Diacritic for writing short E and O in Grantha script

Malayalam and Tamil scripts differ from Grantha script in representing short E and O vowels. Hence, we cannot use E and O vowels or the corresponding vowel signs from either Malayalam or Tamil script. Note that Grantha long E and O are Tamil or Malayalam short E and O, whereas Grantha short E and O vowel representation employs a diacritic symbol [1].

This has been explained by many specialists on Grantha. For example, Sriramana Sharma's
proposal on Grantha script (L2/09-372)

  09372-sharma-puLLi-dot-for-E-O_Page_3 09372-sharma-puLLi-dot-for-E-O_Page_1 09372-sharma-puLLi-dot-for-E-O_Page_2

Reference [1]: “PuLLi takes the form of a dot above or in the upper part of the akSara. In addition to this normal virama function, puLLi is also used with the vowels e and o in order to mark them as short: in contrast to Sanskrit and most Middle-Indo-Aryan dialects, the Dravidian languages have short as well as long e and o phonemes.” In the Brahmi encoding, puLLi function and its shape “dot” to reduce long /e/ and /o/ to short vowels is allowed in Unicode (S. Baums and A. Glass, L2/07-342, pg. 8, L2/07-342).

It should be noted that Tamil E and O differ from the older Grantha E and O because of the reform introduced by a Jesuit priest from Italy. "The famous Jesuit Beschi (1704-1774) is the author of a great improvement in Tamil orthography – the distinction between long and short e & o.” (pg. 37A. C. Burnell, Elements of South Indian Paleography, 1874).

Table comparing Grantha SHORT E and O vs. Tamil  and Devanagari SHORT E and O.
Note that the vowels and vowel signs are quite different.