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Daisy-chained - all the devices are chained to each other, one-by-one. It is similar to the
bus with spurs, but does not use junction boxes. It is a useful method of connecting de-
vices as new devices can be added by simply daisy-chaining from a close device. The
disadvantage is devices must be disconnected in order to connect a nearby device, unless
a special connector can be used that allows a connected device to be connected.
Tree - type of topology uses a single junction box, with the devices connecting directly
to the junction box. Typically, it is used when devices are added and deleted from the
network on a regular basis.
Daisy
chain
PLC
Sensor/
Actuator
Sensor/
Actuator
Sensor/
Actuator
Sensor/
Actuator
Junction boxes
Bus, with
spurs
PLC
Sensor/
Actuator
Sensor/
Actuator
Sensor/
Actuator
Sensor/
Actuator
Junction
boxes
Tree
PLC
Sensor/
Actuator
Sensor/
Actuator
Sensor/
Actuator
Sensor/
Actuator
Figure 19.3
Fieldbus connection topologies
19.3.2 FOUNDATION Fieldbus layers
Process control communications can be group into three different levels:
Hardware-address buses - this type of bus uses hardware addresses and registers to store
values. Examples are I/O buses, PLCs, SCADA protocols with RTUs (remote termina-
tion units).
Symbolically addressed buses - this type of bus uses addresses that actually have a sym-
bolic name. This works at a higher-level than the hardware-address bus.
Comprehensive user-layer functionality buses - this type of bus operates at a higher level
than hardware and symbolic addressing. It is used in the FOUNDATION Fieldbus and
supports function blocks, standardised parameters, operational modes, cascade initialisa-
tion sequences, antiwindup mechanisms, quality-of-data propagation and response, fail-
state initiation, alarm reporting and control mechanisms, process control data structures,
and so on.
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