A Brief History of Instrument Specifications
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Before there were ‘Life Cycles' - we Wrote Specifications for our 'objects' and drew schematic diagrams of how to connect them.

Our Objects were Physical - they included

Field Instruments - such as transmitters, valves and so on.

Panel Instruments - such as controllers and indicators etc with buttons and dials, and Alarm Annunciators

Back of Panel Instruments - such as relays

Schematic were produced showing the connections including

Logic Diagrams

Loop diagrams

Knowing that the Vendors all sold with similar functions, such as 6 inch by 3 inch PID Controllers, Indicators , Recorders etc we wrote Product Independent Specifications.

We specified these instruments, got Quotes and Then chose the Vendors

Then, we updated the Loop diagrams with the vendor specifics and we put the model numbers on the Specs

A couple of old panels

Object Oriented Control System. Lots of objects - aka Panel Instruments 

It is also worth noting that early Distributed Control Systems (eg Honeywell TDC2000) were designed to emulate the control panel paradigm. 

Similarly PLC's were designed to emulate relay controls such as might be found in the back of the panels. 

These factors are one of the reasons that the use of computers in process automation 'took off', because it was a vast improvement on previous computer controls.

It is also a reason why the IT approach (uml etc) to automation requirements analysis is inappropriate.

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