S        This expression returns a string containing the single ASCII character represented by the indicated byte in the received binary message.

It takes one parameter, which is the 0-based byte index in the binary message to read the ASCII character from.


Both Relay and Blue will return blank if invalid.

In Blue, if you try to read something that is not an ASCII character, you will get an error. Invalid indexes will also cause an error. When doing this outside of a Message event, you will get an error related to invalid index.


In Blue Unicode, if you want to send a single character/code point but it is not ASCII, you should add it using the Binary to send > Add string > Without null terminator, and read it back with Text With Size expression instead, passing size 1.