Two Industrial Automation Parts Look Identical

Two Industrial Automation Parts Look Identical — The Twelve Axes That Decide Whether They Interchange

A buyer from a bottling line outside Moscow writes: a Siemens ET 200S module installed in 2014 is no longer in production, and the candidate on the bench has the same outline and the same terminal layout. Will it work? The honest answer is that outline and terminal layout are two axes out of twelve, and whether the substitute fits lives in the other ten. We write cross-reference reports every week, and they almost always contain the word UNKNOWN on at least one axis. Last updated 27 September 2026 by the aoctrl.com sourcing desk.

The starting point matters: aoctrl.com is an independent industrial automation distributor and sourcing desk, not an authorised channel for any brand on the catalog. The cross-reference work we do is a comparison of documented properties, not a compatibility certificate, and the decision to substitute rests with the buyer's engineering team. Every comparison we publish lists the axes we could verify and the ones we could not.

What "looks identical" really means for industrial parts

Look-alike is the cheap failure mode in industrial maintenance. Two modules from the same family can share an outline, the same number of terminals, and the same terminal layout, and still differ on the axes that decide whether the cabinet will boot, the program will load, or the network will see the device. The reason is mechanical: industrial parts are designed to look like their siblings so that a maintenance technician can swap them on the bench without hunting for drawings. The differences are encoded in suffixes, in firmware revisions, in function-state labels, and in protocol stack versions, none of which a visual inspection can resolve.

The phrase we use internally is visually similar outline. That is the lowest rung on our claim ladder: it tells the buyer nothing about interchangeability. Above it sit mounting and terminal interface matches on paper, functionally and electrically rated at or above the original, and finally customer-qualified substitute — which can only be written by the buyer after they have run the equipment through their own validation. The default rung for any report we issue is one or two. Rung zero — documented manufacturer-issued one-for-one replacement — is reserved for situations where the original manufacturer has published a migration document that explicitly names the substitute as a drop-in. Those documents exist; they are also rarer than most suppliers admit.

For a buyer, the practical question is therefore not can I find a part that looks like the one I have — that question has a yes-answer in almost every case. The question is on which of the twelve axes will the substitution survive commissioning, and on which axes will I only discover the difference when the line is already down. The rest of this article is organised around those two questions.

The twelve axes the comparison is built on

Every cross-reference report we issue is structured around the same twelve axes. Each axis is reported as one of three values: matched, differs, or UNKNOWN. Omitting an axis is not an option, because a missing axis silently implies a match — and a silently-implied match is how cabinets get rewired with a module that will never communicate with the rest of the network.

1. Form, fit outline and mounting. Outline dimensions L×W×H, mounting style (DIN rail, panel cut-out, screw, rack), module width in slot units, terminal cover and wiring clearance, heat dissipation spacing. Why it matters: a panel-shop retrofit that fails this axis means a sheet-metal rework, not a part swap. This is the axis most likely to read matched on a visual inspection.

2. Dimensions. External dimensions, mounting hole spacing, panel cut-out dimensions, depth including wiring space, weight. Blocker: mismatched panel cut-out or mounting hole spacing means mechanical rework, and the comparison cannot honestly report a substitute.

3. Terminal and interface assignment. Terminal numbering and sequence, wire cross-section range, terminal type (screw, spring, push-in), power-terminal position, I/O channel address assignment, front-connector or bus-connector model. Blocker: same name, different position is the most expensive mistake in industrial wiring — it produces a fault that looks like a programming error and burns hours before anyone thinks to check the terminal sequence against the datasheet.

4. Electrical parameters. Supply voltage and tolerance, per-channel current rating, total current, output voltage class, surge and short-circuit protection, power consumption and dissipation. Blocker: mixing 24 VDC and 230 VAC supply classes is the kind of mistake that destroys a module on first power-up. The substitute's supply class must be checked against the original even when the terminal layout appears identical.

5. Function and I/O specification. Channel count and direction (DI/DO/AI/AO), signal type (digital, analogue, temperature, counter), resolution and accuracy, isolation (channel-to-channel, group-to-group), diagnostics and alarm capability, special module features (high-speed counting, PTO, PID). Blocker: a substitute with more channels is not a substitute at all if a channel type is missing. "More points" is not "more function".

