These displays are 16f54 based, PCB smaller than the display.
3-wire serial interface. Master PIC is 16f54 here as well. The protocol is not so easy to use.
You can see the source code here:
http://pic.hitechworld.org/htp1/serialdisplay.html
The InitApp() function is missing but only 3 lines to set up tristate,
I have these PCBs around for months but actually never used them, because the assembler software is so messy to handle.
The firmware = still based on assembler, added decimal points support today.
But now since the assembler source became lost, I had to rewrite it in C. The PCB design also is lost but possible to see on blank PCBs- only one extra part, 1x 22uH inductor, and connections imprinted in the copper side.
For sure PIC with hardware serial port would be better to use.
Actually data transfer is done during blank display phases, every odd phase = blank.
And the master control program must monitor for a 1-0-1 transition on the RDY line.
The serial protocol is synchronous, 5x 6 bits.
MPLAB 8, I was not able to use C for 16f54, but here using MPLAB X, this also works!
And the code for software synchronous serial interface is kind of slow, I looked at the disassembly.
Assembler = much better for this, but not portable.
LEDs displays are only refreshed a few times/second, so does not matter
