EPOS4 error code 0x5281 "Hardware incompatibility" - What to do?

I.) Situation:

An "EPOS4 Compact ..." reports permanently or sporadically the error code 0x5281 "Hardware Incompatibility" after power-on or reset.

• What can be the cause for error state 0x5281?
• What actions have to be taken to clarify and solve the issue?

II.) Technical background:

• The "EPOS4 Module" (= base PCB) of an "EPOS4 Compact" identifies if a matching "EPOS4 Connector Board" is mounted. This is done by checking some defined signal levels at start-up or after device reset.
• The error code 0x5281 is reported in case there is some incompatibility issue in between the "EPOS4 Module" and the "EPOS4 Connector Board" detected, i.e. expected signal levels do not match.

III.) Typical issues causing error code 0x5281:

The error code 0x5281 is typically reported in these cases:

1.  A mismatching type of "EPOS4 Connector board" is mounted, e.g. 
   • ... an "EPOS4 CB 24/1.5 ..." is mounted on an "EPOS4 Module 50/5"
     or
   • ... an "EPOS4 CB 50/5 ..." is mounted on an "EPOS4 Module 24/1.5"
2. There is a damaged or faulty soldering joint, a defective component, or a defective contact pin of an internal connector that is interfering with or interrupting the required signals for "Connector Board "detection.

Add-on remarks:

• In very rare case it had been observed that pins of the connectors in between the bottom "EPOS4 Module" and top "EPOS4 Connector Board" had been damaged due to multiple disassembly / assembly actions or a misaligned mounting once. Such damaged connector pins can force permanently present 0x5281 too. 
• In theory a wrong 180° orientation of a board would also result in an 0x5281 error state but such a completely wrong mounting is almost impossible (or just with a high force) because there is some mechanical coding present too.

IV.) Special issue:
Conductive plastic cover (March 2025 - April 2026)

The "EPOS4 Compact 50/5 ..." has a base plastic cover. This plastic cover has to ensure that ...

• ... bottom-side components of the EPOS4 Module cannot be touched or short-circuited by some metal contact (e.g., of a metal housing or base plate).
  and
• ... protect the electronics of ESD (= Electrostatic discharge).

The plastic cover typically has approximately a resistance of 100 kOhm to comply with both demands. 
 

IV.1.) Issue causing 0x5281 error messages:

During a limited production period from March 2025 to April 2026, an unsuitable material for the plastic ESD protective cover, whose surface resistance was not consistent and could be too low. 

• The resistance of plastic covers produced in between March 2025 - April 2026 may vary in some cases between low values of 1 kOhm up to 20 kOhm (instead of 100 kOhm). The resistance also depends on how close the two measuring points are to each other and how firmly the contacts press into the surface of the plastic.
• Under unfavorable conditions, the snap-fit clips on the installed plastic cover may create a connection that interferes with the correct detection of the hardware configuration during startup or after a reset.
  
  In the worst-case scenario, the resistance of the snap-fit clip, which presses against three signal pins, could be as low as 1 to 20 kOhm. This results in a signal level being detected that corresponds to an "EPOS4 CB 24/1.5 ..,” even though a correct "EPOS4 CB 50/5 ..." is actually mounted. In this state, the EPOS4 reports error 0x5281 "Hardware Incompatibility" due to the incorrect signal and the misleading information regarding the "Connector Board".
• It is also possible that the low-resistance snap-fit clip interferes EtherCAT communication internally, which passes through the soldering pads it contacts (in case of an "EPOS4 Connector Board ... EtherCAT"). Anyway, none of such EtherCAT issues have been observed to date.

IV.2.) Safety aspects of error code 0x5281:

In the event of error state 0x5281, commissioning and operation of the EPOS4 are technically impossible; in other words, the controller and its power stage are in a locked state.
 

IV.3.) Status of plastic cover production and delivery:

The plastic cover material (with possible low resistance) had been just in use for "EPOS4 Compact 50/5 ..." and "EPOS4 Compact 24/1.5 ..." product types produced in between March 2025 and April 2026. The error code and wrong hardware detection can just happen in case of an "EPOS4 Compact 50/5 CAN" p/n 541718 (but for an "EPOS4 Compact 24/1.5 CAN" p/n 546714). 

maxon has immediately reacted in April 2026 and stopped all deliveries. There is new improved plastic cover in use since May 2026. The "old" plastic cover had been replaced by the new one for all units on stock and any returned "EPOS4 Compact 50/5 ...".
 

IV.4.) Possible actions in the event of 0x5281 error messages

IV.4.a) Possible action locally: Cut-off snap-fit clips

If a quick on-site solution is required, users can manually remove the snap-fit clips of the plastic cover using a suitable cutting tool.

Important:
If the snap-fit clips are removed, the plastic cover is no longer securely attached to the "EPOS4 Module". A different type of fastening method is then required:

• Screw connection
  Normally, the "EPOS4 Compact ..." is secured at the installation site with two screws. Holes are provided for this purpose in the circuit board and the plastic cover. Such a screw connection also secures the plastic cover to the "EPOS4 Module".
  or
• Glue connection
  If mechanical fastening with the supplied screw connection is not possible, the plastic cover can also be glued to the bottom side of the "EPOS4 Module". 
  Please note:
  • The adhesive must not be conductive under any circumstances.
  • Gluing makes repairs or subsequent analysis impossible; therefore, maxon generally does not recommend this method.
     

IV.4.b) maxon actions in case of a return

Feel free to return the EPOS4 reporting the 0x5281 error state.
In case of any returns ...

• The plastic cover is replaced by one based on another plastic type. 
• It is checked and ensured that no other hardware issue is present in addition
  (e.g., damaged or failing components, bad solder pads, bad connector pins, etc.)
• The EPOS4s are fully tested and in perfect condition then again.

We regret any inconvenience that may have occurred. The information provided is intended to help you assess the situation and may assist in resolving the observed behavior.
If you have further questions regarding your specific application and requirements, maxon’s local sales organization or the maxon global support team will be happy to assist.

• Use maxon's support form:
  https://support.maxongroup.com -> Submit a request
• Send an e-mail to support@maxongroup.com

Thanks for your attention and understanding.