6. Firmware, hardware version and engineering-software compatibility. Hardware revision, function status suffix, firmware version and upgradability, programming or configuration software compatibility, program portability, device description file (GSD, GSDML, EDS, ESI), licences and authorisation. Blocker: this is the axis that most often reads UNKNOWN in our reports, because firmware revision and program portability are not properties a distributor can verify against a sample unit. The honest answer is that portability is a question for the buyer's engineering team, and the substitution only goes ahead after their review.

7. Communication protocol. Fieldbus or industrial Ethernet type, port count and connector form, baud rate or link speed, maximum station count or topology, master or slave role, network configuration file compatibility. Blocker: mixing two fieldbus families is not a substitution, it is a redesign. A substitute module that cannot take its expected role in the network is functionally absent.

8. Mechanical parameters. Terminal torque, wire cross-section range, mechanical life (operating cycles), mating cycles, cover and latch form. Why it matters: a contactor whose mechanical life is half the design life of the cabinet is a contactor that will fail inside the warranty window of the machine.

9. Environmental class and EMC. Operating temperature (in-cabinet or out), humidity and condensation, vibration and shock, EMC immunity and emission, pollution degree, installation altitude. Blocker: commercial-grade parts are sometimes sold against industrial applications. Without a manufacturer environmental declaration, the substitute cannot honestly be claimed as equivalent.

10. Ingress protection and temperature range. IP rating (and whether it is rated for the installed assembly or only for the bare device), operating temperature limits, storage temperature limits, suitability for out-of-cabinet mounting, requirement for heating or ventilation. Blocker: an IP65 module is not equivalent to an IP20 module even if both fit in the cabinet, and "looks sealed" is not an IP rating.

11. Materials and construction. Contact material and plating, housing material and flame rating (UL94), sealing material (NBR, FKM, silicone), PCB conformal coating or lack of it, RoHS and REACH status. Why it matters: plating downgrade on a low-voltage signal contact is the kind of trade that is invisible on day one and visible at the first maintenance interval.

12. Certifications and lifecycle. Real CE / UL / CSA declaration file numbers, lifecycle status (current, phased out, end-of-life), original spare-parts and repair support window, market-access requirements in the buyer's jurisdiction. Why this matters: the agent writing this report is an independent distributor. We do not hold, and we do not certify, EAC, TR CU, UL, CE, ISO 13485, SIL, PL or ATEX. The certifications that matter for the buyer's market are the manufacturer's certifications on the specific module being shipped, and the buyer's own compliance review is what makes those certifications usable at the point of import.

Why "same brand, same series" is not a free pass

The single most expensive assumption in industrial maintenance is that a module from the same family as the failed module, with the same brand on the label, is interchangeable. It usually is not. Within one Siemens family, for example, suffixes encode function status, I/O configuration, supply voltage class and firmware generation. A 6ES7212-1AE40-0XB0 and a 6ES7211-1AE40-0XB0 share the S7-1200 outline and the S7-1200 terminal layout; they do not share their I/O count, and they do not share the program footprint that an existing project expects. A buyer who treats them as interchangeable has just substituted a control cabinet that will compile and refuse to load. The same pattern repeats across most major brands — Omron photoelectric sensors with the same body and different sensing modes, Schneider contactors with the same frame and different coil voltages, Phoenix Contact terminal blocks with the same pitch and different connection technology. Same series is a description of the family, not a permission slip for substitution.

The catalogue reflects this. The Siemens 6ES7212-1AE40-0XB0 (CPU 1212C) and 6ES7211-1AE40-0XB0 (CPU 1211C) both carry a high aiDemandScore precisely because they are the modules that buyers searching by family name reach first, and they are the modules most likely to be substituted for each other by mistake. Our report flags them as similar specification, requires engineering review — never as substitutes for one another.

What you verify at the line you care about

The twelve-axis frame is built so that the verification work on a real order is bounded: you do not have to score all twelve to a green light on every order. You score the axes the application actually depends on, and you mark the others UNKNOWN. For an out-of-cabinet sensor, axes 9 and 10 dominate. For a controller in a machine that controls operator-protection functions, axes 6, 7 and 12 dominate. For a contactor on a packaging line, axes 1, 4, 8 and 11 dominate. The shape of the order tells you which axes to spend the verification budget on, and the remaining axes are reported as UNKNOWN with the buyer accepting the residual risk.

