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Siemens officially discontinued the S7-300 product line in October 2023. Replacement parts are getting scarce, lead times are stretching, and Siemens no longer issues firmware patches. If you are still running S7-300 stations, the migration path leads to S7-1500.
This guide covers the practical steps: which CPU replaces yours, how to handle your I/O, what happens to your STEP 7 Classic program, and how to phase the migration so your plant keeps running. If you are still evaluating the S7-1500 platform, start with our S7-1500 Complete Buyer's Guide for a full overview of CPUs, I/O modules, and accessories.
Why migrate now?
- End of production: Siemens stopped manufacturing S7-300 CPUs and I/O modules. Remaining stock is finite.
- No new firmware: Security patches and bug fixes are no longer released for S7-300. This is a growing cybersecurity risk for connected plants.
- Spare parts pricing: Third-party S7-300 CPUs now cost as much as — or more than — new S7-1500 equivalents.
- Performance gap: An S7-1500 CPU 1513 outperforms the fastest S7-300 (CPU 319) by roughly 10x in bit instruction speed.
- TIA Portal ecosystem: New HMI panels, drives, and safety systems all integrate through TIA Portal. STEP 7 Classic cannot configure them.
S7-300 to S7-1500 CPU cross-reference
Siemens publishes a migration path for every S7-300 CPU. The table below maps the most common S7-300 CPUs to their S7-1500 equivalents that we stock.
| Your S7-300 CPU | Work Memory | Replacement S7-1500 CPU | S7-1500 Work Memory | Part Number |
|---|---|---|---|---|
| CPU 312 / 312C | 32 KB | CPU 1511-1 PN | 150 KB | 6ES7511-1AK02-0AB0 |
| CPU 313C / 313C-2 | 64 KB | CPU 1511C-1 PN or CPU 1511-1 PN | 175 / 150 KB | 6ES7511-1CK01-0AB0 / 6ES7511-1AK02-0AB0 |
| CPU 314 / 314C-2 | 96–128 KB | CPU 1513-1 PN | 300 KB | 6ES7513-1AL02-0AB0 |
| CPU 315-2 DP | 256 KB | CPU 1515-2 PN + CM 541-1 | 500 KB | 6ES7515-2AN03-0AB0 + 6ES7541-1AD00-0AB0 |
| CPU 315-2 PN/DP | 256 KB | CPU 1516-3 PN/DP | 750 KB | 6ES7516-3AP03-0AB0 |
| CPU 317-2 PN/DP | 1 MB | CPU 1517-3 PN/DP | 2 MB | 6ES7517-3AP00-0AB0 |
| CPU 319-3 PN/DP | 2 MB | CPU 1518-4 PN/DP | 6 MB | 6ES7518-4AP00-0AB0 |
Handling the I/O: three migration strategies
The biggest concern for most engineers is the I/O. S7-300 modules (SM 321, SM 322, SM 331, SM 332) do not physically fit an S7-1500 rack. You have three strategies, and you can combine them.
Strategy 1: Full rack replacement (clean cut)
Replace the entire S7-300 rack — CPU, I/O modules, and backplane — with an S7-1500 rack. Map every S7-300 I/O module to its S7-1500 equivalent.
| S7-300 Module | Function | S7-1500 Replacement | Part Number |
|---|---|---|---|
| SM 321 (6ES7321-1BH02-0AA0) | 16 DI, 24 VDC | DI 16x24VDC HF | 6ES7521-1BH00-0AB0 |
| SM 321 (6ES7321-1BL00-0AA0) | 32 DI, 24 VDC | DI 32x24VDC HF | 6ES7521-1BL00-0AB0 |
| SM 322 (6ES7322-1BH01-0AA0) | 16 DO, 24 VDC/0.5A | DQ 16x24VDC/0.5A HF | 6ES7522-1BH01-0AB0 |
| SM 322 (6ES7322-1BL00-0AA0) | 32 DO, 24 VDC/0.5A | DQ 32x24VDC/0.5A HF | 6ES7522-1BL01-0AB0 |
| SM 331 (6ES7331-7KF02-0AB0) | 8 AI, universal | AI 8xU/I/RTD/TC ST | 6ES7531-7KF00-0AB0 |
| SM 332 (6ES7332-5HD01-0AB0) | 4 AO, voltage/current | AQ 4xU/I ST | 6ES7532-5HD00-0AB0 |
Best for: Planned shutdown windows, new panel builds, or systems with fewer than 10 I/O modules.
Strategy 2: CPU swap + remote I/O (gradual)
Replace only the CPU. Keep existing S7-300 I/O racks in the field by converting them to ET 200M remote I/O stations connected to the new S7-1500 CPU over PROFINET or PROFIBUS.
This works because ET 200M uses the same S7-300 I/O modules (SM 3xx) as slaves. Your existing S7-300 racks become remote I/O stations without rewiring a single field cable.
Best for: Distributed systems, limited shutdown time, or phased migration budgets.
Strategy 3: Hybrid (CPU + partial I/O)
Replace the CPU and the local rack I/O with S7-1500 modules, but keep remote S7-300 I/O racks as ET 200M slaves on PROFINET. Migrate remote racks to ET 200SP or ET 200MP one at a time during future maintenance windows.
