Character codes 128~255 are sometimes called "high ASCII" codes. They can be entered (on PCs at least) by holding the Alt key and typing the number on the numeric keypad, then releasing the Alt key - "Alt-keypad" entry.
The "extended ASCII" characters listed at
www.asciitable.com are the high-ASCII characters used by native ROM-based text video modes on the PC family. They include the box-drawing and background shading characters, which only work properly with monospaced fonts. This character mapping is not used by modern systems.
High ASCII character numbers have various character mappings depending on the character set used.
The following "high ASCII" characters seem to enter and display OK with XenForo for me (I'm running Firefox under Windows 7):
158 × multiplication symbol (little 'x' above the line)
230 µ mu/micro (Greek mu character)
241 ± plus/minus (plus sign with horizontal line joined along the bottom)
248 ° degrees (small 'o' near top of line)
250 · multiplication dot (small dot above the line)
251,253,252 ¹²³ superscript 1,2,3 (or you can use [SUP]1[/SUP], [SUP]2[/SUP] instead)
For me, typing Alt-keypad 234 produces Û which shows (for me) as an upper case U with a caret or circumflex. I can't get an Omega symbol using Alt-numberpad on my machine. Byt pyromaniac's Omega symbol in post #2 shows up as an Omega symbol for me. So there must be at least one level of translation happening.
After a bit of research I've found that the character set used by XenForo appears to be UTF-8 - see
https://en.wikipedia.org/wiki/UTF-8. This encoding standard uses 2-byte and longer sequences to represent most "special" characters and covers the full Unicode character set.
According to
https://en.wikipedia.org/wiki/Omega the upper case Omega (ohms symbol) corresponds to a UTF-8 encoding of 206,169. But when I enter those codes using Alt-keypad I just get ╬® - a box-drawing character and a registered trademark symbol!
.
