Sure, 20-year-old systems don't have all the bells and whistles that modern ones have, and yes, it can be a pain to find spare parts, but many of these legacy systems are like a 1963 fuel-injected, split-window Corvette coupe: That is, rare and expensive, but in its day technically advanced and still capable of performance equal to that of its modern counterparts. So like the Corvette, your legacy system was worth its price then and worth the time, money and effort now to keep it running.
Legacy of Value
For many processors the value of their legacy control system is measured by the huge capital investment made to purchase the original system. Its legacy is also represented by the commitment to all the sensors, transmitters, wiring, cable, cabinets, power supplies, digital and analog I/O, plant networks, redundant data highways, remote terminal units (RTUs), A/D converters, multiplexers, analyzers, distributed control modules and all the other specific elements and technologies that went into its creation. Old yes, but built to last (Table 1). So, while parts may be hard to find for a decades-old DCS, banking on its current value and investing in its future may be well worth it.
Besides, all your legacy system may really need to run like new is a few relatively inexpensive and simple software and hardware upgrades. But because so many factors can influence the success and cost-effectiveness of a legacy control system upgrade, before you step up, system vendors and plant operators recommend you step carefully along the path you take.
Step One: Upgrade Software
Upgrading system software can be one of the easiest and least costly ways to improve the operational aspects of a legacy control system, says Steve Garbrecht, industrial application server product marketing manager for Wonderware. Garbrecht lists an upgrade path in Table 2, where possible upgrades are ranked from the easiest (1) to the most costly (2).
Table 1: The Upgrade Path
He says bulldozing - completely replacing an old system with a new one - is rarely necessary. "Ninety-nine percent of the time, it is not appropriate to bulldoze your existing control system," says Garbrecht. Any vendor that does not provide an incremental capability is not serving their customers' best interests.
Because it is possible to update almost all legacy DCSs, let's start with HMI software, as the first step down the upgrade path.
Many HMI/SCADA software vendors offer plug-in compatibility with dozens of old DCSs. The software can interface with process historians, databases, PLCs, data highways, and even the system's RS232 ports to gain access to real-time or historical data. This almost immediately provides modern PC-based HMI trend displays, alarm screens, and fresh process graphics to replace the old stick-figure black and white displays you've been living with since the 1970s.
Moreover, a modern HMI/SCADA package also connects that creaky old legacy system to today's modern enterprise software, such as ERP, MES, and supply chain management applications. Built-in web servers also allow you to observe the process from a web browser on a PC.
"With our BizViz software, users do not have to rip out and replace older systems," says Tim Donaldson, marketing manager at Iconics. "BizViz is designed to mine data from legacy systems."
If all you think your system needs are better HMI displays and a connection to enterprise software, then modern HMI/SCADA software and a PC might be the best fit and the most cost-effective solution. For example, CRISP Automation, Raleigh, NC, offers an HMI upgrade package for its legacy systems based on commercially available HMI/SCADA software.
"We created a new HMI based on InduSoft's WebStudio software," says Ken Wild, president of CRISP Automation. Although CRISP and its successors Anatec and Square D are all essentially out of the DCS business, together those companies built and installed about 100 CRISP systems during their heyday, Now Wild works to keep them all running. "Our upgrade allows customers to import their existing operator screens into WebStudio," he explains. "It adds state-of-the-art functionality, such as allowing users to access their systems over the Internet or run their HMI on a handheld PDA."
Wild says the old CRISP systems have legendary reliability and are virtually bullet-proof. Originally based on DEC PDP-11 and VAX computers and I/O, these 1970s and 1980s-vintage systems just keep on ticking.
Like many companies with legacy systems, some CRISP users don't want to change. "No change in the application is very important to our pharmaceutical, food and chemical customers," says Wild. "Recertification is difficult, expensive and draining on internal engineering resources."
Jim Meils, critical systems supervisor at Aventis Behring, in Kankakee, Ill., agrees. "While an upgrade to a newer version of the same equipment may be technically easier, this will not improve the rest of the upgrade path, because our systems must be specified, commissioned, validated and submitted to regulatory agencies," he laments. "Most of our machines, including the control systems, remain the same until the entire system is decommissioned."