diff --git a/src/emu/inptport.c b/src/emu/inptport.c
index cb7b1d67de0..5d55b8f5c31 100644
--- a/src/emu/inptport.c
+++ b/src/emu/inptport.c
@@ -108,6 +108,12 @@
 #endif /* MESS */
 
 
+/* temporary: set this to 1 to enable the originally defined behavior that
+   a field specified via PORT_MODIFY which intersects a previously-defined
+   field completely wipes out the previous definition */
+#define INPUT_PORT_OVERRIDE_FULLY_NUKES_PREVIOUS	1
+
+
 /***************************************************************************
     CONSTANTS
 ***************************************************************************/
@@ -3066,7 +3072,7 @@ static void field_config_insert(input_field_config *field, input_port_value *dis
 			config->mask &= ~field->mask;
 
 			/* if the new entry fully overrides the previous one, we nuke */
-			if (config->mask == 0)
+			if (INPUT_PORT_OVERRIDE_FULLY_NUKES_PREVIOUS || config->mask == 0)
 			{
 				field_config_free((input_field_config **)scanfieldptr);
 				scanfieldnextptr = scanfieldptr;