Image Processing Reference
In-Depth Information
message if the IC value reaches . As the data exchange per master is strictly limited, the token
rotation time is upper-bounded, even though it is likely not constant.
A similar implicit method is employed by ControlNet. A node possessing the token may transmit
one single data frame possibly containing several link layer packets. All nodes monitor the source
address of this frame, and at the end of the frame increment this address in an internal implicit token
register. If the value of this register matches the address of a node, it possesses the token and may
senditsowndata.Iftherearenodatatobesent,thenodemustsendanemptyframesothatthe
token can be passed on. This mechanism is used both for scheduled and unscheduled traffic with
two different tokens. While every node with scheduled traic will have a guaranteed bus access every
network cycle, the access right for unscheduled traffic changes in a round-robin fashion, i.e., the first
node to get bus access for unscheduled traffic is rotated every cycle.
20.5.4.3 Time-Slot-Based Access
In time-slot-based methods, the available transmission time on the medium is divided into distinct
slots, which are assigned to the individual nodes. During its time slot, a node may then access the
medium. The difference to cyclic polling strategies—which in fact also come down to partitioning
the polling cycle into time windows—is that the nodes are not requested by a central station to send
their data; they can do it by themselves. Therefore, time-slot-based methods are mostly also called
TDMA (in a narrower sense). he slots need not have the same size, they may be different. Likewise,
they may be dynamically distributed to the nodes according to the amount of data they wish to send.
This way, aperiodic traffic can be incorporated, whereas TDMA in general favors periodic traffic.
A strictly equal distribution of the slots is also called synchronous TDMA and is particularly known
from telecommunications technology, where each end device is assigned a time window in which,
e.g., the correspondingly digitized voice data can be transmitted. There, such systems are referred
toas“synchronousbussystems.”hecasewheretimeslotsaredynamicallydistributediscalled
asynchronous TDMA. Contrary to the synchronous version where the position of the time slot within
the cycle implicitly identifies the sending nodes, some address information must be sent in the case
of asynchronous TDMA.
The fact that nodes may access the medium without prior explicit request does not mean that
TDMA methods are generally multi-master or peer-to-peer techniques. his depends on the way the
synchronization of the time slots is achieved. Evidently, if no handshake procedures are implemented
for data transfer, all nodes must be properly synchronized so that only one node at a time accesses
thebus.hiscanbedoneinacentralizedordecentralizedway.
The simpler version is to have one dedicated bus master sending some sort of synchronization
message at the start of the cycle. Ater that, the nodes exchange their data in their previously assigned
time slots. Such a strategy is used, e.g., by SERCOS, which is therefore regarded as a master-slave
fieldbus (Figure .). In this particular case, the time slots for the slaves are all equal in length, and
the master has a larger time window to send its data to the slaves afterwards. A similar method is
also used by TTP/A. Here, a bus master (apart from having the possibility for simple polling) sends
a synchronization message in the first time slot of a round to start the cyclic data transfer from all
other nodes. In byteflight, a node (the sync master) sends a periodic synchronization signal to start
the data exchange round (in byteflight called slot), and the others initialize slot counters to determine
the position of their predefined microslot. If a node has no data to send, the respective microslot is
shortened. In this respect, the bus access mechanism resembles an implicit token passing method.
The spare time of unused microslots becomes available for aperiodic or lower-priority traffic wards
the end of the round.
Related to the centralized TDMA approaches is the summation frame protocol of INTERBUS
(Figure .). As noted in the previous section, the system is based on a ring structure and acts
like a large shift register. Each slave has a predefined slot in the data frame that is selected in
