Datasheet labelled AT89C8252, an Atmel 8051-family microcontroller document of 316 KB; the same size as the AT89S8252 sheet suggests related or identical content, so open it to confirm which part is covered. Expect information on the 8 KB flash, 2 KB EEPROM, pin assignments, timers and programming modes. Technicians use it to identify pins and programming requirements when replacing or reflashing controllers on older industrial boards. Compare the C or S marking on the actual chip, since parallel and serial programming methods differ.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @datasheet. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.