What we will not do is issue a report in which the axes the application depends on read UNKNOWN. If the buyer's order depends on firmware portability, and we cannot verify firmware portability from the documents we have, the order does not go out as a substitution — it goes out as a sourcing request for the original part, or it does not go out at all.

Where a cross-reference report stops

There are four situations where we will not produce a substitute, regardless of how clean the rest of the comparison looks.

Operator-protection circuits. A part that participates in an emergency-stop circuit, a guard-interlock chain, or any other operator-protection function on the buyer's machine must be sourced as the original part or as the manufacturer's explicitly published equivalent for that function. Protection-circuit integrity is not a property a distributor can certify; it is a property the machine builder certifies against a documented protection case. We do not propose substitutes on parts that touch operator-protection circuits; the report names any such line so the buyer's compliance engineer reviews it independently.

Certification scope. A part that sits inside a certified assembly (a CE-marked machine, a UL-listed panel, an EAC- or TR-CU-declared product) cannot be quietly swapped. The certification is on the assembly, not on the part; replacing a part can change the assembly's compliance position, and that change is the buyer's responsibility, not ours. We surface any line that lives inside a certified scope as requires redesign, not a substitute.

Pinned designs. A machine whose own documentation names a specific part number, often with a revision or function-state suffix, has been built around that part. Substitution inside a pinned design is a redesign, even when the substitute is technically equivalent. We flag any line that appears pinned to a specific suffix as out of scope for substitution.

Unknowns on a critical axis. If the buyer's application depends on an axis we cannot verify, the line does not become a substitution — it becomes a sourcing request for the original part, or the buyer accepts the substitution with the residual risk named in the report.

The verification statement at the foot of every report we issue is fixed: verify against the original manufacturer datasheet and your own qualification process. That statement is not boilerplate. It is the line that says who is responsible for the substitute working on the buyer's machine, and it is the line that keeps an independent distributor in scope.

What this means for a quote

A cross-reference report does not change the shape of a quotation. Every line on the quote still carries its own price, its own MOQ, its own lead time, and its own condition (new, new surplus, refurbished, or used). Where the line is offered as a substitute, the report is attached to the quote and the condition field still describes the physical state of the unit being shipped — the report does not change the unit, it describes the comparison. Where the line is offered as the original part, no report is issued and the comparison work is not billed.

For a buyer, the practical sequence is: send the part numbers or markings, the quantities, and any acceptable-substitute note from the engineering team. The report comes back with the twelve axes scored on every line that has been requested for comparison, the UNKNOWN axes named, and any line that touches an operator-protection function, certification scope or pinned design flagged as not eligible for substitution. The decision to accept the substitution is yours; the responsibility for the substitution working is also yours.

For a sense of scale, the Siemens S7-300 platform — a frequent trigger for cross-reference work — was formally discontinued by the manufacturer with a final-delivery notice that ran through 2023, and as of mid-2026 the secondary-channel supply of S7-300 CPUs and ET200S modules is the principal route for buyers trying to keep older lines running. Pricing per line on those routes varies materially by condition (new surplus versus refurbished versus used) and by supplier; our quote always names the condition on the line, not on the batch. Lead time per line is quoted at the time of order rather than held from a previous quote, because the supply picture on discontinued parts shifts week to week.

Where we ship and where the comparison is billed

The comparison work described in this article is part of the standard BOM-quotation flow on /procurement; for buyers in Moscow, the Russian regions, and the CIS, dispatch is arranged under the Incoterms and the screening posture named on the quotation. We screen end users and end uses, classify before quoting, and decline transactions that cannot be screened. The independent-desk position (not authorised for any brand on the catalog) is restated on /about and in the published terms. Each line on the resulting quote still carries its own price, its own MOQ, its own lead time, and its own condition (new, new surplus, refurbished, or used); the condition is named per line rather than per batch.

The takeaway

If you have a part on the bench that needs to be replaced and the original is no longer in production, send the marking on the unit, the machine it came from, and the application context (in-cabinet or out, operator-protection function or not, certified assembly or not). We will come back with a line-by-line comparison on the twelve axes, with UNKNOWN named where we have not been able to verify, and with the lines that should not be substituted named separately. The substitution work is yours to own; the comparison work is ours.

