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Jonni Lazzerini · Guest

Hi,

for my next project I have to build a framework to communicate with several
Programmable Logic Controllers and use it to build a Process Visualization
Software. Basically this framework should give me the possibility to
establish a connection to a PLC, subscribe the variables I need, specifying
update rate, scaling and so on, and have a set of objects that reflects the
data on the plc and inform me when something changes. This should happen in
the reverse direction too, allowing me to set properties on my "local" data
layer managing itself the update on the plc. I think on some stage I'll
need to implement the observer pattern to reflects some changes on the GUI,
but my first two problems are the PLC Communication Layer and my "shadow"
Data Layer

1) The PLC Communication Layer.

There are some different methods I can play with to speak with a PLC. Each
PLC brand has his proprietary way to do that. Mosts implements the OPC
standard, but sometimes the way they do that, penalizes heavy the
performance. Sometimes I should go "native" without such overhead. There are
common operations that best fit on a base abstract class or set of classes
but others that are simply dedicated to only one system. I would use some
pattern something like the one used on a TXMLDocument for his operations (I
can select the DOMVendor to one available on the system and go with it) or
the one used on the ADO components allowing me to change DB drivers without
influencing the upper layers. In my case I should be able to go OPC,
XNative, YNative and so on with the possibility to use some XNative Methods
that OPC does not have! What is / are the right patterns? Facade? Strategy?
Where can I find some examples on somewhat similar applications?

2) The "shadow" Data Layer.

That's probably the more delicated part! I know that data on PLCs may be of
a basic type such an integer, a float, a string and so on but complexes type
too such as as records or arrays (in Delphi jargon) or a file. Sometimes I
can only subscribe a single entity throught the communication layer such as
an 32bit integer sometimes I can subscribe a whole structure such as a 100
bytes big block (may be consisting in an array of 10 records each of them
having five 16 bit integer field). With such complexes structures i need a
shadow object that mimic the real structure on the PLC. Here I'm thinking on
a composite pattern in which each element may be subscribed on the
Communication Layer. Such composition may allow me to add new types and
implement them in the future.I don't know if this is the right way to
implement it. I would be pleased to receive some suggestions here.

Thank you very much.

Jonni Lazzerini.

Eric Hill · Guest