A very important feature in order to build reusable descriptions is to define and reuse partial models. For Example A common property of many electrical components is that they have two pins. This means that it is useful to define an interface model class TwoPin, which has two pins, p and
n, and a quantity, v, which defines the voltage drop across the component.


partial model TwoPin
Pin p, n;
Voltage v;
equation
v = p.v - n.v; p.i + n.i = 0;
end TwoPin;


The equations define common relations between quantities of a simple electrical component.
The keyword partial indicates that the model class is incomplete. To be useful, a constitutive equation must be added. To define a model for a resistor, start from TwoPin and add a parameter for the resistance and Ohm’s law to define the behavior.


model Resistor "Ideal resistor"
extends TwoPin;
parameter Resistance R;
equation
R*p.i = v;
end Resistor;


A string between the name of a class and its body is treated as a comment attribute and is
intended to describe the class. Tools may display this documentation in special ways. The
keyword parameter specifies that the quantity is constant during a simulation experiment,
but can change values between experiments.