How to use Fixed PDO mapping on Lichuan LC Servo

I have a question. How to use Fixed PDO mapping on Lichuan LC Servo?

I used 10 entries in PDO 1600h ( Variable PDO ) in ethercat-conf.xml ( 10 entries are maximum )
So I just moved some <pdoEntry>  to FixedPDO (1704h )
But it didn't work.

Do I have to do something to use FixedPDO ?
Are there any good example for it ?
 

My understanding is that you can't change the pdoentries in a fixed PDO, they are hmm fixed.
The variable PDOs can be populated with pdoentries according to your need.