Troubleshooting notes from the workbench — written for search, kept honest for engineers.
A CRC failure means the frame arrived but the math didn't. Noise, termination, clock drift, bad reframing or a second master — each leaves a different fingerprint. Here's the 5-minute triage.
Nine times out of ten it's the 40001-versus-0-based offset. The tenth time is more interesting.
The five-minute physical-layer check that saves an afternoon of protocol debugging.