Suggestion: Look to your existing vendor for a migration plan for this special software. However, the vendor may not have sold many of these applications and, therefore, may have decided not to support a migration solution. Third-party companies often can offer a solution, but you should understand the amount of re-configuration work that will be required. Some seamless solutions are available for certain batch management applications.

4. Engineering library
If the time comes to expand the plant and introduce a new controller, you will have to configure from scratch. However, the availability of a familiar set of function blocks can ease the job and avoid special training. So, look for a vendor that supplies in the engineering tool for the new controller a library of IEC1131 function blocks that emulate the blocks or other languages of the existing controller library (Figure 4).

Figure 4. The availability of a familiar set of function blocks can make configuration of a new controller easier. (click to enlarge)

Hurdle: The new controller library must use the same algorithms that the old controller used. Some DCS vendors have this library. Most do not.

Suggestion: Ask your controller vendor if its new controller configuration tool has a library that emulates the old controller’s code.

5. Controller application conversion

When the time comes to replace a legacy controller, use a tool to convert the existing process configuration (Figure 5).

Figure 5. Tools for use with older controllers not based on IEC1131 can be hard to find and generally are limited. (click to enlarge)

Hurdle: Most new controllers are based on IEC1131. However, older controllers are not, heightening the complexity of creating an automatic software solution. As a result, some vendors have abandoned this solution due to cost.

Suggestion: Ask your controller vendor if it has a controller application conversion tool. Most vendors, at best, will only have a tool for their own legacy systems. Siemens is working on a generic tool that can be used on APACS as well as other vendors’ controller code.

6. Controller network gateway

If you have old and new controllers mixed together in your system, a communication gateway providing peer-to-peer communications between them allows for more-complex control algorithms (Figure 6). It also enables the old and new systems to be online simultaneously, providing for a phased, stepwise migration.

Figure 6. Peer-to-peer communication between old and new controllers allows both to be online simultaneously.

Hurdle: Communication protocols may differ between old and new controllers and the vendor of the older controller may not have the tools necessary to communicate on newer media.

Suggestion: The incumbent vendor may have the best tools for communicating to its own newer controllers. If your controller vendor can’t help, another control system company may be able to provide the communication translation know-how and necessary hardware and software products.

7. I/O gateway
One of the easiest ways to get on board with new equipment is by using new input/out (I/O) modules (Figure 7). The most open method is to use a fieldbus (Profibus, for example) card. This allows new I/O modules to accommodate any remote I/O or expansion need.

Figure 7. A fieldbus card can provide an easy and open way to add new I/O modules to a legacy system.

Hurdle: This solution requires your DCS vendor to offer a fieldbus board for the legacy controller.

Suggestion: Contact your controller vendor to see if it has a fieldbus interface. The incumbent or new DCS vendor can supply the I/O modules, provided they are fieldbus compatible.

8. I/O replacement

This approach retains the termination and existing I/O rack. The replacement I/O is part of the new system, allowing field signals to bypass the legacy controller and move directly to the new controller (Figure 8). The old controller board is removed. The vendor of the new system takes responsibility for the old rack where the new module is mounted.

Figure 8. An existing rack can be retained if special I/O modules are installed in the legacy controller slots.

Hurdle: This solution requires special hardware modules that fit into the legacy controller slots. The drawback to this approach is that it puts a specialized I/O module (which may not be supported as well as other mainline products) into an old backplane. Also, this solution forces you to migrate completely, eliminating the potential to keep older HMIs online at the same time as new HMIs. The new controller must be used to control the process, so other tools are required to move the legacy controller configuration to the new controller.

Suggestion: Consider phased options first. This solution may look easy to implement, but it requires wholesale movement of controller application configuration and a full replacement of HMIs.