My Teensy won't communicate via USB Serial - TeensyUser/doc GitHub Wiki

Cables
- Did you check your cable? There are a lot of charge only cables out there which have no communication lines. NOTE: good cables can go bad.
- Teensy 4.x uses 480 Mbps USB unlike prior Teensys that work at 12 Mbps. So, cheap/bad cables which worked fine with a T3.x might fail with a T4.x.
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- Did you wait for an established USB connection before sending data (see here for details)?