Siemens TIA Portal FB instance DBs vs Allen-Bradley AOIs

For Siemens TIA Portal V18+.

TIA Portal Function Blocks with instance Data Blocks are the Siemens answer to AOIs. The mental model is similar but the memory rules are different. This page is the working engineer's read — what the menu paths actually are in TIA Portal V18+, what the keystrokes do, and the mistakes that bite once the program is on a real CPU. We program S7-1200 / S7-1500 ourselves, daily; we are not a Siemens sales channel.

What this is and when you need it

TIA Portal Function Blocks with instance Data Blocks are the Siemens answer to AOIs. The mental model is similar but the memory rules are different. The walkthrough below is the same sequence we use when teaching this on the simulator. Every step names the exact menu path or keystroke; if a name has changed in your version of TIA Portal V18+, it is called out. The simulator runs the same logic flow without the licence cost — ladder, FBD, and ST in a browser, with a virtual CPU you can download to.

Walkthrough

1. Understand FB vs FC vs OB

Three block types do reuse work in Siemens: OB (Organization Block, called by the runtime — OB1 for cyclic, OB35 for cyclic interrupt), FC (Function — stateless, no instance memory), and FB (Function Block — stateful, gets an instance DB). FBs are the AOI equivalent. An FC is closer to a static C function; an FB is closer to a class instance.

2. Create a Function Block

Right-click Program blocks > Add new block > Function Block. Name it Motor_Start_FB. Choose LAD, FBD, STL, or SCL. Click OK. The interface declarations open at the top: Input, Output, InOut, Static (your AOI Local equivalent — persists between calls), Temp (scratch, cleared each call), Constant. Static is where you put the seal-in latch.

// Motor_Start_FB interface
// Input:  StartPB Bool, StopPB Bool, OL_Trip Bool
// Output: RunCmd Bool, Fault Bool
// InOut:  MotorData "udtMotor"
// Static: SealLatch Bool

3. Call the FB and pick instance DB style

When you call the FB from OB1, TIA Portal asks for the instance type: single instance (creates a dedicated DB — DB10 for the first call), multi-instance (the static memory lives inside the parent FB's instance DB), or parameter instance V17+ (instance pointer passed in, similar to AOI InOut). Single instance is the default and the most predictable for first projects.

4. Reference instance memory correctly

Inside the FB, parameters are referenced as #StartPB, #SealLatch (the # prefix marks them as interface-scoped). From outside the FB, instance values are reachable as DB10.SealLatch or Motor1_DB.SealLatch when symbolic. Optimised access DBs (the V13+ default) hide raw addresses — you can't address bytes by offset, only by symbol.

5. Compile and watch the cross-reference

Ctrl+B compiles. The Cross-reference (View > Cross-references) shows every place an FB is called and every instance DB it owns. If you see two call sites pointing at the same instance DB, that's the equivalent of the Studio 5000 'shared instance tag' bug — the second call clobbers the first call's state on every scan.

Common mistakes

• Putting persistent state in Temp variables thinking they survive scans — Temp is scratch and is undefined at the start of every call
• Manually editing the instance DB number to share it across two call sites — the static memory gets overwritten each scan and the FB behaves erratically
• Mixing optimised and standard access DBs in one project without realising Siemens treats addressing differently — optimised hides offsets entirely
• Calling an FB without an instance DB — TIA Portal will compile only if you call as a parameter instance, otherwise the call site is invalid

Each of these mistakes shows up in real projects every week. The simulator catches the first three at compile time; the fourth one only surfaces on hardware, which is why we recommend running the cert packs against a real CPU once you have completed the curriculum modules.

How this fits the broader curriculum

Siemens TIA Portal FB instance DBs vs Allen-Bradley AOIs is one of the building blocks. The full Siemens curriculum on the simulator covers: programming-language fundamentals (ladder, FBD, ST), tag and variable scope, HMI tag binding, comms setup (Profinet / EtherNet/IP / Modbus depending on the platform), and the brownfield troubleshooting pathway. Each is its own module with worked examples and a portfolio piece. The cert packs at the Pro tier align to the ISA CCST exam content outline. Reference: isa.org.

For the platform-pick decision — when Siemens is the right call versus a different brand — see the brand hub. For region-specific context on where Siemens dominates the SA install base, see the relevant city pages under /brands/siemens/training-in-* and the sector pages under /industries.

Where this sits in a working week

A technician who has finished this module typically spends the next three to four working days running the same logic flow on hardware. The simulator's value is the dry run — getting the keystrokes and the IDE conventions into muscle memory before you sit down with a live CPU. The first time you build this on hardware, expect the IO mapping and the addressing conventions to slow you down for a session or two; the simulator's project tree mirrors the same shape so the transition is short.

The full Siemens curriculum runs roughly 60 to 100 hours of focused practice. That breaks into bit logic and timers in the first 20 hours, FBs and structured data in the next 20, comms and HMI in the next 20, and a portfolio piece in the last block. Pace yourself — three or four hours per session, four sessions a week, and you finish in eight weeks. Most of our learners report that the bottleneck is not understanding the IDE, it is building reflex around the conventions: where Siemens expects you to put state, how it scopes variables, what naming patterns the OEMs in the sector use.

Vendor reference

Siemens's own documentation is the canonical reference once you are working on real hardware: Siemens Industry Online Support. The simulator covers the basics; the vendor docs cover everything specific to a hardware revision, a firmware update, or a CPU-specific quirk. Bookmark both. The IEC 61131-3 standard that governs all the Siemens programming languages is at iec.ch.

What we don't claim

This site is not SAQA-registered, not MerSETA-accredited, and not an NQF-registered qualification provider. Our completion certificates are course-level only — they describe what you covered, not an NQF Level X qualification. The CCST cert from ISA is the portable industry credential we recommend; we are not an ISA cert delivery partner either, but our cert packs are CCST-aligned. The walkthrough above is brand-specific because Siemens's tooling has its own conventions; do not assume the same menu paths exist in another brand's IDE.

How to start

You can be running siemens tia portal fb instance dbs vs allen-bradley aois in the simulator in 5 minutes. Free tier covers the basics, no card, no install. Once you are 20 minutes in you will know whether the platform fits how you learn. The full Siemens curriculum is the Basic tier (USD 12 / month). The cert packs and portfolio export sit in the Pro tier (USD 29 / month). For institutional buyers — TVET colleges, private training providers, in-house engineering training departments — the bulk-licence option is the Teams tier, USD 199 per seat per year, minimum 5 seats. The training-centres page has the institutional pitch and the contact form.

Honest expectations on the local job market

Petrochem, mining, FMCG, automotive, and water-utility sectors all carry Siemens install bases somewhere in their stack. Knowing the IDE conventions on this page does not get you a job by itself; it gets you past the first technical screen. The portfolio piece — a working program you built yourself, with a wiring track, a tag list, an HMI screen, and a short README explaining the design choices — is what lands the second interview. The simulator's portfolio export bundles all of that into a single folder you can hand a hiring engineer. Recruiters in this space skim the README first; if your design choices are coherent, they read the code.

Load-shedding has reshaped what gets built first in Siemens programs across SA. Power-recovery patterns — controlled shutdown on UPS hold, state recovery from retentive memory, sequenced restart of motor groups — now belong in the same module as the basics. Siemens TIA Portal FB instance DBs vs Allen-Bradley AOIs fits into that shape: every line of code you write needs to consider what state the controller is in when it powers up after a 2.5-hour cut, not just what state it is in when running. The simulator's restart-from-cut mode lets you exercise this without bricking real hardware.