Control Engineering

David Chappell

A Device is a Device is a Device; But what if it is not?
February 28, 2008

As the Make2Pack ISA88 Part 5 continues its efforts to bring understanding to the terms and models used in the industry we continue to stumble over the term “Device.” Following is the definition for a device found in the ANSI/ISA–51.1–1979 (R1993) Formerly ANSI/ISA–S51.1–1979 (R1993) standard:

device: An apparatus for performing a prescribed function.

In the ISA88 Part 1 standard the term device is referenced eight times. A common theme is that a device is independent of the control that causes it to function as indicated in the following passage:

“In Process Control, control functions are discussed that deal directly with equipment actions such as the need to implement control functions using regulating devices and/or state-oriented devices.”

When the device(s) is/are combined with control it becomes a Control Module which is represented as the lowest level of the entity and equipment models.

In many other implementations outside of where the ISA88 standard is applied the term Device is used such that the Device also contains the control that in the ISA88 the Control Module contains. Of course this leads to a great deal of confusion and conflict when the ISA88 standard comes in conflict with these other uses of the term Device and Control Module.

The ISA Part 5 will not make this discrepancy disappear, but at least we expect to be able to clearly identify where and how the terms conflict and how to successfully deal with all the different uses!

A PDF draft 1 of the March 2006 Make2Pack WBF Report includes a graphics, terms and early background of about ISA88 Part 5.

Want to provide input or ask questions? Please use the post comment feature. Here's a little information about the Make2Pack ISA88 Part 5 meeting next month in Dayton.

Posted by David Chappell on February 28, 2008 | Comments (1)