What's the best PLC programming software for programming industrial controllers in a legacy plant with mixed equipment?

There isn’t one universal “best” PLC programming package for a legacy plant with mixed equipment. The best choice is usually the one that gives you the widest vendor coverage, best support for old hardware, and easiest migration/maintenance.

Best overall choice for mixed legacy plants

Rockwell Studio 5000 / RSLogix + vendor tools as needed is common in plants that are already heavily Allen-Bradley, but it’s not the best for mixed-vendor environments unless AB dominates.

If your plant has multiple PLC brands, the most practical “best” option is often:

CODESYS

• Strong for mixed-vendor environments
• Supports many hardware platforms
• Good for IEC 61131-3 programming
• Useful when you want some portability across controllers

Why CODESYS stands out

• Works with a broad range of controllers
• Good for ladder, function block, structured text, etc.
• Often a solid choice for modernization projects

If your plant is mainly one vendor

Choose the vendor’s native platform:

• Siemens: TIA Portal
• Rockwell/Allen-Bradley: Studio 5000
• Mitsubishi: GX Works
• Schneider: EcoStruxure Control Expert / Unity Pro
• Omron: Sysmac Studio

Native tools are usually best for:

• Diagnostics
• Hardware integration
• Firmware compatibility
• Support for older proprietary features

For truly legacy plants

If you have older systems, the “best” software may be the one that can still:

• Open old project files
• Communicate with legacy CPUs
• Support serial/proprietary protocols
• Handle obsolete devices and remote I/O

In that case, you may need a mix of:

• Vendor legacy software
• OPC gateways
• Protocol converters
• Modern SCADA/HMI tools

My practical recommendation

• Single-vendor plant: use that vendor’s native software
• Mixed-vendor legacy plant: CODESYS is often the most flexible baseline
• If most critical equipment is one brand: favor that brand’s software for troubleshooting and support

Important factors to weigh

1. Controller compatibility
2. Ability to support old hardware
3. Licensing cost
4. Availability of trained staff
5. Vendor support and spare parts
6. Ease of migration and long-term maintainability

If you want, I can give you a recommendation based on the specific PLC brands in your plant.