Siemens identifies almost everything it makes – PLCs, drives, contactors, breakers, motors – with an order number. Siemens has called it the MLFB (from the German for “machine-readable product designation”) and, more recently, the article number; on labels and in catalogs it is the same string. Two modules with the same description can have different order numbers and behave differently, so the number, not the description, is what you match. This guide explains how to read it; for sourcing discontinued S5, S7-300 and older drives, see finding obsolete Siemens parts.
Key takeaways
- The full order number (MLFB or article number) identifies one exact product; the description does not.
- The first characters give the product range – for example 6ES7 for SIMATIC S7, 6SL3 for SINAMICS drives, 3RT for SIRIUS contactors and 3VA for molded case breakers.
- The product version (E-Stand or FS) is the hardware release; the firmware version (V…) is the software inside.
- For CPUs and drives, check firmware and software compatibility before a swap; for contactors, check the coil voltage code.
How a Siemens order number is built
Take 6ES7 315-2AG10-0AB0, an S7-300 CPU 315-2 DP. It reads in blocks:
6ES7– the product range: SIMATIC S7 automation.315– the product type within that range: here, the CPU 315.-2AG10– the variant: which version of CPU 315 it is.-0AB0– further options and details.
Each position has a meaning that Siemens defines for that product range, so a character that means one thing on a PLC module means something else on a contactor. Decode with the catalog or the Siemens online product pages, not by comparing with another range. Many labels and catalogs print a space after the first block (6ES7 315-…) and others do not; the space has no meaning, so search both ways.
Common prefixes
The first block tells you which family you are holding. A few you will see constantly:
| Prefix | Product range |
|---|---|
6ES5 | SIMATIC S5 PLCs and I/O (legacy) |
6ES7 | SIMATIC S7 PLCs and I/O; the next digits narrow the family, for example 6ES7 3.. for S7-300 and 6ES7 4.. for S7-400 |
6AV | SIMATIC HMI operator panels |
6GK | SIMATIC NET industrial communication |
6SE | Older drives, such as 6SE70 MASTERDRIVES and 6SE64 MICROMASTER 4 |
6SL3 | SINAMICS drives |
3RT | SIRIUS contactors |
3RH | SIRIUS contactor (control) relays |
3RU / 3RB | SIRIUS overload relays, thermal and electronic |
3RV | SIRIUS motor starter protectors |
3VA / 3VL | SENTRON molded case circuit breakers |
5SY | Miniature circuit breakers |
3SU / 3SB | Pushbuttons and pilot lights |
Within a range, the digits after the prefix often encode the generation and size. On SIRIUS contactors, for instance, 3RT1 and 3RT2 numbers belong to different generations, and the digits that follow encode the size and rating. Treat these patterns as a way to recognise a part, and confirm the details in Siemens' documentation.
Low-voltage controls: the suffix matters
On contactors and relays, part of the last block encodes the coil voltage. Two contactors that differ only in the coil code are the same size and rating, but one may be for 24 V DC control and the other for 230 V AC – they are not interchangeable in the same circuit. On breakers, the later characters describe the trip unit, rating and connection type. Match every character, or check exactly what the differing one means.
Product version: E-Stand and FS
The order number stays the same when Siemens releases a revised version of the same product. The product version tracks those hardware releases. On many labels it is marked “E-Stand” (from the German Erzeugnisstand) and printed as a row of numbers with the current version crossed or marked; some labels show it after “FS” instead. A later product version is normally intended to replace an earlier one, but Siemens' product information for the order number is where differences are documented – read it for controllers, communication modules and drives.
Firmware version
CPUs, communication modules and drives also carry a firmware version, printed as a V-number such as V2.6 or V3.3 and readable online with the engineering software. The hardware configuration in a STEP 7 or TIA Portal project records both the order number and the firmware version of each module. As a rule:
- a module with newer firmware can usually run a configuration made for an older version;
- a module with older firmware than the project expects may refuse the configuration;
- firmware can often be updated with files and instructions from Siemens' support site, but not every module can be taken back to an older version.
Check which software your project needs, too. S7-1200 and S7-1500 are programmed in TIA Portal; S7-300 and S7-400 projects may be in classic STEP 7 or in TIA Portal. A replacement that needs a newer software version than you have is not a quick swap.
Before you replace a Siemens part
- Photograph the label: order number, product version and firmware in one frame.
- Match the full order number. If Siemens names a successor, read what changed – a successor can belong to a different generation with different wiring or software needs.
- Back up the program and settings: CPU program and memory card, HMI project, drive parameters.
- Check memory cards: many S7 CPUs need a memory card, and it may hold the program. Find out whether yours moves across or a new one is needed.
- Check firmware and software against your project before the outage, not during it.
Safety first: replace modules, drives and contactors only if you are qualified to do so, following your site's lockout/tagout procedure and the manufacturer's instructions. Drives hold a dangerous charge after power is removed; wait the time on the label and verify zero voltage with a properly rated meter.
Buying Siemens parts as surplus
Surplus is often the quickest source for discontinued Siemens parts and for current ones with long lead times. Every item we stock is photographed, so you can read the order number, product version and, where printed, firmware on the label before you buy. Search by the full number, such as 6ES7 SIMATIC parts, 6SL3 SINAMICS drives or 3RT contactors, or send a list of order numbers as an ISO list or RFQ for an official written quote in under 24 hours on business days. Other makers number their parts differently; see our guide to Allen-Bradley catalog numbers and the MRO glossary.
Frequently asked questions
What is an MLFB number?
MLFB is Siemens' name for its machine-readable product designation: the order number, now usually called the article number, such as 6ES7 315-2AG10-0AB0. It identifies one exact product, including its variant and options.
What does E-Stand mean on a Siemens label?
E-Stand (from the German Erzeugnisstand) is the product version: the hardware release of that order number. It is often printed as a row of numbers with the current version marked; some labels show the version after FS instead.
Does the space in a Siemens order number matter?
No. Siemens often prints order numbers with a space after the first block (6ES7 315-…) and elsewhere without it. The characters and hyphens are what matter, so search both ways if you get no result.
Is a later product version of the same order number a drop-in replacement?
Usually a later product version is intended to replace an earlier one, but check Siemens' product information for that order number: firmware, software support or accessories can differ between versions.