This information is part of the Modelica Standard Library maintained by the Modelica Association.

The Modelica standard library defines the most important elementary connectors in various domains. If any possible, a user should utilize these connectors in order that components from the Modelica Standard Library and from other libraries can be combined without problems. The following elementary connectors are defined (the meaning of potential, flow, and stream variables is explained in section "Connector Equations" below):

domain	potential variables	flow variables	stream variables	connector definition	icons
electrical analog	electrical potential	electrical current		Modelica.Electrical.Analog.Interfaces Pin, PositivePin, NegativePin
electrical polyphase	vector of electrical pins			Modelica.Electrical.Polyphase.Interfaces Plug, PositivePlug, NegativePlug
electrical space phasor	2 electrical potentials	2 electrical currents		Modelica.Electrical.Machines.Interfaces SpacePhasor
quasi-static single-phase	complex electrical potential	complex electrical current		Modelica.Electrical.QuasiStatic.SinglePhase.Interfaces Pin, PositivePin, NegativePin
quasi-static polyphase	vector of quasi-static single-phase pins			Modelica.Electrical.QuasiStatic.Polyphase.Interfaces Plug, PositivePlug, NegativePlug
electrical digital	Integer (1..9)			Modelica.Electrical.Digital.Interfaces DigitalSignal, DigitalInput, DigitalOutput
magnetic flux tubes	magnetic potential	magnetic flux		Modelica.Magnetic.FluxTubes.Interfaces MagneticPort, PositiveMagneticPort, NegativeMagneticPort
magnetic fundamental wave	complex magnetic potential	complex magnetic flux		Modelica.Magnetic.FundamentalWave.Interfaces MagneticPort, PositiveMagneticPort, NegativeMagneticPort
translational	distance	cut-force		Modelica.Mechanics.Translational.Interfaces Flange_a, Flange_b
rotational	angle	cut-torque		Modelica.Mechanics.Rotational.Interfaces Flange_a, Flange_b
3-dim. mechanics	position vector orientation object	cut-force vector cut-torque vector		Modelica.Mechanics.MultiBody.Interfaces Frame, Frame_a, Frame_b, Frame_resolve
simple fluid flow	pressure specific enthalpy	mass flow rate enthalpy flow rate		Modelica.Thermal.FluidHeatFlow.Interfaces FlowPort, FlowPort_a, FlowPort_b
thermo fluid flow	pressure	mass flow rate	specific enthalpy mass fractions	Modelica.Fluid.Interfaces FluidPort, FluidPort_a, FluidPort_b
heat transfer	temperature	heat flow rate		Modelica.Thermal.HeatTransfer.Interfaces HeatPort, HeatPort_a, HeatPort_b
blocks	Real variable Integer variable Boolean variable			Modelica.Blocks.Interfaces RealSignal, RealInput, RealOutput IntegerSignal, IntegerInput, IntegerOutput BooleanSignal, BooleanInput, BooleanOutput
complex blocks	Complex variable			Modelica.ComplexBlocks.Interfaces ComplexSignal, ComplexInput, ComplexOutput
state machine	Boolean variables (occupied, set, available, reset)			Modelica.StateGraph.Interfaces Step_in, Step_out, Transition_in, Transition_out

In all domains, usually 2 connectors are defined. The variable declarations are identical, only the icons are different in order that it is easy to distinguish connectors of the same domain that are attached at the same component.

Hierarchical Connectors

Modelica supports also hierarchical connectors, in a similar way as hierarchical models. As a result, it is, e.g., possible, to collect elementary connectors together. For example, an electrical plug consisting of two electrical pins can be defined as:

connector Plug
   import Modelica.Electrical.Analog.Interfaces;
   Interfaces.PositivePin phase;
   Interfaces.NegativePin ground;
end Plug;

With one connect(..) equation, either two plugs can be connected (and therefore implicitly also the phase and ground pins) or a Pin connector can be directly connected to the phase or ground of a Plug connector, such as "connect(resistor.p, plug.phase)".

Connector Equations

The connector variables listed above have been basically determined with the following strategy:

1. State the relevant balance equations and boundary conditions of a volume for the particular physical domain.
2. Simplify the balance equations and boundary conditions of (1) by taking the limit of an infinitesimal small volume (e.g., thermal domain: temperatures are identical and heat flow rates sum up to zero).
3. Use the variables needed for the balance equations and boundary conditions of (2) in the connector and select appropriate Modelica prefixes, so that these equations are generated by the Modelica connection semantics.

The Modelica connection semantics is sketched at hand of an example: Three connectors c1, c2, c3 with the definition

connector Demo
  Real        p;  // potential variable
  flow   Real f;  // flow variable
  stream Real s;  // stream variable
end Demo;

are connected together with

connect(c1,c2);
connect(c1,c3);

then this leads to the following equations:

// Potential variables are identical
c1.p = c2.p;
c1.p = c3.p;

// The sum of the flow variables is zero
0 = c1.f + c2.f + c3.f;

/* The sum of the product of flow variables and upstream stream variables is zero
   (this implicit set of equations is explicitly solved when generating code;
   the "<undefined>" parts are defined in such a way that
   inStream(..) is continuous).
*/
0 = c1.f*(if c1.f > 0 then s_mix else c1.s) +
    c2.f*(if c2.f > 0 then s_mix else c2.s) +
    c3.f*(if c3.f > 0 then s_mix else c3.s);

inStream(c1.s) = if c1.f > 0 then s_mix else <undefined>;
inStream(c2.s) = if c2.f > 0 then s_mix else <undefined>;
inStream(c3.s) = if c3.f > 0 then s_mix else <undefined>;