MOD TCL User Programming
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TCL is based on PASCAL. Because of this, it is very similar to IEC611 Structured Text. It's quite an easy language to learn. If you have done any sort of text based programming you should be able to read TCL programs without any problems.
Once you understand how to declare variables and the basic syntax for commanding CCF loops and devices then you should find it very easy.
The standard MOD300 manuals contain a very good description of TCL.
Your best option would be to take participation in a ABB University Course. There are many courses which could bring you through a Begginer level to a Advanced Level. Please see the course detail in the link.