Q: Polled Networks

A Polled Network consists of a central master station (Base) that is responsible for all radio communications. In Fig 1 the Base is broadcasting a message to all the outstations on the system, (Outstation 1 through 6) addressing each outstation in turn whether there is a change in status on their inputs.  Each slave will only respond if it receives a message specifically addressed to itself and if there has been a change in status on the inputs, and will not respond to a message not address to it or if there is no change of status.  The master is continuously broadcasting to all of the nodes, but addressing each node in turn and only receiving an update from the addressed node.


Typical Polled Network





In Fig 2, Outstation 2 has been addressed and it has had a change of status on the inputs. Outstation 2 will now send this data back to the Base.  Whilst this is happening, the Base will be switch to receive mode.  When the data has been received by the Base, it will switch back into transmit mode and send out a broadcast to the outstations again. This is obviously desirable for systems where only a change in status is critical or needed instantaneously. The diagram below shows more obviously how this type of network operates.


Fig 2


Event driven Networks


The Event Driven Networks allows much more sophisticated communication systems to be configured. In a basic events driven system, it is normally the slave that initiates communications. Instead of continuous transmission, each outstation will transmit at specific timed intervals, or if there is a change of status on the inputs.  This type of network is also used for power saving systems, relayed nodes and one way systems. The main advantage of this network is that all radio communications can be controlled from the software configuration, which makes it very flexible for customised applications.


By means of a serial communications link it is possible to link both types of network together offering the advantages of speed and configurability together.


Example of an Event Driven network


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