I'm researching how people source obsolete/legacy industrial automation parts.



For example: discontinued Allen-Bradley PLCs, Siemens S7-300/S7-400 modules, old HMIs, drives, etc.



When a production line goes down and you need a specific part urgently:



-> Where do you usually look first?

-> How many suppliers do you typically contact?

-> What's the biggest problem: price, availability, lead time, trust/counterfeits, finding the exact revision, something else?

-> Do you usually buy from the same supplier/broker, or compare multiple options?

-> Have you ever had trouble finding a part that was supposedly in stock?

-> For an urgent $1k–$10k+ part, would you consider buying from a marketplace that let multiple specialised suppliers quote on the same RFQ?



Also interested in hearing what would make you NOT trust such a marketplace.

 

Direct answers first, then the part that actually decides whether it works.

- Where I look first: the OEM's authorised distributor, even for a discontinued item. They hold last-time-buy stock, and they will tell you what supersedes it. Then a specialist obsolete-parts stockist — someone who owns the stock rather than searches for it. Marketplaces and auction sites last, and only with the checks below.
- How many suppliers: two or three, deliberately chosen for different things — one authorised (provenance), one specialist stockist (availability), one repair house (because repairing your own module is sometimes faster than buying one).
- Same supplier or compare: a small stable set, and I stay with them because they have been proven. Comparison shopping is useful for price, not for trust. Trust is decided by how they behave on the first problem, not on the first quote.
- Supposedly in stock: yes, repeatedly. "In stock" frequently means "we believe we can get it". Ask whether the unit is physically in their warehouse, and ask for a photo of the actual unit before you pay.

Now the answer to your third question, which is where I would push back

Price is rarely the binding constraint. Availability and lead time are real, but they are visible and you can plan around them. The constraint that actually hurts is the one that is not on your list: it arrives, it is the right part number, and the machine still does not run. Everything below is about that.

Five ways an urgent legacy purchase fails

1. Same part number, different revision or firmware. The label looks identical; the behaviour is not. Firmware too new may be refused by the existing CPU or the engineering software you still have; firmware too old may lack a function your program relies on. Put hardware revision and firmware version into the RFQ as mandatory fields, and require a photo of the actual label before shipping.

2. Refurbished sold as new. Very common on discontinued items. Things that give it away: date codes inconsistent with "new", flux residue or rework around the board edges, mismatched screws or bent tabs, a new label applied over an old one. Ask directly — new, refurbished, or pulled? — and make them write the answer into the quote.

3. The part is not just the part. What makes it work is everything around it: the memory card, the retentive battery, the terminal cover, the connector set, the licence, the documentation. A CPU that arrives without its program is a paperweight. Specify the scope as "CPU plus program plus licence plus battery plus connectors", not "CPU".

4. You buy it and cannot program it. Old projects need an old version of the engineering software, and those versions are frequently no longer licensable, or need an operating system you no longer have. Settle this before you pay, not after: which software version, which OS, and do you actually hold a licence?

5. It was never the fault. Urgency pushes people to buy the module they suspect. Often the fault is a supply, a wiring problem, an earthing issue or a peripheral, and the replacement behaves exactly the same. An evidence-based diagnosis before ordering saves more money than any negotiation — and it is the step that gets skipped when a line is down.

What separates a supplier from a broker

A broker answers the availability question. A supplier answers the questions you did not ask. The ones worth keeping do the following: they fill in the revision and firmware fields instead of ignoring them; they send a photo of the actual unit rather than a stock image; they volunteer that it is refurbished when it is; they state warranty and DOA terms in writing with a return window; they can tell you what else in your system will have to change if you fit it; and they will offer to bench-test it — a willingness to test is a sign of confidence, not an upsell.

What would make me distrust a marketplace

Since you asked specifically, these are the ones that would lose my business:

- The RFQ has fields for revision and firmware and the quotes come back blank. That tells you the seller is a search engine, not a stockist.
- Stock status that is "in stock" right up until you pay, and "three to four weeks" the moment you have.
- No way to see the actual unit before purchase. A stock photo of the same model, taken in 2014, tells you nothing about the 20-year-old board you are about to receive.
- Price as the primary ranking signal. On legacy parts the cheapest quote is very often the refurbished one described as new, and you find out at commissioning.
- No identifiable seller — company, address, registration — and no stated return path.
- No DOA policy in the listing. It should be there before you ask, not negotiated after it arrives dead.
- "We can source anything" listings that clearly hold no stock of their own.

Checklist before you pay

1. Freeze the requirement as exact part number plus hardware revision plus firmware version plus the list of items that must be included.
2. Demand label photos and the date or batch code.
3. Ask new, refurbished or pulled — in writing, in the quote.
4. Get warranty and DOA return window in writing.
5. Confirm which engineering software version and licence you will need.
6. Have a bench-test plan for day one, not "fit it on the live line and see".
7. After it works, keep a cold spare and write down the restore procedure.

One thing worth saying about the marketplace idea

The fastest way to get a line running again is often not to buy the discontinued part at all. If you know what that module does in the sequence — its I/O, its role, what changes if a substitute is fitted — you can sometimes restore production with a modern equivalent or a documented interim arrangement, and then source the legacy part at leisure instead of at panic price. That capability comes from the I/O list and the sequence documentation rather than from a supplier, and that documentation is exactly what most plants cannot find on the day the line stops. Worth considering as a feature: a marketplace that helped with the interface documentation would be solving a problem the price-comparison ones cannot reach.

 

For obsolete automation parts, you need to verify the exact MPN and revision first. Availability can also be misleading, especially with legacy equipment. It's better to contact several suppliers when downtime is costly. Photos, testing details, warranty, and return terms would affect trust. For urgent $1k–$10k purchases, consider multiple supplier quotes. Always ask for verified inventory and supplier credentials before paying. Unikeyic can be referenced when buyers need to verify MPNs, datasheets, stock status, and alternative sourcing options.