Migrating from SIMATIC S7-300 to S7-1500

S7-300 went out of production in October 2023 after twenty-eight years. Siemens continues spare parts and repair service into the 2030s, and the surplus market will outlast that, so nothing forces a migration this year. But every S7-300 in a plant is now a machine whose control system has a known end date, and the migration is much cheaper when it is planned than when a CPU dies on a Friday.

What you are actually replacing

The successor family is S7-1500, engineered in TIA Portal rather than STEP 7 Classic. The migration touches four things, in rising order of effort.

The CPU. An S7-315 or S7-317 maps onto a CPU 1511 / 1513 / 1516 depending on memory and performance. The 1500 series is roughly an order of magnitude faster on bit operations, so a program that was tuned to fit the old scan time will comfortably fit the new one.

The I/O. This is the decision that sets the budget. ET 200M distributed I/O carries over to S7-1500 unchanged via PROFIBUS or PROFINET, and so does most ET 200S. Central S7-300 racks (SM 321/322/331/332 modules on a rail) do not: S7-1500 uses ET 200MP modules on a different rail with a different backplane. If your machine is centrally wired, you are rewiring the cabinet or fitting an adapter rail, and that is the real cost.

The network. MPI and PROFIBUS DP are supported on S7-1500 via a CM module, so drives and remote I/O on PROFIBUS do not have to move on day one. PROFINET is native. Plan the field bus as a separate step from the CPU swap where you can.

The HMI. Comfort and Basic panels port with the project; classic OP/TP 170 and 270 panels do not, and a 1500 migration is usually when they get replaced too.

The code

TIA Portal ships a migration tool that converts an S7-300 STEP 7 project into a TIA project. It does a good job on the structure and a variable job on everything else. Expect to touch:

• Absolute addressing. S7-300 code is full of MW, DB10.DBW4 and direct I/O addressing. S7-1500 defaults to optimised data blocks with symbolic access and no fixed offsets. Optimised blocks are faster and safer; converting them means every AT construct, every BLKMOV over a DB and every pointer needs to be revisited. You can leave blocks non-optimised to get running faster, then convert selectively.
• Indirect addressing and pointers. ANY pointers and area-crossing pointers are restricted on S7-1500. The replacements (VARIANT, Slice access, REF/DEREF) are cleaner but are not a textual substitution.
• Timers and counters. S5 timers (S_ODT and friends) are emulated but should be replaced with IEC timers; the retentivity behaviour differs.
• System functions. Many SFC/SFB calls changed number and signature. The migration tool flags them; it does not always fix them.
• Data types. S7-1500 is strict about implicit conversions that STEP 7 accepted silently. Most first-compile errors are this.

Budget the code conversion as roughly a quarter of the work and the I/O and commissioning as the rest.

Keeping the old machine alive in the meantime

Until the migration is scheduled, treat S7-300 as a stocked-spares machine:

1. Hold a spare CPU of the exact order number, with the same firmware where the program depends on it.
2. Hold the two or three signal modules that carry the most channels; SM 321/322 failures are more common than CPU failures.
3. Replace backup batteries on schedule. A CPU 31x with a flat battery and no memory card loses the program on power loss.
4. Keep an archived, restorable copy of the STEP 7 project — and check that you can actually open it. A project you cannot open on any machine you still own is the most expensive part of any migration.

Surplus and refurbished S7-300 stock is plentiful today and prices are reasonable. That is exactly the window in which to buy the spares, rather than after everyone else has decided to migrate at once.

A realistic sequence

For a typical line the order that causes the least downtime is: audit the I/O and decide central versus distributed; convert and compile the project offline against a simulated CPU; pre-build the new cabinet or rail alongside the old one; swap and commission during a planned stop; keep the old CPU and I/O on the shelf for one production cycle before selling or scrapping them.

Two weeks of commissioning is a normal number for a single machine with a few hundred I/O points. Anyone promising two days has not yet found the analogue scaling that was hidden in a FC from 2004.