UART 3 Stop bits is undefined

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"According to the DMX 512 standard, the MTBF is set to high (1) and can be from zero to 250 kilobits long, or zero to one second long."

So if you're sending it, you don't need to include it at all (as it can be zero), and if you're receiving it, it simply looks like there is some idle time between two bytes, and generally your UART will ignore that.

Also, not sure if you saw it, but sending/receiving DMX with an ESP has been done before.