S5 to S7 Migration: What You Need to Know
By Sync Motion — industrial software and OT/IT integration from Austria. We modernize existing plants across the manufacturing industry worldwide: PLC migration, HMI renewal, documentation of undocumented systems.
The SIMATIC S5 is one of the longest-lived PLC families Siemens ever built. Introduced in 1979, it still runs reliably in many plants today. This article covers what's actually involved — technically and organizationally — when you decide to migrate off S5, and what to watch out for along the way.
Where the S5 stands today
Siemens ended the SIMATIC S5 lifecycle on October 1, 2020. The official status is P.M500 — End of Lifecycle. That means no spare parts from Siemens, no repairs, no technical support. S5 spare parts are still available through third-party suppliers, though selection is narrowing and prices are rising.
At the same time, STEP 5 expertise is becoming less common. New technicians and programmers train on the S7-1500 and TIA Portal. The people who know STEP 5 well learned it 20 or 30 years ago. That's not a problem as long as they're available — but it's a factor worth considering when planning ahead.
Worth noting: the S7-300 reached P.M410 status (discontinuation) in October 2025 and can no longer be ordered new. Anyone planning a PLC migration today should go directly to the S7-1500 with TIA Portal.
What actually gets migrated
The most common misconception: PLC migration means swapping the CPU. In practice, an S5 to S7 migration covers several areas.
The application program — written in STL, FBD, or LAD, typically grown and modified over many years. The HMI connection — operator panels (often x70-generation OP/TP units, also discontinued), including their screens, tag lists, and communication drivers. The communication interfaces — serial, PROFIBUS, or proprietary — to variable frequency drives, scales, load cells, or supervisory control systems. And the documentation that describes the current state of the system.
All of this falls within the scope of a migration. It's worth capturing the full picture early in the project.
What happens during code conversion
Siemens includes a conversion tool with STEP 7. It reads S5 programs (.S5D files), generates .AWL source files for STEP 7, and suggests I/O address mappings. In practice, the tool cleanly converts about 80% of the code. The remaining portion produces compile errors that have to be resolved manually.
The reasons are technical: STEP 5 and STEP 7 are built on different architectures. S5 uses byte-level addressing, S7 uses bit-level. Organization blocks have different numbers and call conditions. S5 communication blocks (AG-SEND/AG-RECV, SINEC couplings) don't exist in S7 — the equivalents are TSEND_C/TRCV_C or PUT/GET over PROFINET. Startup OBs (OB21/OB22 in S5), synchronization blocks, and all communication FBs need to be rewritten from scratch.
With an S7-1500 migration there's more to consider: TIA Portal works entirely symbolically rather than with absolute addressing. Data blocks can be 10 MB instead of 64 KB. Optimized blocks (the default in S7-1500) behave differently in terms of addressing than non-optimized ones. None of this is a blocker, but it's more work than a straight copy. An S7-1500 migration is, at its core, a rebuild of the software based on the existing logic.
Three common strategies
Full changeover. Plant stops, everything gets swapped at once. Works well for manageable machines with clear documentation. Typical downtime: one extended weekend. Advantage: clean cut, no parallel operation. Disadvantage: limited room if something takes longer than planned.
Phased migration. CPU first, S5 peripherals stay connected via adapters or PROFIBUS couplings. Siemens offers the IM 463-2 interface module (for S7-400) and I/O adapters for S7-1500 that accept existing S5 front connectors directly. Field wiring stays untouched. I/O modules get replaced in later phases. Less downtime per step, but a longer overall timeline and more complex parallel operation.
Parallel operation. S5 and S7 run simultaneously, plant sections are migrated individually. Requires communication bridges between systems (typically data exchange via PROFIBUS or Industrial Ethernet) and programmers who can work in both environments. Often the preferred approach for critical processes.
Documentation: the part that's easy to underestimate
A practical observation: most S5 systems have no complete, up-to-date documentation. Electrical drawings that haven't been updated since the last modification. Programs without comments or symbol tables. Sometimes no current backup of the application program at all.
Before any migration begins, there's an assessment phase: read the program from the CPU (requires STEP 5 and a PG with the right adapter), go through signals, document the I/O allocation, trace communication paths. For a typical S5-115U or S5-135U with a mature program, 2 to 5 working days for this step is realistic.
This isn't extra work — it's the foundation for everything that follows. The cleaner the assessment, the more predictable the rest of the project.
What drives the cost
Quoting specific prices without knowing the system would be guesswork. But the cost drivers are clear.
More effort is needed when: documentation is missing, there are many communication interfaces (each one has to be understood and rebuilt), the S5 program contains custom solutions, old operator panels need to be fully recreated, and there are many distributed stations. The TIA Portal license (STEP 7 Professional) is also a line item on its own.
Less effort when: documentation exists, the program is manageable in size, peripherals are standard, and field wiring plus PROFIBUS slaves can be reused.
Planning for downtime
Software development and testing happen beforehand. Siemens offers PLCSIM (part of TIA Portal), a simulator that runs the S7 program on a development machine. This allows most of the logic to be validated before anyone picks up a screwdriver.
For critical systems, weekend changeovers are standard practice: start Friday after the last shift, acceptance by Sunday. This requires solid preparation — materials on site, a clear sequence plan, and a fallback scenario in case the changeover needs more time than expected.
With a phased approach, production largely continues. The overall timeline is longer, but the impact on day-to-day operations is significantly smaller.
Common stumbling blocks
A few things that come up repeatedly in S5 to S7 migrations: the program scope gets underestimated because the machine looks simple from the outside. HMI dependencies surface late — the operator panel is part of the migration too. Communication interfaces to drives or supervisory systems don't get tested until the plant is supposed to go live. And there's no rollback plan, even though one should be standard equipment for every changeover.
What can stay
Not everything needs to be replaced. Field wiring, sensors (4–20 mA, Pt100, incremental encoders), actuators, contactors — all of this is independent of the controller and can typically continue in service. Siemens offers I/O adapters that let existing S5 front connectors plug directly into new S7 modules. Cabling stays where it is.
PROFIBUS slaves — drives, distributed I/O, valve terminals — can often be kept as well, provided the new S7 CPU is configured as a DP master. What works doesn't need replacing.
Next steps
Every plant has its own history — modifications, custom solutions, adaptations that aren't written down anywhere. A generic estimate doesn't help much.
Sync Motion offers a free initial assessment of your S5 system. A technical conversation about what you're actually dealing with and which approach would make sense.