ard_newbie:
I'm using ports SDA20 and SCL21 on both cards This can't work since SDA/SCL have pull-ups (on both cards), you want to connect SDA/SCL to SDA1/SCL1(SDA1/SCL1 don't have pull-ups).
Ok thanks, SDA20 and SCL21 on the slave? Then SDA1 and SCL1 on the master?
What do I have to change in the code?