Best for: Most real-world projects. It gets you off the discontinued CPU immediately while spreading the I/O migration over months or years.
Software migration: STEP 7 Classic to TIA Portal
S7-1500 requires TIA Portal. Your STEP 7 Classic project needs to be converted. Here is the process:
- Export from STEP 7 Classic. In SIMATIC Manager, archive your project (File → Archive). Note the STEP 7 version used.
- Open the Migration Tool in TIA Portal. Go to Project → Migrate project. Select your archived S7 project.
-
Review the migration report. TIA Portal generates a detailed report listing every block that converted successfully and every item that needs manual attention. Common issues:
- Absolute addressing: S7-300 programs often use absolute addresses (I0.0, Q4.0). S7-1500 uses symbolic addressing by default. TIA Portal converts these but flag them for review.
- SFBs and SFCs: Some S7-300 system function blocks have different numbers or behavior on S7-1500. TIA Portal maps most automatically.
- STL-specific instructions: A few STL instructions (like direct register access) are not supported on S7-1500. These require manual rewriting.
- Update hardware configuration. Replace the S7-300 CPU and I/O modules in the TIA Portal hardware view with their S7-1500 equivalents. Re-assign I/O addresses.
- Compile, download, and test. Use TIA Portal's simulation (PLCSIM Advanced) to test logic before going to the physical hardware.
Addressing changes: what you need to know
The biggest software difference between S7-300 and S7-1500 is how I/O addresses are assigned.
- S7-300: I/O addresses are determined by the physical slot position. Slot 4 = starting address for that module's I/O range.
- S7-1500: I/O addresses are assigned freely in TIA Portal. Module position on the rail does not determine addresses.
This means you can keep the same I/O addresses your program uses even if the S7-1500 modules are in different physical slots. Assign addresses in TIA Portal to match your existing program and you avoid rewriting I/O references throughout your code.
Communication considerations
| S7-300 Feature | S7-1500 Equivalent | Notes |
|---|---|---|
| MPI | Not available natively | Use CM 541-1 for MPI connectivity during transition. Plan to migrate MPI links to PROFINET. |
| PROFIBUS DP (integrated) | CPU 1516-3 or 1517-3 PN/DP | Built-in DP Master port. Or add CM 541-1 to PN-only CPUs. |
| PROFINET | All S7-1500 CPUs | Native. 2-port switch built into every CPU. |
| S7 Communication (PUT/GET) | Supported, but disabled by default | Enable in CPU properties → Protection & Security → "Permit access with PUT/GET." Siemens recommends migrating to OPC UA instead. |
Migration checklist
- Inventory all S7-300 hardware: CPU model, I/O module part numbers, communication modules, and remote I/O stations
- Map each S7-300 component to its S7-1500 replacement (use the cross-reference tables above)
- Decide your migration strategy: full replacement, CPU swap with remote I/O, or hybrid
- Archive your STEP 7 Classic project and run the TIA Portal migration tool
- Review the migration report and resolve any flagged items
- Order S7-1500 hardware — CPU, I/O modules, power supply, mounting rail, memory card, front connectors, and U-connectors
- Build and configure the new rack in TIA Portal
- Test with PLCSIM Advanced before commissioning
- Schedule the cutover during a planned maintenance window
- Commission, verify I/O, and validate safety functions
Frequently asked questions
Can I reuse my S7-300 I/O modules with an S7-1500 CPU?
Not on the same rack — S7-300 and S7-1500 use different backplane connectors and form factors. However, you can keep S7-300 I/O modules in the field as ET 200M remote I/O stations connected to your S7-1500 CPU over PROFINET or PROFIBUS. This lets you reuse existing I/O wiring and migrate modules gradually.
How long does a typical S7-300 to S7-1500 migration take?
For a single-rack system with under 10 I/O modules, the software migration typically takes 2–5 days of engineering time. The physical cutover can be done in a single maintenance window (4–8 hours) if the new rack is pre-built and pre-tested. Multi-rack distributed systems take proportionally longer, but the CPU swap can be done first while remote I/O racks migrate over weeks or months.
Will my STEP 7 Classic program run on S7-1500 without changes?
Not directly. You must convert the project using TIA Portal's built-in migration tool. Most LAD, FBD, and STL blocks convert automatically. Manual work is usually limited to: replacing a few unsupported STL instructions, updating hardware configuration, and reviewing absolute addresses that may need symbolic names. Expect 80–95% of your code to migrate without manual changes.
Do I need a new TIA Portal license for S7-1500?
Yes. S7-1500 requires STEP 7 Professional (TIA Portal) V14 or later. If you currently use STEP 7 Classic, you will need to purchase a TIA Portal license. Siemens offers upgrade pricing for existing STEP 7 Classic license holders. You will also need a Comfort/Advanced license for WinCC if you are migrating HMI panels simultaneously.
Ready to start your migration?
We stock the full range of S7-1500 CPUs, I/O modules, communication modules, and accessories needed for S7-300 replacements. All parts ship with a 2-year warranty.
Browse our Siemens PLC Systems collection or request a quote with your existing S7-300 BOM — we will cross-reference every part to its S7-1500 equivalent.