Answer / mahesh

Nodes are I/O communication protocols.
Eg: DCS is the master controller, then Nodes slaves.
Nodes are normally receives data from field instruments and processes in DCS.

Answer / ranjith

In Mahesh's answer, it says 'Nodes are I/O communication protocols' this part is wrong.

In a typical DCS architecture from top you have got,
0. MES/SAP/
1. Operator station
2. Runtime/Database/Historian/Batch/Gateway & Engineering Servers
3. Controller
4. IO nodes
5. Marshalling Panel
6. ...(I'll stop here)

The IOs are generally not clustered with the Controllers, IOs will be in a different panel communicating with the controller in various communication protocols(exceptions are there).
There will be a slave module communicating to the master(Controller) and also communicating to the IOs connected to its Backplane/Bus of the slave. This slave module and the IOs are collectively called as 'Node'.
There will be multiple nodes due to segregation of IOs(Plant/unit/Process part) or to distribute the slave module's load (different manufacturer has different IO density specified or may be limitation of protocol).

Now imagine wiring the whole bunch of field signal cables to your control room...!
An alternative is to take a communication cable from 'Main control Room/Center' to 'Remote/Sub station' may be kilometers apart and enjoy limited wiring.

Total number of nodes are limited by the communication protocols itself. (Payload, Noise, Cycle Time ...)

Further reading:
Industrial Communication Protocols /Fieldbus

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