wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

WorldFip 34-9
t r (20 µs)
ID_DAT(25.6 µs)
RP_DAT(32 µs)
F
E
D
C
B
A
1 2 3 4 5 6 7 8 9 10 11 12 13 T (ms)
Microcycle
Macrocycle
FIGURE 34.7 Schedule for the periodic transfers of Table 34.2.
3. The length of the macrocycle can induce a memory size problem, since the table parameters
must be stored in the BA. For instance, if the scanning periodicities of variables E and F were,
respectively, 5 and 7.ms, the length of the macrocycle would be 420 microcycles instead of
just 12 microcycles.
Both the communication jitter and memory size problems have been addressed in the literature. In [1],
the authors discuss different methodologies to reduce the BAT size, without penalising the communication
jitter. The idea is very simple, and it basically consists on reducing some of the scan periodicities in
order to have a harmonic pattern. The problem of table size has also been addressed in other works [2,3],
however, in a different perspective. In the referred work, the authors discuss an online scheduler (instead
of storing the schedule in the BA’s memory), which is not directly applicable to the WorldFIP case.
It is also worth mentioning that Figure 34.7 represents a macrocycle composed of synchronous
microcycles, that is, for the specific example, each microcycle starts exactly 1.ms after the previous one.
Within a microcycle, the spare time can be used by the BA to process aperiodic requests for buffer
transfers, message transfers, and padding identifiers. A WorldFIP BA can also manage asynchronous
microcycles, not transmit padding identifiers. In such case, a new microcycle starts as soon as the periodic
traffic is performed and there are no pending aperiodic buffer transfers or message transfers. Initial
periodicities are not respected since identifiers may be more frequently scanned.
34.5.4 WorldFIP Aperiodic Buffer Transfers
The BA handles aperiodic buffer transfers only after processing the periodic traffic in a microcycle. The
portion of the microcycle reserved for the periodic buffer exchanges is denoted as the periodic window of
the microcycle. The time left after the periodic window until the end of the microcycle is denoted as the
aperiodic window of the microcycle. The aperiodic buffer transfers take place in three stages:
1. When processing the BAT schedule, the BA broadcasts an ID_DAT frame concerning a periodic
variable, say identifier X. The producer of variable X responds with a RP_DAT and sets an apeÂ
riodic request bit in the control field of its response frame. The BA stores variable X in a queue of
requests for variable transfers. Two priority levels can be set when the request for aperiodic transfer
is made: urgent or normal. The BA has two queues, one for each priority level.
2. In the aperiodic window, the BA uses an identification request frame (ID _RQ) to ask the proÂ
ducer of the identifier X to transmit its list of pending aperiodic requests. The producer of X
responds with a RP _RQ frame (list of identifiers). This list of identifiers is placed in another BA’s
queue, the ongoing aperiodic queue.
© 2011 by Taylor and Francis Group, LLC