Awesome, thank you @peekay123
The library I’ve used successfully on Teensy is https://github.com/PaulStoffregen/DmxSimple
Note that while it would be awesome if the Photon could simultaneously transmit DMX and be connected to the cloud, I’d still be very happy with an initial (interim?) solution where the cloud needed to be disabled during DMX use, and DMX disabled during cloud use. I would, however, like to have the Photon be able to respond to I2C (as a slave) while DMX transmission is happening (or, at least be able to interrupt or abort its DMX transmission in order to not miss the incoming I2C command, and then resume its DMX activity).