Siemens S7-300 and S7-400 End of Life: Spares Strategy and Migration to S7-1500

If you run a plant built between 1998 and 2018, you almost certainly own S7-300 CPUs, ET 200M racks and probably a couple of S7-400s controlling the big lines. Siemens announced the S7-300 phase-out (PM400) in October 2023 and cancelled the product (PM410) on 1 October 2025; S7-400 central I/O moves to spares-only status on 1 October 2026. Siemens has committed to spare-part availability for several years after cancellation, which sounds comfortable until you notice that "available" and "available at the price and lead time you planned for" are different things. We have been quoting S7-300 parts weekly for two years; this is what the market looks like from the supply side and how we advise customers to handle it.

What "spares only" means in practice

Siemens product milestones are precise. PM300 is the phase-in, PM400 is the announcement of phase-out (the product is still orderable), PM410 is product cancellation (no new orders at list price through normal channels) and PM420 marks the end of repairs and spare-part delivery. For S7-300 the PM410 date has passed; PM420 is typically ten years after PM410, so repairs and some spares run to around 2035. Three things change on the day of PM410, and they matter more than the far-off PM420 date:

• Allocation replaces stock. Distributor stock of popular CPUs (6ES7315-2EH14, 6ES7314-6CH04, 6ES7317-2EK14) was drawn down during the last-time-buy wave of 2024–2025. What remains is held by brokers and specialists at prices set by scarcity, not list price.
• Firmware and revision matter. A replacement CPU must match the firmware family the project was compiled for. A V3.3 program will not load on a CPU shipped with V2.6 firmware without a project upgrade, and some memory cards (MMC) are not interchangeable across generations. Specify the full order number and firmware version when sourcing, not just "CPU 315-2 PN/DP".
• Lead times stop being predictable. A CPU that shipped in three days in 2023 may take six weeks from a European broker today, or be in stock in Vietnam tomorrow. Treat every S7-300 part as a strategic spare with a defined reorder trigger.

Step one: inventory what you actually have

Most plants do not have a complete list. The quickest way to build one is from the engineering side, not the storeroom: open every STEP 7 or TIA Portal project, export the hardware configuration (HW Config → Export to CSV, or the TIA "Print hardware catalogue" function) and consolidate by order number. Add the physical check for things that never made it into a project — spare racks, panel-mounted CPUs on test rigs, the OP 277 in the corner. The result is a table of order number, quantity installed, quantity in stock, firmware version and criticality (does this CPU stop the whole plant or one filler?). We offer this as a free service for customers who send us their project archives; it takes an afternoon and it is the single most useful document in the whole exercise.

Step two: decide what stays, what gets spares and what migrates

Not every S7-300 needs to go. A CPU 312 running a pump station with twenty I/O points can be covered by two spare CPUs and will probably outlive the pumps. The decision rule we use:

Situation	Recommendation	Why
Small CPU, < 100 I/O, no fieldbus beyond PROFIBUS, no HMI integration	Buy spares, keep running	Migration cost exceeds ten years of spares
Medium CPU (315/317), PROFINET, several ET 200M stations, WinCC flexible HMI	Spares now, migrate within 2–3 years	HMI software (WinCC flexible) is already unsupported; panels fail before CPUs
CPU 319 or S7-400 running a main line, Safety (F-CPU), or PCS 7	Plan migration now	F-CPUs and PCS 7 libraries are the hardest to source and re-certify later
Any system where the original integrator is gone and no project archive exists	Migrate — the archive risk is bigger than the hardware risk	A failed CPU with no project is a rebuild, not a replacement

Step three: the spares calculation

For each order number you keep, hold spares according to failure rate and downtime cost, not by gut feeling. S7-300 CPUs have a field failure rate in the order of 1–2 % per year in a clean panel, higher where temperatures exceed 45 °C or power quality is poor. For a fleet of ten CPU 315s over a planned five-year remaining life, expect 0.5–1 failures; two spares plus one for the test bench is a defensible number. Signal modules fail more often than CPUs — relay outputs (SM 322 DO relay) and analog inputs with 4–20 mA shorts are the usual victims — so hold spares per module type in proportion to the quantity installed. Our last-time-buy formula turns this into a spreadsheet.

Step four: migration — what carries over and what does not

The S7-1500 is the designated successor for both S7-300 and S7-400, with the S7-1200 covering the small end. TIA Portal includes a migration path for STEP 7 V5.x projects, and in our experience about 70–80 % of a typical program converts automatically. The remaining 20 % is where the effort goes:

• Absolute addressing and pointers. Programs written with ANY pointers, indirect addressing in STL or direct access to the process image by byte offset need rework. STL is still supported on S7-1500 but it is not optimized; we usually translate the critical routines to SCL.
• System functions. SFC/SFB calls such as SFC 20 BLKMOV or SFB 52 RDREC map to new instructions, mostly automatically, but timers (S5 timers) and counters are replaced by IEC timers. Timing behaviour can shift if the program depended on scan-cycle quirks.
• I/O. ET 200M stations keep working behind an S7-1500 over PROFIBUS or PROFINET (IM 153-4 PN), so you can migrate the CPU first and the I/O later. For new I/O the ET 200SP (IM 155-6 PN) is smaller and cheaper per channel; plan the terminal re-wiring as its own work package, because that is where the hours go.
• HMI. WinCC flexible panels (MP 277, TP 177) must be replaced by Comfort or Unified panels. Budget this as a separate project; it is often larger than the CPU swap.
• Safety. F-CPU programs migrate but the safety program must be re-validated and the signature re-recorded. Allow for a safety engineer's time and a new acceptance test.
• PROFIBUS devices. Drives and instruments on PROFIBUS DP continue to work through the S7-1500's DP interface (CP 1542-5 or CPU with DP). No reason to replace them because the CPU changed.

A realistic timeline and budget

For a line with one CPU 317, six ET 200M stations and two panels we typically see: two days of audit and project conversion, five days of program clean-up and testing on a simulator, a weekend shutdown for the CPU and panel swap with I/O left in place, and two weeks of follow-up support. Hardware is usually 30–40 % of the total; engineering is the rest. The common mistake is to buy the hardware first and discover the project archive is incomplete, so we insist on the archive check before any purchase order.

Where Vietnam fits in

Our stock of tested S7-300 CPUs, signal modules, IM 153 interface modules and MMC cards comes from end-of-line purchases and from plants that migrated early. Every unit is tested on a reference rack, firmware is recorded and the Siemens serial number is checked against the manufacturer database before it ships, with photographs of the label in the quotation. For migration we supply the S7-1500/ET 200SP hardware and, where wanted, the conversion engineering. Send us the HW Config export and we return a priced spares-and-migration plan within a week.

Quick answers

Can I still buy new S7-300 CPUs from Siemens?

Not through normal ordering since 1 October 2025 (PM410). Siemens delivers spares and repairs for several more years, typically through the Siemens Industry Online Support spare-parts service, at spare-part pricing and lead time. Most plants source from distributor residual stock and tested surplus.

Does my ET 200M I/O work with an S7-1500?

Yes. ET 200M stations with IM 153-2 (PROFIBUS) or IM 153-4 PN (PROFINET) are supported by S7-1500 CPUs, so CPU migration can be done without touching field wiring. ET 200M itself is also in phase-out, so plan the I/O replacement as a second stage.

How long does Siemens supply S7-300 spare parts?

Siemens states a spare-part availability period after product cancellation, generally up to ten years (around 2035 for S7-300), subject to component availability. Treat that as the outer limit, not a guarantee of every order number.