Impacted by shrinking municipal budgets, many water/wastewater plants and facilities operate frugally, often running on systems plagued with obsolete hardware and software and patched with years of Band-Aid fixes to remain functional. These patches are not always recorded properly when they are made, and over time, the initially accurate documentation about a facility’s control system can become scarce or unreliable, making it difficult to troubleshoot or upgrade the system when necessary.

A brief history of PLC programming

Looking at the history of programmable logic controllers (PLCs) often used in these facilities, the intent for these devices was to create an industrialized controller to replace electromechanical relay logic (see Figure 1). The world of the electrician/technician was transitioning into the world of the computer programmer. Even as this occurred, electricians maintained a strong influence on PLCs, which were destined to stay in their domain.

Electricians wanted a visual representation of the hardwired logic, and many decisions associated with PLC coding methodologies were impacted by this desire. However, PLC coding means and methods were not standardized among manufacturers, making PLC automation upgrades difficult.

Existing functional code may not be well defined and is never directly portable into new or different systems, which can drive up the risk and cost of migrations. This article examines some ways to migrate legacy PLC programs into more modern platforms.

Challenges of aging systems

PLC coding practices have evolved throughout the years with the development of the IEC 61131 standards for PLC languages. Address-based programming evolved into tag-based programming. Tag-based programming evolved into smarter structures such as user-defined data and program blocks. Newer coding methodologies eased scalability and consistency in coding efforts. Smarter coding practices accounted for handling each instrument or process element uniformly from the standpoint of controls, interlocks and alarms.

Maintaining, let alone specifying, PLC programming standards poses challenges. For many end users, PLCs are mysterious boxes that run so well they may not even have the latest backup programs stored in their possession, or the ability to upload, download or troubleshoot the code.

This means when the need does arise to understand how the automation system functions, technicians must often dive through undocumented program code and out-of-date drawings. In many cases, they must rely on dogeared printouts of the original code, along with dated drawings, to reverse engineer the actual control strategies.

As the system ages, the facility’s system patches may not be enough to maintain a functional process or the fragile status quo any longer. Automation technology continues improving and technical support and aftermarket parts for obsolete hardware and software can come to an end. Facilities need to bring their systems up to speed with the latest technology to avoid problems.

Click here to read full article.