# Connector Definitions¶

## Syntax¶

As we have seen several times already, Modelica definitions share a considerable amount of syntactic similarity. This is just as true with connector definitions.

The general syntax for a connector definition is:

connector ConnectorName "Description of the connector"
// Declarations for connector variables
end ConnectorName;


Unlike a model or function, a connector is not allowed to include any behavior. So there can never be an equation or algorithm section present in a connector.

## Variables¶

### Causal Variables¶

In our previous discussion of Block Connectors, we showed that variables within a Modelica connector definition can have a causality associated with them. If the signal is expected to be computed externally, then the variable should have the input qualifier associated with it. If, on the other hand, a signal is expected to be computed internally (and then transmitted to other components), it should have the output qualifier associated with it.

### Acausal Variables¶

In our discussion of Simple Domains and Fluid Connectors, we saw numerous examples of connector definitions that included through and across variables. These variables always occurred in pairs with the through variable being prefixed by the flow qualifier while the across variable had not qualifier associated with it.

As we will see in the coming chapters, such connector definitions are very convenient when modeling physical systems because they enable the Modelica compiler to automatically generate conservation equations for networks of components. Furthermore, they allow quantities like, mass, momentum, energy, charge, species and so on to flow bi-directionally through a network.

### Parameters¶

A variable in a connector definition can also have the parameter qualifier associated with it. This qualifier means the same thing that it meant when we first discussed Parameters, i.e., the value of the variable cannot change during a simulation. A parameter variable is frequently used in connector definitions to indicate the size of an array contained in the connector.

### Final Remarks¶

It should be noted that a connector definition can mix causal, acausal and parameter variables all in the same connector. In fact, a variable in a connector can itself be a connector as well. This richness of expressiveness in Modelica allows users to model a range of different types of interactions and choose, on a variable by variable basis, the semantics that make the most sense for each potential interaction.