Image Processing Reference
In-Depth Information
In strict polling, there is no room for aperiodic traffic, which is a severe drawback for fieldbus
systems designed to be more than simple sensor-actuator busses. Therefore, most fieldbus systems
foresee some possibility for the master to handle sporadic data exchange for configuration data or
process variables that require transfer only on an event basis. he important constraint is that this type
oftraicmustnotinterferewiththeperiodicone.Inpractice,thisisdonebyreservingsomeband-
width in addition to the scheduled periodic messages for a priori unknown aperiodic data transfer.
PROFIBUS-DP/PA and many Ethernet-based automation networks (such as PROFINET) use a ded-
icated portion of the bus cycle after the periodic traffic for aperiodic traffic. To keep the cycle period
constant, there is usually some spare time after this window to account for possible retransmissions
of messages. In WorldFIP, aperiodic traffic can be scheduled in the empty slots of the elementary
cycles, so that the macrocycle needs not be extended.
Another disadvantage of polling is that slaves cannot become active by themselves. If all of a sudden
a slave—for instance in the event of an alarm condition—needs to transmit additional data, it first
has to signal this alarm condition to the master to initiate an aperiodic request. In most cases (like in
WorldFIP), the slaves have the possibility to set a flag in the ordinary cyclic data exchange messages to
indicate the need for additional service (in the old CAMAC system, this flag had the descriptive name
“look at me”). he master then typically starts a handshake procedure to retrieve these data, possibly
by first requesting information about their amount to allocate an appropriate time slot for the transfer
(also with respect to other pending requests). his basic mechanism can be further extended to allow
a communication among slaves. Again, two possibilities exist. he first is to handle it indirectly via
the master which gets the message from the sending slave and forwards it to the addressee (as done in
the Measurement bus). his permits the master to maintain full control over the bus. he alternative
is that the master delegates the bus access to the slave for a while until the slave has sent all its data and
returns the access right to the master (as in WorldFIP, Foundation Fieldbus, or PROFIBUS-DP-V).
In this case, the master has to implement some timeout mechanism to ensure that the procedure is
properly terminated.
20.5.4.2 Token Passing
One way to coordinate medium access between several peer stations in a network is token passing.
In this method, the right to control the network is represented by a special piece of information
called token, which is passed on from node to node. Only a node possessing this token may initiate
data transfer. A set of rules ensures that the token is passed in a fair manner and that errors such as
lost tokens or duplicate tokens are detected and resolved. Compared to time-slot mechanisms, token
passing has the advantage that if a node has no data to send, it will hand over the token to the next
station immediately, which saves time. Essentially, there are two ways of implementing the token:
either explicitly by means of a dedicated short message or implicitly by distributed, synchronized
access counters (AC) in all nodes. Token passing is often combined with an underlying master-slave
mechanism to control a subset of nodes.
The explicit form of token passing is employed by PROFIBUS, both FMS and DP/PA (Figure .).
Even though PROFIBUS-DP in its basic variant is a single-master system, several DP networks may
coexistonthesamebus.Acrucialpointhereisthecorrectsettingofatimingparametercalledtarget
tokenrotationtime
T
TR
. his parameter indirectly determines how long a master may occupy the bus.
When a master receives the token, it starts a timer to measure the real token rotation time. When it
receives the token the next time, it is allowed to exchange messages with other masters and slaves
only while the timer has not reached the
T
TR
.Assoonasthishappens,thetokenmustbehanded
over to the next master in the list. If a master receives the token after the timer is expired, it may send
just one high-priority message before having to pass on the token again. he time available for each
master therefore also depends on the amount of time all the other masters in the network hold the
token, and it might vary from round to round if aperiodic data have to be sent or retransmissions
