I recently purchased 8 BrightSign AU335 players.
All of the devices had a systemic defect. They are equipped with 8 GPIO pins that can be configured as inputs or outputs, numbered 0 through 7.
On all of the devices, GPIO 4 and 5 were not functioning.
At the time of purchase, the players came with firmware version OS 8.2.17.3
If you update them to the latest versions currently listed by BrightSign for the AU335 (latest release: OS 9.1.173 or Beta OS 10.0.31), the players stop working entirely.
After researching the list of older firmware versions provided by BrightSign (https://docs.brightsign.biz/releases/brightsignos-releases#au335), I updated the players to OS version 8.3.46, which is slightly newer than an update that was supposed to fix the GPIO 4 and 5 defect. The players are now working properly. All GPIOs are functional.
Good to know for anyone who might encounter this problem!