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

INTERBUS 33-9
4.5
4
3.5
PL = 1 byte
PL = 4 byte
3
T IB (ms)
2.5
2
1.5
1
0.5
0
0 25 50 75 100 125 150
Number of nodes k
FIGURE 33.8
INTERBUS cycle times as a function of the number of nodes and the used payload size.
The frame overhead of 6 bytes is caused by a 2 byte long loopback word and a 4 byte long FCS. Each octet
at layer 2 is transmitted within a 13 bit data telegram at layer 1. Depending on the total amount of input
and output data, the total payload size can be calculated by
i= k
i=
k
⎛
Input
n = max ⎜∑PLi
,
∑PLi
⎝
i
where
n is the total payload size: sum of all user data [bytes] of all devices k with n ≤ 512 bytes and k ≤ 512
T Bit is the bit time: 2.μs at 0.5.Mbps or 0.5.μs at 2.Mbps
T SW is the software runtime of the master (depending on implementation)
PL i is the layer 2 payload size of the ith device [bytes] where 1 ≤ i ≤ k and k ≤ 512
In Figure 33.8, the INTERBUS cycle time at the MAC level with a bit rate of 2 Mbps and a software runtime
of 0.7.ms is illustrated as a function of the number of nodes k and their corresponding payload size PL.
33.5 Summary
INTERBUS is a serial bus system developed for the time-critical data transfer at the field level of industrial
automation systems. The physical topology of INTERBUS is based on a ring structure, that is, all devices are
actively integrated in a closed transmission path. The data forward and return lines in the INTERBUS system
are led to all devices via a single cable. This means that the general physical appearance of the system is an
“open” tree structure. Due to a total frame approach, the cyclic data transfer of INTERBUS is very efficient
in comparison to fieldbus systems based on an individual frame approach. As a result, small cycle times can
be achieved with small bit rates. Acyclic messages are transmitted using preconfigured time slots within the
total frame in conjunction with a segmentation and recombination mechanism at layer 2.
References
1. IEC, Digital data communication for measurement and control—Fieldbus for use in industrial
control systems, 2001, IEC 61158 Ed.3, Type 8 (INTERBUS), IEC, Geneva.
2. INTERBUS Club, www.interbusclub.com, visited May 2009.
3. Baginski, A. and Müller, M., INTERBUS Basics and Practice, Hüthig-Verlag, Heidelberg, Germany,
2002 (German).
i
Output
⎞
⎟
⎠
© 2011 by Taylor and Francis Group, LLC