How to control a Bus LED display

[Ju]

Good evening, I need some help. I have 14 LED displays for buses that could be used to show destinations on the side of a small bus (they are 90x8 LEDs) or to display upcoming stops inside the bus. These displays come from a company that went out of business and were used for the Tisseo network in Toulouse and its surroundings, in France.
I am desperately looking for a way to control them from a PC. I don't have a device to load configurations like Hanover systems do. I've created a power supply for the display using a printer power supply with a 24V, 0.8A output. A LED lights up on the motherboard, so I believe the power supply is working, but communication with the device is impossible.
There are 8 ports on the display: 24V, 0V, A+, B-, BP1, RxD, GND, and TxD. I've already tested communication between the PC and the GND, TxD, and RxD ports using a USB to RS-232 adapter, and communication between the PC and the GND, A+, and B- ports using a USB to RS-485 adapter. Neither solution worked; I was never able to display anything on the screen.
I'm asking for your help to understand whether communication with the display requires a specific method, a different connection setup, or if it's even possible to send data directly from a PC to a display. Thank you in advance for your assistance!