converting multiple lines of text to single line of text with comma separting them

I have a file called abc.txt

00:00:00:00
00:02:59:90
00:09:08:50

I would like it to read as follows:

00:00:00:00,00:02:59:90,00:09:08:50

I have tried
Code:tr "n" " " < abc.txt > xyz.txtbut to no avail.
Any help or assistance is greatly appreciated.
Thank you.

Cogiz