ISA TR50.02, PARTS 3&4 PDF

Introduction

Incomprehensibility

A long story of incomprehensibility has followed (and often also preceded) the ISA Fieldbus Data Link Layer (DLL) documents. That is due to several reasons: some real ones, others only imagined but repetitively suggested and spread around by some of the people who did not agree with the proposal.

Standard style

First of all, the fear of compelling something relevant only to the implementation domain always pushes standard documents not to describe "why" or "how" but just "what" to do.

That, in this particular case of fieldbus, made things even worse than usual because of the existing experiences and approaches of many experts being radically different; it led to real problems (and it eased many imagined ones) in mutual understanding.

Features variety

Due to the differences between the existing fieldbus approaches, the technical and diplomatic need of merging them has always been the key issue of all the ISA DLL efforts and has so increased the variety of features and the complexity of the structure within such a document that tries to harmonize them all.

Many updates

Then, the long path followed in order to chisel a proposal that could be really agreed by all the different existing approaches has caused the need to frequently update, partially re-write, or delete, or add many parts of the ISA Fieldbus DLL drafts along several years.

And this, of course, did not give the documents the same clear structure as an immediate paper would have had.

Effort to understand

Finally, we have to remind how only "if" and "when" we really want to implement something, then we make the effort to understand it. When we already have an existing, even if not universal, alternative why should we make such an effort?