diff --git a/src/mame/video/vtvideo.cpp b/src/mame/video/vtvideo.cpp
index 1356efc70ca..61e7f601c8e 100644
--- a/src/mame/video/vtvideo.cpp
+++ b/src/mame/video/vtvideo.cpp
@@ -236,20 +236,18 @@ WRITE8_MEMBER(vt100_video_device::dc012_w)
 {
 	// Writes to [10C] and [0C] are treated differently
 	// - see 3.1.3.9.5 DC012 Programming Information (PC-100 spec)
-	if ((offset & 0x100) && (data == 0) ) // MHFU is disabled by writing 00 to port 010C.
+	if ((offset & 0x100) ) // MHFU is disabled by writing a value to port 010C.
 	{
-		if (MHFU_FLAG == true)
-						printf("MHFU  *** DISABLED *** \n");
+//		if (MHFU_FLAG == true)
+//						printf("MHFU  *** DISABLED *** \n");
 		MHFU_FLAG = false;
-		MHFU_counter = 0; // ?
 	}
 	else
 	{
-		if (MHFU_FLAG == false)
-			printf("MHFU  ___ENABLED___ %05x \n", space.device().safe_pc());
-
+//		if (MHFU_FLAG == false)
+//			printf("MHFU  ___ENABLED___ %05x \n", space.device().safe_pc());
 		MHFU_FLAG = true;
-		MHFU_counter = 0;
+		MHFU_counter = 0;  
 	}
 
 	if (!(data & 0x08))
@@ -278,13 +276,14 @@ WRITE8_MEMBER(vt100_video_device::dc012_w)
 			m_write_clear_video_interrupt(0);
 			break;
 		case 0x0a:
-			// set reverse field on
-			m_reverse_field = 1;
-			break;
-		case 0x0b:
-			// set reverse field off
+			// PDF: reverse field ON
 			m_reverse_field = 0;
 			break;
+		case 0x0b: 			
+			// PDF: reverse field OFF 
+			// SETUP: dark screen selected 
+			m_reverse_field = 1; 
+			break;
 
 			//  Writing a 11XX bit combination clears the blink-flip flop (valid for 0x0C - 0x0F):
 		case 0x0c:
@@ -835,49 +834,40 @@ void rainbow_video_device::video_blanking(bitmap_ind16 &bitmap, const rectangle
 	bitmap.fill(((m_reverse_field ^ m_basic_attribute) ? 1 : 0), cliprect);
 }
 
-
+#define MHFU_IS_ENABLED 1
+#define MHFU_COUNT -1
+#define MHFU_VALUE -2
+#define MHFU_RESET_and_ENABLE   -100
+#define MHFU_RESET_and_DISABLE  -200
+#define MHFU_RESET              -250
 
 int rainbow_video_device::MHFU(int ASK)
 {
 	switch (ASK)
 	{
-	case 1:         // "true": RETURN BOOLEAN (MHFU disabled or enabled?)
+	case MHFU_IS_ENABLED:		// "true": RETURN BOOLEAN (MHFU disabled or enabled?)
 		return MHFU_FLAG;
 
-	case -1:        // -1: increment IF ENABLED, return counter value (=> Rainbow.c)
-		//if (MHFU_FLAG == true)
-			if (MHFU_counter < 255)
+	case MHFU_COUNT:		// -1: increment IF ENABLED, return counter value (=> Rainbow.c)
+		if (MHFU_FLAG == true)
+			if (MHFU_counter < 254)
 				MHFU_counter++;
 
-	case -2:
+	case MHFU_VALUE:
 		return MHFU_counter;
 
-	case -250:          // -250 : RESET counter (NOTHING ELSE!)
+	case MHFU_RESET:		// -250 : RESET counter (NOTHING ELSE!)
 		MHFU_counter = 0;
 		return MHFU_FLAG;
 
-	case -100:          // -100 : RESET and ENABLE MHFU counter
+	case MHFU_RESET_and_ENABLE:	// -100 : RESET and ENABLE MHFU counter
 		MHFU_counter = 0;
-		if(0) //if (VERBOSE)
-			printf("-100 MHFU  * reset and ENABLE * \n");
-
-		if(0) // if (VERBOSE)
-		{
-			if (MHFU_FLAG == false)
-				printf("-100 MHFU  ___ENABLED___\n");
-		}
 		MHFU_FLAG = true;
 
 		return -100;
 
-	case -200:          // -200 : RESET and DISABLE MHFU
+	case MHFU_RESET_and_DISABLE:	// -200 : RESET and DISABLE MHFU
 		MHFU_counter = 0;
-
-		if(0) //if (VERBOSE)
-		{
-			if (MHFU_FLAG == true)
-				printf("MHFU  *** DISABLED ***xxx \n");
-		}
 		MHFU_FLAG = false;
 
 		return -200;