Own PID controller - ET200S

I wanna make own PID just to practise. So far I have implemented following stuffs:
1. Sample time is always fixed and it's determed by OB35 cycle.
2. Derivative of the Error is equal to negative derivative of Input - to avoid big changes in the output once we change Reference point.
3. Smooth transfer once I change the Ti.
4. High and Low limits - for integration part and as well on output.
5.Man/Auto -> Setting Output manually.
6.Initialization -> Always when I'm switching between Man/Auto -> initialize things for a smooth transition, to keep the output from jumping.
7. Deadbeand -> By not chaning only output value if the error is in some borders(limits)


My question is what else I should do implement to make my PID better, and how of cource.
Any advices are welcomed!

There are lots of discussions on PID loops here, just search for PID. A good thread with PID equations from Peter Nachtwey is here:
http://www.plctalk.net/qanda/archive/index.php/t-237.html
search for 'pickle the PID' in that thread.

If Peter answers this post you will find that there are no simple answers, because, first you need to understand the problem. The problem that you are needing to solve means understanding the physical system that you are trying to get a PID loop to control. When you understand the system, then you can choose an appropriate PID equation/s to match it. The bit that I haven't been able to find on the web is a simple-ish method for understanding the problem. So I tend to bumble along like most people who use PID and just make the best of it that I can.

More options

Implement the "velocity" or incremental form of PID. The only down side is that the integrator gain must be used with the incremental form. The upside is that gains can be changed on-the-fly without big steps in the control output.

Above you indicated that you implemented the PI-D form of PID. I often use the I-PD form where the P and D gains operate on the feed back only. I prefer to use the I-PD in the inner loop of a cascaded loop control system.

Make it easy to implement a low select.