August 2007

Has the system been 'burned in?' 

By Michael Whitt

Specification of control system components can be as much of an art as it is a science.  In many cases, "legacy" equipment limits the choices by virtue of its existing presence. For example, if a plant has a lot of old Allen-Bradley equipment, the default should be to stay with Allen-Bradley to minimize the retraining cost. This default position may not persist, however, if the site's historical experience with the legacy system is negative.

So, let's make an assumption. Assume a worst-case scenario in which a legacy system exists, but with a checkered past. 

The operations team likes the old system because they are used to it. The maintenance department hates the old system because they cannot get parts, it is hard to work on, and they get very poor support from the manufacturer. 

This places the specification writer squarely in the middle. How should the writer proceed? The PLC and the HMI should receive equal consideration. The following list, in order of precedence, should apply to both elements individually:

  • Legacy benefit: Will the retention of the legacy manufacturer be a benefit or a liability? What are the issues surrounding the existing system? Can one place a dollar value on them or are they preferential in nature?
  • Technical support: Is local support available? If so, how effective is it likely to be? If not, will the local sales staff be able to assist in getting the attention of the factory if necessary?
  • Local sales support: Is sales their motivation or are they motivated to help you succeed? How long have they been "pushing" this product? What is their customer base in the area? How does a sampling of this customer base rate both the sales and technical support services of this system?
  • Investment: How much will the system cost initially, and what is the projected long-term cost? Is a maintenance agreement a possibility? If so, how much benefit is it likely to provide?
  • Hardware: Is the hardware accessible to the maintenance person? Is it hardy? Are replacement parts available locally, or will they need to be stored on-site? Do the power requirements fit the existing power distribution scheme, or will there be a need for new power?
  • Software: Has the software been "burned in," or is it a new version? Is it user-friendly and well documented? Is it scalable toward your particular future needs?
  • Training investment: How much retraining is necessary? What is the cost of the training? Is it local? How often are there classes? How effective is the student likely to be after the course? In this case, again, sampling the local user population can be helpful.

After one has pared the list of likely systems down, one should view the PLC/HMI pair from a system-wide standpoint. Following are some issues to consider from that viewpoint:

  • Can the PLC and the HMI share a database? If not, is there a third-party software solution that would manage the two databases?
  • Does the HMI have a standard interface driver that works well with the PLC, or will one need to be developed? (This was a bigger issue in previous years.)

Don't forget to consider the aftermarket for your legacy equipment. Frequently, your sales representative can point you to warehouses that store legacy equipment for resale. Therefore, it is possible to recoup some of the original investment provided your old system is serviceable.


Michael Whitt ( is an ISA Senior Member and the I&C (instrumentation and controls) manager at Mesa Associates. His book is Successful instrumentation and control systems design, ISA Press, 2004.