Loose & Tight Systems

When I gave Drew, our fun and entertainment director, the title of this piece, he had lots of off color remarks about its content. Loose and tight lend themselves to a lot of “interpretation”. It might be fun to go there but we’d have to be drinking brews to have that conversation.

Instead I’d like to talk about difference between “loosely-coupled” systems and “tightly-coupled” systems. I’m sure that these are not new concepts but I think they maybe haven’t been explained in light of the current trend toward the integration of factory floor systems and enterprise systems.

I would argue that factory floor systems should be labeled tightly-coupled. Systems that use Profibus, Profinet IO, DeviceNet, EtherNet/IP or any Modbus version have a very strict architecture. These are really I/O systems much as the ODVA and PI (Profinet International) would have you believe otherwise. [Those folks like to argue that you can extend these protocols all the way up the factory architecture to the enterprise but that’s just not workable.]

Tightly coupled systems have these kinds of characteristics:

1.       A strictly defined communication model where device communication is inflexible and tightly regulated

2.       A strictly defined data model with a limited set of proprietary objects

3.       A requirement for human intervention to change the communication or data model

Tightly coupled systems provide much needed, well-defined functionality in a highly specific domain. Expanding operation to other domains or trying to provide more general operation is difficult. Making more generic data and functionality available requires significant programming resources that result in a very fragile interface. And that’s why tightly coupled systems are wrong for Enterprise communications.

Loosely coupled systems, on the other hand, provide exactly the right kind of interface for enterprise communications. Loosely coupled systems decouple the platform from the data, the data from the data model and provide a much more dynamic mechanism for moving data.

Loosely coupled systems have these kinds of characteristics:

1.       A widely used, standards based transport layer; TCP and HTTP.

2.       An open, platform independent, easy to implement encoding; XML

3.       An extensible operating interface; SOAP

What I’ve described here is web services. Web Services is the backbone of everything we do on the internet. It is extensible, flexible, platform independent – all required for the ever expanding internet.

The challenge is to how to best connect the tightly coupled factory floor architectures with the loosely coupled web services architecture of the Internet. Rockwell has its Factory Talk product line. The ODVA promotes EtherNet/IP. PI promotes Profinet IO. I don’t believe any of these are good solutions.

The solution? I’ve thought a lot about it and I think it’s OPC UA for reasons that I’ve explained in earlier articles.