The client calls: "The US parent has told us to switch from Siemens TIA Portal to Rockwell Studio 5000 within a year. What will it cost?" This is the typical start of a conversation where the real answer doesn't lie in engineer-hours, but in the question that hasn't been asked yet: why. And when it's asked, in nine out of ten cases it isn't a technical reason.
A migration from Siemens (S7-300, S7-1200, S7-1500) to Allen-Bradley (CompactLogix, ControlLogix) is one of the most expensive decisions a production plant makes — and most often made for a reason that has nothing to do with whether the PLC works. This article names the real reasons behind the decision, describes six concrete things that always break in migration, and gives you three situations where the right answer is "don't migrate."
The real reasons clients migrate
In practice we meet a migration decision driven by three real reasons and one stated reason:
**Stated reason:** "we want a unified platform across plants." That's true, but it's only the façade — underneath it lies one of the following three.
**Real reason 1 — corporate mandate.** The US parent has a standardised Rockwell + FactoryTalk + Studio 5000 + ThinManager stack. The Slovak plant historically has Siemens. The global CFO wants one contract, one vendor support, one licence price. This is a business decision, not technical. The argument "technically better" or "migration costs €400k" doesn't help — the decision was already made at a level where these numbers don't look big.
**Real reason 2 — end-of-life Siemens hardware.** S7-300 (series 313, 315, 317) reached end-of-sale in 2023, end-of-support in 2028. The client has 40+ installs on S7-300 and knows they must be replaced within 5 years. The question is: to S7-1500 (more expensive, but direct porting) or to ControlLogix (more expensive, but the chance to unify with the US parent).
**Real reason 3 — FactoryTalk integration.** The client has a Rockwell MES or SCADA (FactoryTalk View SE, FactoryTalk Historian, FactoryTalk MES). The Siemens PLC integrates via OPC UA or via an RSLinx OPC gateway — it works, but alarms, recipe management and faceplates never sit as cleanly as native ControlLogix. The client is tired of running two worlds.
Only when you name these reasons at the start of the project can you price realistically. Because a migration driven by reason 1 can bend — perhaps you can negotiate "new lines Rockwell, old ones stay Siemens" and save 60% of CAPEX. Migration from reason 2 isn't "whether" but "when and where to." Migration from reason 3 can be sidestepped by an OPC UA bridge for €15–25k instead of €400k of reprogramming.
Six things that always break in migration
1. HMI faceplate library
Siemens WinCC (Comfort, Advanced, Unified) has its own faceplate library for standard objects: motor, valve, analogue loop. Rockwell FactoryTalk View SE has its own — Process Library 5.0, PlantPAx. **They are not compatible, conceptually or visually.** The operator who has been clicking a Siemens motor faceplate with 4 buttons (Start / Stop / Auto / Manual) and a Wago status colour for 8 years gets a PlantPAx motor faceplate with 9 tabs, alarm listing and an EPK status indicator. Operator retraining is mostly underestimated — count 8–16 hours of training per operator + 3 months "until they get used to it and stop asking every shift."
2. Timer / counter precision and paradigm
Siemens IEC timers (TON, TOF, TP) in S7-1500 have 1 ms resolution and use IEC 61131-3 syntax. Rockwell ControlLogix has a timer as a structure (Timer.PRE, Timer.ACC, Timer.EN, Timer.TT, Timer.DN) with 1 ms resolution too — but **scan-time dependency** is different. Siemens handles timers in a dedicated thread, Rockwell in the scan cycle. At a scan time of 20 ms and Timer.PRE = 50 ms you actually get 40–60 ms on Rockwell, 50 ms ±1 on Siemens.
Impact: every time-critical cycle (pneumatic cycle, dosing, conveyor sync) must be **recalculated and retested**, not just copied with the same time constant.
3. Alarm handling paradigm
Siemens (TIA Portal) has **System Alarms** (PLC errors, network errors) separated from **Process Alarms** (operator alarms), with alarm classes (Error, Warning, Tolerance) and alarm areas (logical groups). Alarms are configured in the Alarm Editor in TIA Portal, transferred to WinCC via the Alarm S7 protocol.
Rockwell has **FactoryTalk Alarm and Events** (FTAE) — a separate server that collects alarms from all PLCs in the ecosystem. Alarms in the PLC are defined as tags with an ALARM_DIGITAL or ALARM_ANALOG instruction, the FTAE server subscribes to them.
Migration: doesn't translate 1:1. A client with 800 alarms in the Siemens project finds that 200 of them were "informational messages" that in the Rockwell paradigm belong in a logger, not in the alarm system. Re-architecting the alarm logic is a separate phase — 80–160 hours of an engineer who understands both systems.
4. OPC tag mapping and addressing
Siemens uses symbolic addressing (DataBlock.Tag, Marker.M10.0) via OPC UA server in S7-1500 (built-in) or via SIMATIC NET Softnet/OPC. Rockwell uses tag-based addressing in ControlLogix (Program:MainProgram.MotorRun, Local:1:I.Data[0].0) via RSLinx Enterprise or FactoryTalk Linx.
