I had a Flipdot Display for a while now and most of those are controlled with the help of the FP2800A IC. Since this IC isnt produced anymore you can only buy used ones. But when you get those you need to test them. So you can either pop them into a working display, risking to damage it if the IC doesn’t work or you can go through 28 addresses by hand and measure.
Or you use this little tool I made specifically for this task. It uses an Arduino Uno (or compatible) to address the IC and shows the state on LEDs.
Sources for Hard- and Software can be found on Github and the hardware sources are ready to be produced by JLCPCB (not a sponsor) including the SMD assembly.