For DeviceNet, several types of connectors can be used, sealed ones as well as open ones. The definition of the
type to be used depends on the application and on the equipment operation environment. The CFW500 uses a
5 wire plug-in connector, and its pinout is showed in the section 2. For a complete description of the
connectors used with DeviceNet, consult the protocol specification.
1.3.1 Data Link Layer
The DeviceNet data link layer is defined by the CAN specification, which defines two possible states; dominant
(logic level 0) and recessive (logic level 1). A node can bring the network to the dominant state if it transmits any
information. Thus, the bus will only be in the recessive state if there where no transmitting nodes in the dominant
state.
CAN uses the CSMA/NBA to access the physical medium. This means that a node, before transmitting, must
verify if the bus is free. In case it is, then the node can initiate the transmission of its telegram. In case it is not,
then the node must await. If more than one node access the network simultaneously, a priority mechanism
takes action to decide which one will have priority over the others. This mechanism is not destructive, i.e., the
message is preserved even if there is a collision between two or more telegrams.
CAN defines four types of telegrams (
data
,
remote
,
overload
and
error
). Among them, DeviceNet uses only the
data frame
and the
error frame
.
Data is moved using the data frame. This frame structure is showed in the figure 1.1.
Interframe
Space
1 bit 11 bits 1 bit 6 bits 0-8 bytes 15 bits 1 bit 1 bit 1 bit 7 bits ≥ 3 bits
Start of Frame
Identifier
RTR bit
Control Field
Data Field
CRC Sequence
CRC Delimiter
ACK Slot
ACK Delimiter
End of Frame
Interframe Space
Figure 1.1: CAN data frame
Errors, however, are indicated by means of the error frames. CAN has a very robust error verification and
confinement. This assures that a node with problems does not impair the communication in the network.
For a complete description of the errors, consult the CAN specification.
1.3.2 Network and Transport Layer
DeviceNet requires that a connection be established before data exchange with the device takes place. In order
to establish this connection each DeviceNet node must implement the
Unconnected Message Manager
(UCMM) or the
Group 2 Unconnected Port
. These two allocation mechanisms use messages of the explicit type
to establish a connection, which will then be used for process data exchange between one node and the other.
This data exchange uses messages of the I/O type (refer to item 1.3.5).
The DeviceNet telegrams are classified in groups, which define specific functions and priorities. Those telegrams
use the identifier field (11 bits) of the CAN data frame to uniquely identify each one of the messages, thus
assuring the CAN priority mechanism.
A DeviceNet node can be a client, a server or both. Furthermore, clients and servers can be producers and/or
consumers of messages. In a typical client node, for instance, its connection will produce requests and will
consume answers. Other client or server connections will only consume messages. In other words, the protocol
allows several connection possibilities among the devices.
The protocol also has a resource for detection of nodes with duplicated addresses (Mac ID). Avoiding that
duplicated addresses occur is, in general, more efficient than trying to locate them later.
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