First of all, thank you so much for your reponse!
I have read HOWTO about SPI and I have no questions about it.
PID is also realised in ladder logic here:https://cq.cx/ladder-forum.pl?action=viewthread&parent=190
But as you mentioned PID will cause oscillations. So, I should use Take Back Half algorithm.
But regarding TBH algorithm I never have deal with it.
Tried to Google it and found this your post:
"It would represent a minimum problem to implement the algorithm in LDmicro".
Could you show an example in LDmicro? I'm pretty sure that many newbies (like me) will be very grateful for that.