Though not advised, keeping a legacy system as-is may retain a system’s existing functionality, but it leaves the system vulnerable to obsolescence, operational and maintenance issues. Some users scour the internet in hope of finding replacement components and stockpile spare equipment parts as a stopgap measure, but this type of approach is risky, costly, and can’t work indefinitely.

There are two main approaches to address legacy PLC control systems:

1. Rewrite new code from scratch in a new hardware and software platform.
2. Migrate existing legacy code into the new hardware and software platform.

These two approaches may require, at the very least, modifications to the control panel and upgrades to any platforms running SCADA or HMI software.  In some cases, it is advisable to replace either or both.

Rewriting new code from scratch provides an opportunity to break away from the old patches and introduce modern coding philosophies, such as software standards. This is a clean slate approach to upgrading a system, one that does not lock you into the same manufacturer,  but this typically requires a bit more programming time than a migration.

Migrating existing legacy code into new hardware and software, typically from the same manufacturer,  preserves a system’s complex control strategies.  New and consistent programming standards can’t be fully applied using this approach.