@technix : My goal is not to "use/program" a 87c521 but to dump an existing one, what have (probably) been programmed years ago in order to recover the code.
So far, to be fully complete, I've found in a garage sale a small unknown box with a PCB containing all the hardware necessary to make a Minitel external display (French ancestral videotext terminal). As it don't seems to be working, I'd like to try to dump the code in the hope that something remains readable ...
Hervé