1. Record the complete MLFB, not just “CPU 315”
Photograph the existing label and record the entire Siemens order number (MLFB), including every suffix. For example, “CPU 315-2 DP” identifies a processor family, but the full 6ES7 code identifies a much more specific version. Check the hardware revision, firmware version and any functional differences that matter to the installed project. A similar front panel or CPU family label does not prove interchangeability.
Also record the currently fitted CPU's serial and order identifiers for your own maintenance records. Ask the seller for clear photographs of the replacement label and connectors; an incomplete or obscured label is a reason to pause the purchase.
2. Confirm the PLC project and restoration method
Before removing anything from a functioning machine, establish whether the original STEP 7 project and hardware configuration are available, whether they correspond to the installed programme, and whether the software and required licence can still be used. A memory card or backup file is not automatically a verified, restorable machine backup.
Check the CPU's memory medium and the correct handling requirements for that model. Some older S7-300 configurations use specific memory arrangements that cannot be inferred from the shape of a memory card. Record whether an original memory card is included with the replacement, whether it is required, and how application data is backed up. Do not assume a second-hand processor arrives with the programme for your machine.
3. Review communications and connected equipment
An S7-300 rack may interact with PROFIBUS-DP equipment, PROFINET devices, HMIs, drives, distributed I/O or a plant supervisory system. The intended replacement must support the communication interfaces and the configured network behaviour. Document modules, addresses, physical ports and any relevant network settings before a change.
Where a system includes failsafe PLC functionality, safety-rated communications or safety-related devices, treat the replacement as a safety-affecting engineering change. Obtain the appropriate programme, documentation and validation rather than assuming a processor change is routine.
4. Establish the actual condition and test evidence
| Listing description | What it does (and doesn't) tell you |
|---|---|
| Untested | There is no verified functional performance claim. Price and risk should reflect that uncertainty. |
| Powers up | Power-up was observed; programme execution, interfaces and application functions may remain unverified. |
| Online recognised | A programming tool or diagnostics system identified the unit; this does not prove every input, output or network function. |
| Tested working | Ask for the written test scope and results. A brief bench check is not equivalent to a full machine functional test. |
Where the unit has been removed from a working machine, that is useful history but not a substitute for present inspection. Check for missing front covers, damaged terminals, contamination, bent pins, faults indicated by status LEDs and omitted accessories. Confirm returns and warranty terms for the specific listing before purchase.
5. Check the rack and surrounding spares
CPU selection is only part of keeping a legacy machine operational. Confirm the power supply, interface modules, memory cards, front connectors and connected I/O catalogue references. If the installation relies on ET 200M or specialist communications hardware, identify those devices as separate critical spares.
When planning a longer-term spares holding, keep the part-number register, tested backup, electrical drawings, communications configuration and recovery method together. Make a sensible decision about stocking one known-compatible spare versus an unverified job lot.
6. Decide whether to replace, repair or modernise
For an otherwise healthy installation, a confirmed compatible spare may buy time at relatively low cost. But recurring failures, no recoverable programme, scarce interfaces or repeated downtime may justify an engineered migration plan instead. Moving to S7-1500 is not a direct plug-in CPU swap; hardware, software, networking, HMI and commissioning all need review.
Siemens documents lifecycle changes for the S7-300 and ET 200M families. The general family is in a spares-oriented phase, but the exact product record takes precedence: read AO's S7-300 lifecycle explanation and confirm manufacturer notices through Siemens Industry Online Support.
Details to send when requesting a replacement quote
- Complete Siemens order number and revision, plus photographs of both existing and proposed units.
- Quantity, required condition and whether a memory card or connectors are needed.
- CPU firmware, programming software, accessible project backup and key communication interfaces.
- Delivery country, deadline and whether the machine is currently stopped.
- The test scope you require; state any non-negotiable functional or safety requirements.
Automation Outlet can check current listed stock and request quotations for your specific 6ES7 order code. Availability, pricing, compatibility and test scope are confirmed for each proposed unit rather than assumed from a catalogue family.
Editorial review: 18 October 2026 · Automation Outlet, Cambridgeshire, UK. Guidance is general engineering information and does not replace manufacturer documentation, risk assessment or site procedures.