ASCII standardizes characters between 0 and 127.

P

Ascii is not a protocol - it describes a computer system's character set. Communication with a Modbus PLC requires an understanding of how to communicate and the protocol (set of rules) does describe this.

Ascii is a set of values describing the Latin codepage set that can represent certain characters in data. There are no communications "rules" with Ascii, just a data representation.

ASCII characters do represent a numerical codes of letters and other alphabetical signs.

Computers do not understand only numbers so they use this numerical codes to interpret letters into their own "language".

Websites such as asciitable.com and ascii-code.com provide ascii tables on their websites, along with toher information about ascii codes, their uses, and how to use them.