Image Processing Reference
In-Depth Information
23.2.2.1 Pre- and Postprocessing Times
The preprocessing time at the source node depends on the device software and hardware character-
istics. In many cases, it is assumed that the preprocessing time is constant or negligible. However,
this assumption is not true in general; in fact, there may be noticeable differences in processing time
characteristics between similar devices, and these delays may be signiicant. he postprocessing time
at the destination node is the time taken to decode the network data into the physical data format
and output it to the external environment.
In practical applications, it is very difficult to identify each individual timing component. How-
ever, a very straightforward experiment can be run with two nodes on the network. he source node
A repeatedly requests data from a destination node B, and waits until it receives a response before
sending another request. Because there are only two nodes on the network, there is never any con-
tention, and thus the waiting time is zero. he request-response frequency is set low enough that no
messages are queued up at the sender's bufer. he message traffic on the network is monitored, and
each message is time-stamped. he processing time of each request-response pair, i.e.,
T
post
T
pre
,
canbecomputedbysubtractingthetransmissiontimefromthetimediferencebetweentherequest
and response messages. Because the time-stamps are recorded all at the same location, the problem
of time synchronization across the network is avoided.
Figure . shows the experimentally determined device delays for DeviceNet devices utilizing the
aforementioned setup [,]. Note that for all devices, the mean delay is significantly longer than the
minimum frame time in DeviceNet
+
, and that the jitter is often signiicant. he uniform distri-
bution of processing time at some of the devices is due to the fact that they have an internal sampling
time, which is mismatched with the request frequency. Hence, the processing time recorded here is
the sum of the actual processing time and the waiting time inside the device. The tested devices
include photoeyes, input-output terminal blocks, mass flow controllers, and other commercially
available DeviceNet devices.
A key point that can be taken from the data presented in Figure . is that the device process-
ing time can be substantial in the overall calculation of
T
delay
. In fact, this delay often dominates over
network delays. hus, when designing industrial network systems to be used for control, device delay
and delay variability should be considered as important factors when choosing the components. In
the same manner, controller devices such as off-the-shelf PLCs typically specify scan times and inter-
scan delays on the order of a few milliseconds, thus these controller delays can also dominate over
network delays.
(
µs
)
23.2.2.2 Waiting Time at Source Nodes
A message may spend time waiting in the queue at the sender's buffer and could be blocked from
transmitting by other messages on the network. Depending on the amount of data, the source node
must send and the traffic on the network, the waiting time may be significant. The main factors
affecting waiting time are network protocol, message connection type, and network traffic load.
For control network operation, the message connection type must be specified. In a master-
slave (MS) network,
∗
there are three types of message connections: strobe, poll, and change of state
(COS)/cyclic. In a “strobe” connection, the master device broadcasts a message to a group of devices
and these devices respond with their current condition. In this case, all devices are considered to
∗
In this context, an MS network refers to operation from an end-to-end application layer perspective. Master node appli-
cations govern the method by which information is communicated to and from their slave node applications. Note that,
as it will be described further in Section .., application layer MS behavior does not necessarily require corresponding
MS behavior at the MAC layer.
