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

WorldFip 34-17
1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9
AP
F
E
D
C
B
A
Length of the ASI
1 2 3
Microcycle
4 5 6 7 8 9 10 11 12 13 Time (ms)
Macrocycle
Periodic elementary transactor length periodic traffic
ID_RP, RP_RQ transactor length, aperiodic traffic
ID DAT, RP DAT transactor length, aperiodic traffic
FIGURE 34.10
Example of aperiodic traffic schedule.
The number of microcycles (N′) in an ABI is then
N ′
∑
N′ = min{ Ψ}, with Ψ = nap( l)
∧ Ψ ≥ 2 × na
l = 1
(34.6)
that is, the minimum number of microcycles within which the number of available “slots” (each “slot”
with the length of Ca*) is at least 2 × na.
Knowing N′, the length of the aperiodic busy interval (len _ abi) may be evaluated as follows:
np
N ′−1
⎛
⎞
len_ abi = ( N′ − 1) × µ Cy + ∑( bat[ i, N] × Cpi
) + ⎜2
× na −∑nap( l)
⎟ × Ca *
⎝
⎠
i=
1
l=
1
(34.7)
where
Σ i=1,…,np (bat[i, N] × Cp i ) gives the length of the periodic window in microcycle N′
(2 × na − Σ l=1,…,N′−1 nap(l).) × Ca* gives the length of the aperiodic window, with respect to the
aperiodic busy interval, also in microcycle N′
Therefore, the worst-case response time for an aperiodic transfer requested at station k is
Ra
k
k
= σ + len_ abi
(34.8)
and the maximum admissible interval between consecutive aperiodic requests of an aperiodic variable
in a station k is
MIT( Va k ) ≥ Ra k = σ k + len_
abi
(34.9)
34.5.5.9 Example of Aperiodic Traffic Scheduling
Assume that a system configured for six periodic variables (Table 34.1) must also support nine aperiodic
buffer exchanges. Assume also that the length of each Vp i , ∀ i , is Cp i = Cp = 0.0976.ms, and that for sporadic
traffic Ca* = 0.1.ms. If the BAT is implemented as shown in Table 34.2, transactions concerning the
nine aperiodic variables are scheduled as shown in Figure 34.10.
© 2011 by Taylor and Francis Group, LLC