Image Processing Reference
In-Depth Information
Message frame
Bus idle
Arbitration field
Control
Data field
CRC field
ACK
EOF
Int
Bus idle
11-bit identifier
r1 r0
DLC
Data (0-8 bytes)
15 bits
SOF
RTR
Delimiter Delimiter
Slot
FIGURE .
Message frame format of DeviceNet (standard CAN format).
the right to continue to send its message, and the other wins the arbitration. With this method, an
ongoing transmission is never corrupted, and collisions are nondestructive [].
DeviceNet is an example of a technology based on the CAN specification that has received consid-
erable acceptance in device-level manufacturing applications. The DeviceNet specification is based
on the standard CAN with an additional application and physical layer specification [,].
The frame format of DeviceNet is shown in Figure . []. The total overhead is bits, which
includes start of frame (SOF), arbitration (-bit identifier), control, cyclic redundancy check (CRC),
ACK, end of frame (EOF), and intermission (INT) fields. The size of a data field is between and
bytes. he DeviceNet protocol uses the arbitration field to provide source and destination addressing
as well as message prioritization.
The major disadvantage of CAN compared with the other networks is the slow data rate, limited
by the network length. Because of the bit-synchronization, the same data must appear at both ends of
the network simultaneously. DeviceNet has a maximum data rate of kb/s for a network of m.
Thus, the throughput is limited compared with other control networks. CAN is also not suitable for
transmission of messages of large data sizes, although it does support fragmentation of data that is
more than bytes into multiple messages.
23.3.4 Ethernet-Based Networks
The proliferation of the Internet has led to the pervasiveness of Ethernet in both homes and busi-
nesses. Because of its low cost, widespread availability, and high communication rate, Ethernet
has been proposed as the ideal network for industrial automation [,]. Some question whether
Ethernet will become the de facto standard for automation networks, making all other solutions
obsolete [,]. However, standard Ethernet (IEEE .) is not a deterministic protocol, and net-
work QoS cannot be guaranteed [,]. Collisions can occur on the network, and messages must be
retransmitted after random amounts of time. To address this inherent nondeterminism, many differ-
ent“lavors”ofEthernethavebeenproposedforuseinindustrialautomation.Severaloftheseadd
layers on top of standard Ethernet or on top of the TCP/IP protocol suite to enable the behavior of
Ethernet to be more deterministic [,,]. In this way, the network solutions may no longer be
“Ethernet” other than at the physical layer; they may use the same hardware but are not interopera-
ble. As noted in Ref. [], message transmission does not always lead to successful communication:
“just because you can make a telephone ring in Shanghai doesn't mean you can speak Mandarin.” A
more effective and accepted solution in recent years has been the utilization of switches to manage
the Ethernet bandwidth utilizing a TDM approach among time-critical nodes. Rather than repeat
the survey of current approaches to industrial Ethernet in Ref. [], in this section, the general MAC
protocol of Ethernet is outlined, and the general approaches that are used with Ethernet for industrial
purposes are discussed.
