Siemens S7-300 to S7-1500 migration. A working guide for Australian plants.

The timeline that matters.

When did your team last try to source a spare S7-300 CPU?

If the answer is "we haven't tried lately," the timeline below is more urgent than the calendar makes it look. Two dates run the clock. The S7-300 PM400 power supplies entered active phase-out in October 2023. The PM410s followed in October 2025. Siemens has committed to spare parts availability until October 2033 (Siemens Industry Online Support 109809890). That is the window. About eight years from now, the manufacturer no longer guarantees it can ship you a replacement card for a CPU that fails on a Saturday.

Eight years sounds comfortable until you put a brownfield project plan against it. A multi-PLC line migration typically runs 12 to 26 weeks from kickoff to stable commissioning, plus discovery and capex sign-off before that. Plants that defer the migration decision until 2030 are quietly committing to a panic project, not a planned one. The economic case is rarely the migration spend. It is the production cost of a controller failure between 2031 and 2033 with no shelf-ready replacement and no rehearsed switchover.

Why S7-1500 is the obvious target.

Siemens designed the S7-1500 as the direct successor to the S7-300 and S7-400. The development environment is TIA Portal Professional (the Basic edition will not open Step 7 Classic projects or run the migration tool). The platform extends what the 300-series did well: deterministic batch and continuous process control, native Profinet, larger memory ceilings, faster scan times, better diagnostic tooling, and a programming model that aligns with current IEC 61131-3 practice.

A few plants look at alternatives. Rockwell ControlLogix is the most common pivot, usually when the engineering team has stronger Rockwell muscle memory or when the existing FactoryTalk SCADA makes a Siemens-side investment harder to justify. Schneider M580 sees occasional consideration in process industries with a Modicon history. Both are legitimate choices but roughly double the integration scope: you are not migrating a project, you are rewriting it.

For plants where the existing investment is on Siemens, the S7-1500 wins on the simple economics of inherited muscle memory. Operators, electricians, and the on-call engineer already speak the platform. The migration is a project. A platform switch is a program.

What the migration tool does, and what it does not.

Siemens publishes a free migration guide and tool that converts a Step 7 Classic project into a TIA Portal project targeting the S7-1500. For straightforward projects, the conversion runs in an afternoon. The temptation is to treat that as the migration. It is not.

What the tool converts cleanly: standard FB and FC blocks, the tag database, project structure, and most LAD or FBD logic. Symbolic tags survive the move. Comments survive. Hierarchy survives.

What needs manual review or rework:

• S5 timer and counter functions. The S7-300 inherited these from the older S5 platform. The S7-1500 only supports the IEC-compliant equivalents, so anything written against S5 timers needs to be rewritten by hand.
• Absolute addressing. Code that references IO by absolute address (I0.0, Q4.0, MW100) gets flagged for review. The S7-1500 expects symbolic addressing. Auto-converted absolute references work but are flagged as a portability smell.
• STL instructions with direct register access. A small subset of STL accumulator operations is not supported on the S7-1500 because the underlying CPU architecture is different. These show up as conversion errors.
• Custom libraries and bespoke FBs. Anything written by the original engineer with non-standard patterns. The migration tool can convert the code but cannot vouch that the converted behaviour matches the original intent.
• Optimised vs non-optimised data blocks. The S7-1500 introduces optimised data blocks (up to 10 MB, faster access) alongside the legacy non-optimised format. Mixing them in one program degrades performance. The migration tool leaves blocks in their original format. Cleanup is a manual decision per block.
• Hardware configuration. The standard advice from Siemens is to run the migration with the "Include hardware configuration" checkbox unticked, then rebuild the hardware config manually in TIA Portal. The reason: S7-1500 IO addressing is freely assigned in software, not slot-position-determined as on the S7-300. Auto-migrating the hardware config produces a configuration that will not match the physical install.

The migration tool gets the project across the line. The integrator's job is everything between the tool's last green tick and a controller that actually runs a line.

