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

34-10 Industrial Communication Systems
Start of aperiodic
window
Requests in the
urgent queue?
No
Aperiodic ongoing
queue empty?
Yes
Yes
No
Time left to process
buffer exchange?
Yes
ID_DAT(Aperiodic)
RP_DAT(Aperiodic)
No
Time left to process
buffer exchange?
Yes
ID_RQ
RP_RQ
No
No
Time left to process
padding frame?
PAD
Yes
End of aperiodic
window
FIGURE 34.8
Sequence of transactions during the aperiodic window.
3. Finally, the BA processes requests for aperiodic transfers that are stored in its ongoing aperiodic
queue. For each transfer, the BA uses the same mechanism as the used for the periodic buffer
transfers (ID _DAT followed by RP _DAT).
It is important to note that a station can only request aperiodic transfers using responses to periodic
variables that it produces and that are configured in the BAT. For the sake of simplicity, in
the following sections we consider that all aperiodic requests concern the use of the AL service
L_FREE_UPDATE.req(ID_*,Urgent), thus, only the urgent queues (both at the requesting station and
at the BA) are considered. Finally, it is important to stress that the urgent queue in the BA is only processed
if, and only if, the BA’s ongoing aperiodic queue is empty, as detailed in Figure 34.8. Also, it is important to
note that at the end of a microcycle, a transaction is processed if, and only if, there is still time to complete it.
34.5.5 Setting the WorldFIP BAT: Rate Monotonic Approach
34.5.5.1 Model for the Periodic Buffer Transfers
Assume a system with np periodic variables (Vp i , i = 1, …, np). Each periodic variable is characterized
as
Vp
= ( Tp , Cp )
(34.2)
i i i
where
Tp i corresponds to the periodicity of Vp i (assume a multiple of 1.ms)
Cp i is the length of the transaction corresponding the buffer transfer of Vp i (as given by
Equation 34.1)
34.5.5.2 Building the WorldFIP BAT (RM Approach)
For the periodic traffic, end-to-end communication deadlines can be guaranteed by an a priori test since
the BAT implements a static schedule of the periodic variables. In this section, we provide tools and
analysis for setting the WorldFIP BAT according to a rate monotonic (RM) approach [4].
© 2011 by Taylor and Francis Group, LLC