What happened?
On the Prusa CORE One, communication with the xBuddy extension board is achieved using the MODBUS protocol. In MODBUS, there are two types of devices: the master and the slave, also known, respectively, as client and server. On Prusa CORE One firmware, the master is the xBuddy, and each slave is called puppy.
The CORE One firmware includes all the needed files for xBuddy and puppies, including bootloaders. In case an incompatible bootloader is flashed, the printer will show the message: "Puppy uses incompatible bootloader protocol #, Buddy FW requires #".
Error name: Puppy incompatible bootloader
Error code: #31515
The error indicates most likely a configuration problem that might have stemmed from the firmware, or a faulty connection of the xBuddy extension board.
How to fix it?
Access the xBuddy and xBuddy extension boards by loosening six M3x4bT bolts to remove the cover. Once visible, check the boards for visible damage on their surface, and check the connection between xBuddy and xBuddy extension for any irregularities.
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