the Master owning the DeviceNet slave mimics the messages of an unconnected slave device but only accepts the Explicit Message connection.
Messages for the owned slave are received by this Master and sent to the slave. The response from the slave is received and relayed to the original requestor, just as if the original Master device was communicating with the Slave directly. In this fashion a device, like a configuration tool, can get or set an attribute of a DeviceNet slave even if a Master already owns it device. DeviceNet devices are configured using external hardware or software configuration tools.
External hardware can include rotary switches, thumbwheels, dipswitches and other fixed input devices. Software configuration tools access the internal configuration of the device over the DeviceNet network or other communication port. Generic tools that can configure any DeviceNet device use the DeviceNet network. Vendor specific tools that configure just the devices of that vendor may either use the DeviceNet network or some other communication port on a device.
Some end users prefer configuration using switches. Their philosophy is that a device can be easily replaced if all it takes is to set switches and connect the replacement devices. Other end users prefer software configuration. These users view switches as a potential source of device failure. DeviceNet requires termination resistors at each end of a DeviceNet trunk line. The DeviceNet specification expressly recommends that these termination resistors not be included within a device.
Isolation of DeviceNet devices is recommended for devices with connections to external power supplies. DeviceNet vendors are required to provide some type of documentation specifying how their device is configured. The document may be a set of printed instructions or some electronic file.
This assembly provides a single attribute, a single point of reference where all device parameters can be read and written. An electronic listing of the Attributes that configure a DeviceNet device is usually provided by a vendor.
These EDS files sometimes provide little to no information on the device or they can be very lengthy and complex. The more extensive EDS files allow configuration tools to precisely configure the device using text identifiers for bit values and other very informative text strings. DeviceNet Gateways convert data from another protocol to DeviceNet.
There are innumerable protocol converters for DeviceNet. The difficult issue for a DeviceNet gateway is mapping data in the other protocol to the object structure of DeviceNet.
For example, the Modbus protocol represents its data as a series of bit integers while CIP represents data as attributes that are part of objects. To do that it must know the particular register interface of a selected CAN controller. Bringing you the latest news, trends, innovations and opinion from across the process industry, our exclusive newsletter gives you all the industry insights of the moment in one, easy-to-digest bulletin.
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