Panel and IO realities.

An S7-1500 CPU is physically larger than a typical S7-300 CPU and uses a different backplane connector. The S7-300 IO modules (SM 321, SM 322, SM 331, SM 332 and friends) will not fit an S7-1500 rack. That is not a small problem on a panel that has been packed since 2007.

The standard workaround is to convert the existing S7-300 IO into ET 200M remote stations connected back to the new S7-1500 CPU over Profinet (preferred) or Profibus (if the network already exists). The field wiring stays. The IO modules stay. The CPU and its rack get replaced. This is the cheapest path for a panel that is otherwise in good electrical condition, and it preserves the operator's familiarity with the IO layout.

The cost the ET 200M approach hides is the network commissioning. Profinet is more reliable and easier to diagnose than Profibus, but it is also a new network layer that needs IP addressing, switch infrastructure (industrial-rated, ring topology for any line that matters), and segmentation from the corporate IT side under AS IEC 62443. Plants migrating from a flat-Profibus topology to Profinet on a brownfield site routinely discover that the network rework is more work than the CPU swap.

The harder conversation is the panel that cannot host both the existing IO modules and a new S7-1500 CPU at the same time. In those cases, a retrofit becomes a partial replacement, billed like a retrofit. See the retrofit-vs-replace decision framework in the brownfield guide for how that conversation usually lands.

Project span and cost.

The variable that moves cost and schedule is rarely the hardware. It is the state of the existing documentation and the IO count.

Single-PLC migration on a clean panel with full documentation. Hardware: S7-1500 CPU, replacement power supply, Profinet switches, ET 200M conversion if needed. Engineering: discovery, code migration and cleanup, hardware build, FAT, commissioning, post-cutover support. This is the best-case shape, with the fewest unknowns at the start.

Multi-PLC line with SCADA refresh and Profinet rebuild. Five or six controllers across a single line, a refreshed Wonderware or Ignition SCADA front end, network segmentation under AS IEC 62443. The discovery phase alone is often the longest single segment.

Plant-wide migration. Multiple lines, ERP integration, full safety and cyber rework. An entirely different order of magnitude to the scopes above.

The single biggest cost variable on any brownfield migration is documentation. Plants that hand the integrator a current code backup, a current IO list, and a current network diagram start from a fundamentally better position. Plants that hand over a years-old USB stick and a hand-marked drawing pay for the gap in integration hours, at discovery rates.

Spares strategy if you are not migrating yet.

Not every plant can or should migrate immediately. For sites with three to five years of useful S7-300 life ahead of them, the right interim move is a deliberate spares strategy rather than vendor faith.

The minimum spares position for an S7-300 plant in 2026: one of each critical module variant on the shelf (CPU, PS, every SM type in use), sourced from Siemens or an authorised reseller while the spares pipeline is still active, and verified before shelving. Opportunistic eBay sourcing in 2028 is not a strategy. It is a future panic story.

The maintenance side of the bench-spares position is checking that the modules on the shelf will actually power up. Spare cards that have sat unenergised for three years sometimes do not. Periodic bench testing matters.

Common questions.

Sources and further reading.

Every dated claim in this guide is anchored to a verified source, retrieved on 18 May 2026.

• Siemens Industry Online Support. Product phase-out of S7-300 / ET 200M components, document 109809890. Retrieved 18 May 2026. Siemens IOS 109809890
• Siemens. Guide for Migrating SIMATIC S7-300/S7-400 to S7-1500, document 109478811. Retrieved 18 May 2026. Siemens migration guide PDF
• Industrial Cyber. Australia adopts AS IEC 62443 as national cybersecurity standard for critical infrastructure, July 2025. Retrieved 18 May 2026. industrialcyber.co

This article sits under the Brownfield PLC Upgrade Guide. For the broader decision framework on retrofit-vs-replace, cutover models, and integrator selection, the full guide is the place to start.