Change to unspecified connector type, add semantics

Author: pmai

docs/5___ssd.adoc

@@ -119,7 +119,7 @@ Note that there is no requirement that connectors have to be present for all var
 At least those connectors *MUST* be present which are referenced in connections inside the SSD.
 
 |kind a|
-This attribute specifies the kind of the given connector, which indicates whether the connector is an input, an output, both (inout), acausal (in e.g. Modelica), a local variable, a constant, a parameter, a calculated parameter (i.e. a parameter that is calculated by the component during initialization), or a structural parameter (i.e. a parameter that can be set during (re-)configuration mode).
+This attribute specifies the kind of the given connector, which indicates whether the connector is an input, an output, both (inout), unspecified, a local variable, a constant, a parameter, a calculated parameter (i.e. a parameter that is calculated by the component during initialization), or a structural parameter (i.e. a parameter that can be set during (re-)configuration mode).
 
 For components this *MUST* match the related kind of the underlying component implementation.
 For referenced FMUs it *MUST* match the combination of variability and causality:
@@ -138,6 +138,9 @@ However, if it is connected, the semantics are same as for an `output`.
 For SignalDictionaryReferences, the kind of a given connector can additionally be inout, which indicates that the semantics of the connector are derived from the connections going to the connector.
 This can be used for example to allow a connector to function as both an input and output within the same SignalDictionaryReference.
 
+Connectors of kind `unspecified` are used to define connectors for which the flow of information is either not yet specified, or is determined at runtime, for example for acausal connections of Modelica models.
+Such connectors can be connected to any other connector under the rules of the underlying modeling language.
+
 |===
 
 The following XML child elements are specified for the Connector element:
@@ -156,7 +159,11 @@ Boolean / String / Enumeration / Binary / Clock |Exactly one of these elements *
 
 The type of the Connector is identified by the presence of one of the XML child elements Real, Float64, Float32, Integer, Int8, UInt8, Int16, UInt16, Int32, UInt32, Int64, UInt64, Boolean, String, Enumeration, Binary, or Clock.
 
-When Modelica models are represeented in SSP, built-in input and output connectors shall be mapped as follows.
+The dimensionality of the Connector is given by the presence of one or more Dimension elements.
+
+The association of a connector to a clock is given by the presence of one or more Clock elements.
+
+When Modelica models are represented in SSP, built-in input and output connectors shall be mapped as follows:
 
 [width="100%",cols="25%,25%,50%",options="header",]
 |===
@@ -171,15 +178,13 @@ When Modelica models are represeented in SSP, built-in input and output connecto
 |StringOutput |ssc:String |output
 |===
 
-Modelica connectors of more advanced types are mapped the same way as Modelica component models.
+Modelica connectors of more advanced types are currently mapped in the following way:
 
 * The connector type is ssc:Binary.
-* The media type is "text/x-modelica" and the "path" attribute designates the path of the Modleica connector..
-* Acausal Modelica connector types are in SSP of kind="acausal".
+* The media type is `text/x-modelica` and the `path` parameter of the media type designates the path of the Modelica connector.
+* Acausal Modelica connector types are mapped to connectors of kind unspecified.
 
-The dimensionality of the Connector is given by the presence of one or more Dimension elements.
-
-The association of a connector to a clock is given by the presence of one or more Clock elements.
+_[ Note that the current opaque mapping of more advanced types to Binary connectors is a temporary solution, and a more detailed mapping may be provided in future versions of the standard supporting more complex data types. ]_
 
 ===== ConnectorGeometry
 
@@ -551,6 +556,10 @@ Note that source and destination in the following table indicate the resulting d
 Implementations *MUST NOT* specify connections that are not of one of the allowed combinations in the following table.
 Implementations *MUST* ensure that data flow is specified unambiguously, including ensuring that not multiple connections with inbound data flow enter into a connector signifying an input, inout, parameter, or structuralParameter connector of an element, or a local, constant, calculatedParameter, or output connector of an enclosing system.
 
+For connectors of kind unspecified it is ultimately implementation-defined whether and how connections are allowed, as the exact semantics depend on the underlying modeling language.
+For the purposes of the following table, connectors of kind unspecified are treated as connectors of whatever kind is needed to make the connection allowed, i.e. they serve a wild-card role.
+The handling of conflicts that arise in transitive connections from conflicting wild-card assignments is implementation-defined.
+
 It is implementation-defined whether connections between connectors of different types are allowed, and to what extent conversions are performed.
 This includes type conversions that can be performed without data loss, e.g. converting from an output of type `Float32` to an input of type `Float64` or `Real`, as well as conversions that can potentially lead to data loss, e.g. converting from an output of type `Float64` to an input of type `Float32` or `Int8`.
 It is also implementation-defined how and whether any error-handling at runtime is performed in those cases.
@@ -576,7 +585,6 @@ For all connections of an element connector of type `Clock` to a system connecto
 |System |parameter |System |local
 |System |input |System |output
 |System |input |System |local
-|System|acausal|System|acausal
 |System |structuralParameter |Element |structuralParameter
 |System |structuralParameter |Element |parameter
 |System |structuralParameter |Element |input
@@ -586,7 +594,6 @@ For all connections of an element connector of type `Clock` to a system connecto
 |System |parameter |Element |inout
 |System |input |Element |input
 |System |input |Element |inout
-|System|acausal|Element|acausal
 |Element |constant |Element |structuralParameter
 |Element |constant |Element |parameter
 |Element |constant |Element |input
@@ -599,7 +606,6 @@ For all connections of an element connector of type `Clock` to a system connecto
 |Element |local  |Element |input
 |Element |local  |Element |inout
 |Element |inout |Element |input
-|Element|acausal|Element|acausal
 |Element |constant |System |constant
 |Element |constant |System |calculatedParameter
 |Element |constant |System |output
@@ -613,7 +619,6 @@ For all connections of an element connector of type `Clock` to a system connecto
 |Element |local  |System |local
 |Element |inout |System |output
 |Element |inout |System |local
-|Element|acausal|System|acausal
 |===
 
 The following XML child elements are specified for the Connection element: