diff --git a/parse.y b/parse.y
index 950910368..b7f880b8c 100644
--- a/parse.y
+++ b/parse.y
@@ -352,6 +352,13 @@ static void current_function_set_statement(const YYLTYPE&loc, std::vector<Statem
       current_function->set_statement(tmp);
 }
 
+static void port_declaration_context_init(void)
+{
+      port_declaration_context.port_type = NetNet::PINOUT;
+      port_declaration_context.port_net_type = NetNet::IMPLICIT;
+      port_declaration_context.data_type = nullptr;
+}
+
 Module::port_t *module_declare_port(const YYLTYPE&loc, char *id,
 			            NetNet::PortType port_type,
 				    NetNet::Type net_type,
@@ -705,6 +712,7 @@ Module::port_t *module_declare_port(const YYLTYPE&loc, char *id,
 %type <decl_assignments> list_of_variable_decl_assignments
 
 %type <data_type>  data_type data_type_opt data_type_or_implicit data_type_or_implicit_or_void
+%type <data_type>  data_type_or_implicit_no_opt
 %type <data_type>  simple_type_or_string let_formal_type
 %type <data_type>  packed_array_data_type
 %type <data_type>  ps_type_identifier
@@ -1337,7 +1345,11 @@ scalar_vector_opt /*IEEE1800-2005: optional support for packed array */
   ;
 
 data_type_or_implicit /* IEEE1800-2005: A.2.2.1 */
-  : data_type_opt
+  : data_type_or_implicit_no_opt
+  | { $$ = nullptr; }
+
+data_type_or_implicit_no_opt
+  : data_type
       { $$ = $1; }
   | signing dimensions_opt
       { vector_type_t*tmp = new vector_type_t(IVL_VT_LOGIC, $1, $2);
@@ -4466,39 +4478,16 @@ list_of_port_declarations
 	tmp->push_back($3);
 	$$ = tmp;
       }
-  | list_of_port_declarations ',' IDENTIFIER initializer_opt
-      { Module::port_t*ptmp;
-	perm_string name = lex_strings.make($3);
-	ptmp = pform_module_port_reference(@3, name);
-	std::vector<Module::port_t*>*tmp = $1;
-	tmp->push_back(ptmp);
+  | list_of_port_declarations ',' attribute_list_opt IDENTIFIER dimensions_opt initializer_opt
+      { std::vector<Module::port_t*> *ports = $1;
 
-	if ($4) {
-	      switch (port_declaration_context.port_type) {
-		  case NetNet::PINOUT:
-		    yyerror(@4, "error: Default port value not allowed for inout ports.");
-		    break;
-		  case NetNet::PINPUT:
-		    pform_requires_sv(@4, "Default port value");
-		    ptmp->default_value = $4;
-		    break;
-		  case NetNet::POUTPUT:
-		    pform_make_var_init(@3, name, $4);
-		    break;
-		  default:
-		    break;
-		}
-	}
-	  /* Get the port declaration details, the port type
-	     and what not, from context data stored by the
-	     last port_declaration rule. */
-	pform_module_define_port(@3, name,
-				 port_declaration_context.port_type,
-				 port_declaration_context.port_net_type,
-				 port_declaration_context.data_type,
-				 nullptr, nullptr);
-	delete[]$3;
-	$$ = tmp;
+	Module::port_t* port;
+	port = module_declare_port(@4, $4, port_declaration_context.port_type,
+				   port_declaration_context.port_net_type,
+				   port_declaration_context.data_type,
+				   $5, $6, $3);
+	ports->push_back(port);
+	$$ = ports;
       }
   | list_of_port_declarations ','
       { yyerror(@2, "error: Superfluous comma in port declaration list."); }
@@ -4506,9 +4495,21 @@ list_of_port_declarations
       { yyerror(@2, "error: ';' is an invalid port declaration separator."); }
   ;
 
+  // All of port direction, port kind and data type are optional, but at least
+  // one has to be specified, so we need multiple rules.
 port_declaration
   : attribute_list_opt port_direction net_type_or_var_opt data_type_or_implicit IDENTIFIER dimensions_opt initializer_opt
-      { $$ = module_declare_port(@2, $5, $2, $3, $4, $6, $7, $1);
+      { $$ = module_declare_port(@5, $5, $2, $3, $4, $6, $7, $1);
+      }
+  | attribute_list_opt net_type_or_var data_type_or_implicit IDENTIFIER dimensions_opt initializer_opt
+      { pform_requires_sv(@4, "Partial ANSI port declaration");
+	$$ = module_declare_port(@4, $4, port_declaration_context.port_type,
+			         $2, $3, $5, $6, $1);
+      }
+  | attribute_list_opt data_type_or_implicit_no_opt IDENTIFIER dimensions_opt initializer_opt
+      { pform_requires_sv(@3, "Partial ANSI port declaration");
+	$$ = module_declare_port(@3, $3, port_declaration_context.port_type,
+			         NetNet::IMPLICIT, $2, $4, $5, $1);
       }
   | attribute_list_opt port_direction K_wreal IDENTIFIER
       { real_type_t*real_type = new real_type_t(real_type_t::REAL);
@@ -4613,7 +4614,8 @@ cont_assign_list
 
 module
   : attribute_list_opt module_start lifetime_opt IDENTIFIER
-      { pform_startmodule(@2, $4, $2==K_program, $2==K_interface, $3, $1); }
+      { pform_startmodule(@2, $4, $2==K_program, $2==K_interface, $3, $1);
+        port_declaration_context_init(); }
     module_package_import_list_opt
     module_parameter_port_list_opt
     module_port_list_opt