Datasheet for the Microchip PIC16C55x family, a 2.18 MB PDF. These are 18-pin OTP or EPROM microcontrollers, and the x placeholder indicates multiple related part numbers grouped together. The document contains pinouts, the memory map, timer and port descriptions and electrical characteristics. It helps a technician confirm supply, clock and reset lines on equipment built around these controllers. Because the program memory is not flash, a failed chip needs a pre-programmed replacement with the original code.
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.
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The standard format for schematics, block diagrams, power-sequence charts and service manuals.