Basic RPL question

[Graan]

I just tried in a first step to load the global values on the stack in the program and then assigning them to local variables then just put the local variables on the stack and it worked as expected.

<< KP KI KD
-> kp ki kd
<<
kp ki kd
>>
>>

I get the local variables on the stack.
Now I want to do some calculations.

So if I want to use kp ki kd in some calculation like kp + ki + kd should I use 'kp + ki + kd' eval or kp ki + kd + ?

Im working on a PID controller for controlling the temperature on my espresso machine and have already the solution but got the wierd idea to control it with my HP48GX or HP50GX since they have serial communication. I have already built the interface with serial communication to control the heater with pulswidth modulation and a teperature probe so I can interface anything to the controller using RS232.
The contorl loop is quite simple and does not require a fast controller so it should be possible with a HP calculator.
P = kp * err (set temp - acttemp) Proportional part
I = ki * Sumerr Integral part
D = kd * (oldtemp - acttemp) Derivative part

PID = P + I + D

then som adaptation math to output the pulsewidth value (0 - 100%).

If its possible I want to have the temperature curve and PWM output on a graph in the calculator as I have it now on a laptop wich is the controller for tuning the parameters at the moment.