Image Processing Reference
In-Depth Information
on this CP, a variety of CANopen device profiles can be used in an EPL environment without
changes.
21.4.2.2 TCnet (Profile 11/1)
TCnet (Time-critical Control Network) is a proposal from Toshiba. Like EPL, the TCnet interface
goes between the physical and the data link layer; the standard media access control (MAC) access
carrier sense multiple access with collision detection (CSMA/CD) of Ethernet is modified.
In this proposal, there exists a high-speed-transmission-period composed of an RT (in TCnet
called “time-critical”) cyclic data service, and an asynchronous (in TCnet called “sporadic”) message
data service. The time-critical cyclic data service is a connection-oriented buffer transfer
∗
on pre-
established point-to-multipoint connections on the same local link separated by routers, whereas the
sporadic message services are unacknowledged messages on an extended link allowed to go through
routers.
At the start of the high-speed-transmission-period, a special SYN message is broadcasted to all
RTE-TCnet nodes. After receiving the SYN-frame, the node with the number starts sending its
data frames as planned during the system configuration. After completion of the transmission of
its data frames, it broadcasts a frame called Completed Message (see CMP in Figure .). Node
n
upon receiving the CMP
Completed Message can send out its own data frames. Each node
can hold the transmission right for a preset time and must transfer the transmission right to the next
node within this time. The node holding the transmission right can send cyclic data and sporadic
messages. The cyclic data transmission is divided into high-, medium-, and low-speed cyclic data
transmission. Each node sends at least the high-speed cyclic data when it receives the transmission
right. The other, lower priority, data is send only depending on the circumstances. Thus, the cycle
(
n
−
)
Macrocycle T
Periodic-message transferring phase Tp
Nonperiodic-message transferring phase Tn
Device 1
Device 2
Device 3
Time
Periodic-message transfer
Nonperiodic-message announcement
Nonperiodic-message transfer
FIGURE .
EPA timing.
∗
In a buffered transfer, a new message overwrites the old value of the previous message in the receiving bufer. his is in
contrast to the (standard) queued transfer, where the messages are kept in the receiver in the same order they are send.
Buffered transfer is more suited for control applications than queued; the control application is interested in the actual
buffered value and not in the sequence of values.
