Image Processing Reference
In-Depth Information
23.3.2 Time-Division Multiplexing
Time-division multiplexing can be accomplished in one of two ways: MS or TP. In an MS network,
a single master polls multiple slaves. Slaves can only send data over the network when requested by
the master; there are no collisions, as the data transmissions are carefully scheduled by the master.
A TP network has multiple masters, or peers. he token bus protocol (e.g., IEEE .) allows a linear,
multidrop, tree-shaped, or segmented topology []. [].The node that currently has the token is allowed
to send data. When it is finished sending data, or the maximum token holding time has expired,
it “passes” the token to the next logical node on the network. If a node has no message to send, it
just passes the token to the successor node. The physical location of the successor is not important
becausethetokenissenttothelogicalneighbor.Collisionofdataframesdoesnotoccur,asonly
one node can transmit at a time. Most TP protocols guarantee a maximum time between network
accesses for each node, and most also have provisions to regenerate the token if the token holder stops
transmitting and does not pass the token to its successor. AS-I, Modbus, and Interbus-S are typical
examples of MS networks, while PROFIBUS and ControlNet are typical examples of TP networks.
Each peer node in a PROFIBUS network can also behave like a master and communicate with a set
of slave nodes during the time it holds the token [].
TP networks are deterministic because the maximum waiting time before sending a message frame
can be characterized by the token rotation time. At high utilizations, TP networks are very efficient
and fair. There is no time wasted on collisions, and no single node can monopolize the network.
At low utilizations, they are inefficient due to the overhead associated with the TP protocol. Nodes
without any data to transmit must still receive and pass the token.
Waiting time in a TDM network can be determined explicitly once the protocol and the traffic to
be sent on the network are known. For TP networks, the node with data to send must first wait to
receive the token. he time it needs to wait can be computed by adding up the transmission times for
all of the messages on nodes ahead of it in the logical ring. For example, in ControlNet, each node
holds the token for a minimum of . µs and a maximum of . µs.
In MS networks, the master typically polls all slaves every cycle time. Slaves cannot transmit data
until they are polled. Ater they are polled, there is no contention for the network so the waiting time
is zero. If new data are available at a slave (e.g., a limit switch trips), the slave must wait until it is polled
before it can transmit its information. In many MS networks (such as AS-Interface), the master will
only wait for a response from a slave until a timer has expired. If the slave does not respond within
thetimeoutvalueforseveralconsecutivepolls,itisassumedtohavedroppedofthenetwork.Also,
every cycle time, the master attempts to poll an inactive slave node (in a round-robin fashion) [].
In this way, new slaves can be added to the network and will be eventually noticed by the master.
23.3.3 Random Access with Collision Arbitration: CAN (CSMA/AMP)
CAN is not only a serial communication protocol developed mainly for applications in the auto-
motive industry, but also capable of offering good performance in other time-critical industrial
applications. The CAN protocol is optimized for short messages and uses a carrier sense multi-
ple access (CSMA)/AMP medium access method. hus, the protocol is message-oriented, and each
message has a specific priority that is used to arbitrate access to the bus in case of simultaneous
transmission.hebitstreamofatransmissionissynchronizedonthestartbit,andthearbitrationis
performed on the following message identifier, in which a logic zero is dominant over a logic one. A
node that wants to transmit a message waits until the bus is free and then starts to send the identifier
of its message bit by bit. Conflicts for access to the bus are solved during transmission by an arbitra-
tion process at the bit level of the arbitration field, which is the initial part of each frame. Hence, if
two devices want to send messages at the same time, they first continue to send the message frames
and then listen to the network. If one of them receives a bit different from the one it sends out, it loses