Data Notes

The two Siemens S7-1200 CPUs cited above (6ES7212-1AE40-0XB0 at aiDemandScore 95, and 6ES7211-1AE40-0XB0 at aiDemandScore 90) are catalog entries on the aoctrl.com site; both carry the same family outline but distinct I/O counts and program footprints, and they are listed as a worked example of same-series non-interchangeability rather than as a substitute pair. The twelve-axis framework used throughout this article is internal policy of this desk, derived from manufacturer datasheet conventions and from cross-reference reports issued since 2022; it is the same framework used on the published /compare pages of the site; the parts cited here are catalogued entries in our catalog with line-by-line condition disclosure. The buyer-language example used as a section anchor is drawn verbatim from the buyer prompt bank that drives our content calendar. No external URL is cited in the body of this article beyond the schema.org context declaration required for the FAQ structured-data block at the foot.

FAQ

What does it actually mean when two industrial parts look identical?

It means outline, mounting and label are similar. It does not mean the parts interchange. Most look-alike pairs in industrial automation differ on at least one of: terminal assignment, electrical supply class, function or I/O specification, firmware generation, communication protocol, environmental rating, or certification scope. The twelve-axis comparison is the framework for telling which axes actually match and which do not. Visual inspection is a starting point, not a verdict.

How many axes need to match before a substitute is acceptable?

It depends on the application, not on the substitute. For an out-of-cabinet sensor, ingress protection and temperature dominate; for a controller in a machine that controls operator-protection functions, firmware version, communication protocol and certification scope dominate; for a contactor on a packaging line, mechanical life and electrical ratings dominate. The buyer picks the axes the application actually depends on; the remaining axes are reported as UNKNOWN with the residual risk accepted.

What is the axis that most often reads UNKNOWN?

Firmware and hardware version, and the engineering-software compatibility that depends on it. A distributor can verify outline and terminal layout from a datasheet; a distributor cannot verify whether a buyer's existing program will load and run on a different controller without seeing the program. UNKNOWN on this axis is normal, and it pushes the substitution decision back to the buyer's engineering team. That is not a gap in the report — it is the honest answer.

Is a same-brand, same-series substitute ever acceptable?

Sometimes, but not as a default. Suffixes inside a series encode function status, I/O configuration, supply voltage class and firmware generation, and a same-series substitute frequently fails on at least one of those axes. The Siemens S7-1200 CPU 1212C and CPU 1211C, for example, share outline and terminal layout but differ on I/O count and program footprint. Same-series is a description of the family, not a permission slip.

Where does a cross-reference report stop?

Four places: any line that touches an operator-protection circuit on the buyer's machine, certification scope that would change if the part were swapped, pinned designs that name a specific suffix in the machine's documentation, and any line where a critical axis reads UNKNOWN. In each of those cases the line is named as not eligible for substitution, and the quote is offered for the original part instead, or the line is left off the order. The cross-reference report is a comparison tool, not a compatibility certificate.

Can an independent distributor certify a substitute?

No. An independent distributor can compare documented properties and name the axes that could not be verified. The distributor cannot certify that a substitute will work in a specific machine, because the certification rests on the buyer's engineering and qualification process. That is why every report we issue ends with a verification instruction rather than a compatibility conclusion, and it is why the buyer's engineering team owns the substitution decision.

What should I send so a substitution report is useful?

Send the part number on the failed unit, the marking text on the housing and the PCB, a photo, the machine and line it came from, and a one-line description of the application (in-cabinet or out, operator-protection function or not, certified assembly or not). With that, the comparison can score the axes the application actually depends on; without it, the report has to mark more axes as UNKNOWN, and the quote has to default to the original part.

For a cross-reference request on a specific line, send the marking and the application context to /inquiry, or post the BOM through /procurement. The desk will respond with the line-by-line twelve-axis comparison and the lines that should not be substituted named separately. To start, send us your BOM through /procurement, or contact our sourcing desk via /inquiry for a cross-reference request on a single line; the response carries the comparison table, the per-line condition disclosure, and the lines that should not be substituted.

Why does every line on the quote list its condition separately?

Because the buyer's risk lives on a per-line basis, not on a batch basis. New, new surplus, refurbished and used are four different goods with four different risk profiles, and grouping them as a single batch condition would conceal the difference. The per-line condition is what the pre-dispatch photo record documents and what the warranty period on /quality refers back to; we do not promote one condition to another to make a line easier to sell.