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

INTERBUS 33-5
interface shutdown. The identification cycle is also used to find the cause of a transmission error and to
check the integrity of the SR ring.
The individual registers are switched in the different phases of the INTERBUS protocol via the selector
in the ring.
On layer 1, INTERBUS uses two telegram formats with start and stop bits:
• The 5 bit status telegram
• The 13 bit data telegram
The status telegram is used to synchronize the activities on the medium during pauses in data transmission.
The slave nodes use the status telegram to reset their internal watchdogs, which are used to
control a failsafe state. The data telegram is used to carry a byte of the layer 2 payload. The remaining
bits of both telegrams are used to distinguish between data and identification cycles, as well as
the data transfer and FCS phases within a cycle. This information is used by the selector shown in
Figure 33.4 to switch the relevant register in the ring. INTERBUS uses a physical transmission speed
of 500.kbps or 2.Mbps.
The cycle time, that is, the time required for I/O data to be exchanged once with all the connected
nodes, depends on the amount of user data in an INTERBUS system, that is, on the number of nodes
and on the dimension of the SR inside each node. Depending on the configuration, INTERBUS can
achieve cycle times of just a few milliseconds. The cycle time increases linearly with the number of I/O
points, as it depends on the amount of information to be transmitted. For more detailed information,
refer to the chapter performance evaluation.
The architecture of the INTERBUS protocol is based on the open system interconnection (OSI) reference
model according to International Standards Organization (ISO) 7498. As is typical for fieldbus
systems, for reasons of efficiency, ISO layers 3–6 are not explicitly used (see Figure 33.5) but are combined
in the lower layer interface (LLI) in layer 7. The protocol architecture of INTERBUS provides both
periodic and asynchronous communications.
Application process
PMS services
PNM7 services
PMS
Application layer (layer 7)
LLI
PNM7
Empty (layer 3–6)
Data link layer (layer 2)
PDL
Physical layer (layer 1)
PHY
Management
PNM1/2
FIGURE 33.5
Protocol architecture of an INTERBUS node.
© 2011 by Taylor and Francis Group, LLC