In migration the OPC tag tree **must be redone**. SCADA / MES / IoT bridges that subscribe to specific tags from Siemens by name (for instance `"DB10".LineSpeed`) get a new name (`Program:MainProgram.LineSpeed`) and all historian queries, dashboard widgets, MES recipes — must be reconnected.
Real experience: in a migration of a line with 4,500 OPC tags, 12% of tags were lost in the process (typo, missing, renamed) — and most were only found after 3 weeks of the new line running when the customer reported missing metrics in the weekly report.
5. Function block libraries — Siemens LADDR vs Rockwell AOI
Siemens TIA Portal has **LADDR libraries** (Library Advanced Drives, Library Process, Library Safety) with prefabricated function blocks for standard instrumentation. Rockwell has **Add-On Instructions (AOI)** — its own encapsulation type.
Functionally similar, **the interfaces completely different**. A Siemens function block with 18 parameters (IN, OUT, IN_OUT, STATIC) ports to an AOI with InOut, In, Out parameters — but internal states (static variables between scans) are modelled differently. An AOI has local tags preserved between scans; a Siemens FB has its own DB instance.
FB → AOI migration: for standard motor / valve / PID blocks this is 4–8 hours per block once you understand both languages. For custom FBs with 50+ lines of Siemens SCL (Structured Control Language) code — 16–40 hours per block, plus testing.
6. Motion control structure
This is the hardest part if the line has motion (servo-axes, indexing tables, cam profiles). Siemens uses **Technology Objects** in S7-1500T (TO_Axis, TO_CamTrack) with native integration in TIA Portal. Rockwell uses **Motion Group + Axis tags + MAOC / MAJ / MCD instructions** in Studio 5000.
Cam profiles, gear ratios, electronic line shafting — all must be **rebuilt from scratch**, not ported. The client who had a 6-axis indexing packaging line in Siemens won't get "export → import" — they get 80–160 hours of a motion programmer who understands the Rockwell Motion paradigm.
Plus: motion safety. Siemens SIMATIC Safety Advanced + drive-integrated safety (STO, SS1, SS2 on G120, S120) doesn't migrate to Rockwell GuardLogix + Kinetix safety via a wizard. Recertification under EN ISO 13849-1 PLd/PLe is a separate line item.
Three situations where migration isn't the right choice
Situation A — the line works, no business case beyond "uniformity"
An existing S7-1500 line, 5 years old, OEE 88%, hardware support to 2030+, no retooling planned. Corporate "wants uniformity." Real arithmetic: migration €350–500k, production interruption 3–4 weeks (€800k–1.4M of lost margin at €30k/day of production), risk that OEE drops to 75% during a 3-month ramp-up after migration. Total exposure €1.5–2.5M. Benefit: one SCADA dashboard structure.
Argument: propose an OPC UA bridge between the Siemens PLC and Rockwell FactoryTalk MES for €25–40k. You lose 5% of visual consistency, you save 90% of migration. **This usually passes**, if there's a technical partner who can argue it in front of the CFO.
Situation B — the Siemens line is in the EU with local technicians who don't know Rockwell
A Slovak plant with 12 maintenance technicians, all certified on Siemens, 0 on Rockwell. Migration means 200+ hours of training per technician × 12 = 2,400 hours, plus 12–18 months of real-world experience until they're comfortable in production troubleshooting. Training cost €60–120k. The cost of misdiagnoses during the first year: hard to quantify but not negligible.
Argument: keep Siemens, add a Rockwell skill set into new hires, wait for a generational refresh of personnel.
Situation C — Siemens is deeply integrated into the safety systems
PROFIsafe between S7-1500F and distributed safety I/O (ET 200SP F-modules), F-CPU certified to SIL3 / PLe, safety logic audited by a notified body (TÜV SÜD, TÜV Rheinland). Recertification to GuardLogix + CIP Safety costs €40–80k separately and 8–14 weeks of time with a notified body — and no "equivalency" is recognised.
Argument: if the safety layer is stable and working, **migrate only the non-safety layer** (if corporate mandate allows), and leave safety on Siemens for the remaining certificate period (typically 5 years from audit).
A realistic cost frame for 2026
For an indicative sense of what migration costs in Slovakia:
- Small line (1 PLC, 200 I/O, 0–2 servo-axes, 1 HMI) — €60–120k
- Medium line (1–2 PLCs, 500–800 I/O, 2–6 servo-axes, 2 HMI) — €180–320k
- Large line (3+ distributed PLCs, 1,500+ I/O, 8+ servo-axes, motion synchronized, multiple HMI, MES integration) — €400–800k
- Closed system / OEM line (where the OEM didn't release source code, reverse-engineering needed) — add 40–60%
Timeline: small line 8–12 weeks, medium 16–24 weeks, large 28–48 weeks. Ramp-up after commissioning: 4–8 weeks until OEE reaches pre-migration level.
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*We also do migrations in the reverse direction (Rockwell → Siemens, B&R → Siemens, Mitsubishi → Beckhoff). The first consultation (60 min) surfaces which of the three real reasons stands behind your need to migrate and whether it can be negotiated to a smaller scope than the initial corporate brief suggests.*