February 2008

Developing a control logic specification

By Michael Whitt

Editor's note: January's Channel Talk focused on Section 1 - the Process Overview. In this month's analysis, let us focus on Section 2 - Operability & visualization. 
Operators must not only be able to run the plant, but be able to do it with the confidence that they are getting the right information at the right time. 

They must be able to enable and disable automatic operation and enter set points, but they also need to be able to run reports, monitor alarms, and perform a myriad of other tasks particular to their situation. 

However, simply defining the tasks the HMI must support is only a part of developing an HMI specification. Beyond providing functionality, the HMI design should put operators in their comfort zone. This includes color considerations, screen hierarchy, alarm management issues, and other purely esoteric considerations that otherwise would not be necessary to simply get the job done. 

Therefore, the operator interface specification is a "corporate" task, requiring the active participation of all concerned.

Human-machine interface is a broad term that encompasses the following subsystems:

  • The Graphic User Interface (GUI) is a utility that allows the user to create graphic screens that let the operator view the process and quickly gain an understanding of its condition. Most of the time, these screens appear as "cartoons" depicting the process equipment using animated graphics. Some of these screens also give the operator the ability to modify the process by linking the graphics to action tags that force the PLC to react.
  • The action/animation database dovetails with the GUI package. It provides a means for linking the graphics to the control system and, ultimately, to the process equipment. This database lets the user give the active graphic elements names, or "tags," useful for troubleshooting, define their action/animation characteristics, and define their links to the outside world.
  • The device driver is a software utility that mates the HMI to a control device like a PLC. The driver manages (translates) HMI data requests and transfers them onto the communication network.
  • The alarm manager is a software utility that monitors alarm tags. These tags link up to process sensors and switches and to embedded PLC logic. They log alarm events, date and time stamp them, alert the operator by either audio or visual means, and allow the operator to acknowledge or clear alarms.
  • The data historian is a software utility that stores data collected by historian tags. These tags sample specific process sensors at timed intervals and store the information in data archive files on the workstation.
  • The report generator is a software utility that generates process reports.

A well-written control logic specification will include requirements for each of the subsystems listed above. Some of the tools that may be used are:

  • A Graphics Design plan to provide information on the color palette, shapes, and texture
  • An Animation Plan to provide guidance on situational animation (two-step operator action), active illumination (flash & blink), and active sounds (beep & click)
  • A Navigation Plan to provide guidance on how the operator will move from screen-to-screen
  • An Alarm Management Plan to describe alarm philosophy and expected operator interaction
  • A Security Plan to describe layers of accessibility
  • An Information Management Plan, describing the number of reports needed and the content
  • A Data Communications Plan, describing data flow, devices, and device drivers
  • An Environmental and Ergonomics Plan, describing considerations needed for operator comfort and ease of access

The HMI Control Specification is a key chapter in any Control System Specification, as the HMI becomes, in the end, the most visible part of the control system as a whole. 

Time spent on the front end developing the specification is time well spent.


Michael Whitt (mwhitt@mesainc.com) is an ISA Senior Member and the Manager of Integrated Systems at Mesa Associates, Inc. Section 1 of this series is at www.isa.org/link/channel_talk. His book is Successful Instrumentation and Control Systems Design, ISA Press, 2004.


ISA standards activity relating to HMI

  • ISA-RP60.3-1985 - Human Engineering for Control Centers
  • ISA-RP77.60.02-2000 - Fossil Fuel Power Plant Human-Machine Interface: Alarms
  • ISA-TR77.60.04-1996 (R2004) - Fossil Fuel Power Plant Human-Machine Interface-CRT Displays
  • ISA-RP77.60.05-2001 (R2007) - Fossil Fuel Power Plant Human-Machine Interface: Task Analysis
  • ISA101, Human-Machine Interface

For more information, go to www.isa.org/standards.