diff --git a/src/mame/drivers/mrdo.cpp b/src/mame/drivers/mrdo.cpp
index de2fe93b70f..1463ccc4747 100644
--- a/src/mame/drivers/mrdo.cpp
+++ b/src/mame/drivers/mrdo.cpp
@@ -38,19 +38,6 @@ constexpr XTAL VIDEO_CLOCK = 19.6_MHz_XTAL;
 
 } // anonymous namespace
 
-
-
-/* PAL16R6CN used for protection. The game doesn't clear the screen
-   if a read from this address doesn't return the value it expects. */
-uint8_t mrdo_state::mrdo_SECRE_r()
-{
-	uint8_t *RAM = memregion("maincpu")->base();
-
-	return RAM[m_maincpu->state_int(Z80_HL)];
-}
-
-
-
 void mrdo_state::main_map(address_map &map)
 {
 	map(0x0000, 0x7fff).rom();
@@ -60,7 +47,7 @@ void mrdo_state::main_map(address_map &map)
 	map(0x9800, 0x9800).w(FUNC(mrdo_state::mrdo_flipscreen_w)); // screen flip + playfield priority
 	map(0x9801, 0x9801).w("sn1", FUNC(sn76489_device::write));
 	map(0x9802, 0x9802).w("sn2", FUNC(sn76489_device::write));
-	map(0x9803, 0x9803).r(FUNC(mrdo_state::mrdo_SECRE_r));
+	map(0x9803, 0x9803).r(FUNC(mrdo_state::mrdo_secre_r));
 	map(0xa000, 0xa000).portr("P1");
 	map(0xa001, 0xa001).portr("P2");
 	map(0xa002, 0xa002).portr("DSW1");
@@ -199,6 +186,17 @@ void mrdo_state::mrdo(machine_config &config)
 	SN76489(config, "sn2", MAIN_CLOCK/2).add_route(ALL_OUTPUTS, "mono", 0.50); // Verified
 }
 
+void mrdo_state::machine_start()
+{
+	save_item(NAME(m_pal_u001));
+}
+
+void mrdo_state::machine_reset()
+{
+    // initial outputs are high on power-up
+    m_pal_u001 = 0xff;
+}
+
 void mrdo_state::mrlo(machine_config &config)
 {
 	mrdo(config);
diff --git a/src/mame/includes/mrdo.h b/src/mame/includes/mrdo.h
index 331894c97f7..fdab799f466 100644
--- a/src/mame/includes/mrdo.h
+++ b/src/mame/includes/mrdo.h
@@ -33,6 +33,9 @@ public:
 protected:
 	virtual void video_start() override;
 
+	virtual void machine_start() override;
+	virtual void machine_reset() override;
+
 private:
 	// memory pointers
 	required_shared_ptr<uint8_t> m_bgvideoram;
@@ -47,8 +50,10 @@ private:
 	tilemap_t *m_bg_tilemap;
 	tilemap_t *m_fg_tilemap;
 	int       m_flipscreen;
+	uint8_t   m_pal_u001;
+
+	uint8_t mrdo_secre_r();
 
-	uint8_t mrdo_SECRE_r();
 	void mrdo_bgvideoram_w(offs_t offset, uint8_t data);
 	void mrdo_fgvideoram_w(offs_t offset, uint8_t data);
 	void mrdo_scrollx_w(uint8_t data);
diff --git a/src/mame/video/mrdo.cpp b/src/mame/video/mrdo.cpp
index 51ddcff46f1..b28a1465b73 100644
--- a/src/mame/video/mrdo.cpp
+++ b/src/mame/video/mrdo.cpp
@@ -185,13 +185,59 @@ void mrdo_state::mrdo_bgvideoram_w(offs_t offset, uint8_t data)
 	m_bg_tilemap->mark_tile_dirty(offset & 0x3ff);
 }
 
+/* PAL16R6CN used for protection. The game doesn't clear the screen
+   if a read from this address doesn't return the value it expects. */
+uint8_t mrdo_state::mrdo_secre_r()
+{
+	return m_pal_u001;
+}
+
 void mrdo_state::mrdo_fgvideoram_w(offs_t offset, uint8_t data)
 {
 	m_fgvideoram[offset] = data;
 	m_fg_tilemap->mark_tile_dirty(offset & 0x3ff);
+
+	// protection.  each write latches a new value on IC u001 (PAL16R6)
+	const uint8_t i9 = BIT(data,0);
+	const uint8_t i8 = BIT(data,1);
+//	const uint8_t i7 = BIT(data,2); pin 7 not used in equations
+	const uint8_t i6 = BIT(data,3);
+	const uint8_t i5 = BIT(data,4);
+	const uint8_t i4 = BIT(data,5);
+	const uint8_t i3 = BIT(data,6);
+	const uint8_t i2 = BIT(data,7);
+
+	// equations extracted from dump using jedutil
+	const uint8_t t1 =    i2  & (1^i3) &    i4  & (1^i5) & (1^i6) & (1^i8) &    i9;
+	const uint8_t t2 = (1^i2) & (1^i3) &    i4  &    i5  & (1^i6) &    i8  & (1^i9);
+	const uint8_t t3 =    i2  &    i3  & (1^i4) & (1^i5) &    i6  & (1^i8) &    i9;
+	const uint8_t t4 = (1^i2) &    i3  &    i4  & (1^i5) &    i6  &    i8  &    i9;
+
+	const uint8_t r12 = 0;
+	const uint8_t r13 = (1^( t1 ))      << 1 ;
+	const uint8_t r14 = (1^( t1 | t2 )) << 2 ;
+	const uint8_t r15 = (1^( t1 | t3 )) << 3 ;
+	const uint8_t r16 = (1^( t1 ))      << 4 ;
+	const uint8_t r17 = (1^( t1 | t3 )) << 5 ;
+	const uint8_t r18 = (1^( t3 | t4 )) << 6 ;
+	const uint8_t r19 = 0;
+
+	m_pal_u001 = r19 | r18 | r17 | r16 | r15 | r14 | r13 | r12  ;
 }
 
+/*
+/rf13 :=                                             i2 & /i3 & i4 & /i5 & /i6 & /i8 & i9  t1
 
+/rf14 := /i2 & /i3 & i4 & i5 & /i6 & i8 & /i9   +    i2 & /i3 & i4 & /i5 & /i6 & /i8 & i9  t2 | t1
+
+/rf15 := i2 & i3 & /i4 & /i5 & i6 & /i8 & i9    +    i2 & /i3 & i4 & /i5 & /i6 & /i8 & i9  t3 | t1
+
+/rf16 :=                                             i2 & /i3 & i4 & /i5 & /i6 & /i8 & i9  t1
+
+/rf17 := i2 & i3 & /i4 & /i5 & i6 & /i8 & i9    +    i2 & /i3 & i4 & /i5 & /i6 & /i8 & i9  t3 | t1
+
+/rf18 := /i2 & i3 & i4 & /i5 & i6 & i8 & i9     +    i2 & i3 & /i4 & /i5 & i6 & /i8 & i9   t4 | t2
+*/
 void mrdo_state::mrdo_scrollx_w(uint8_t data)
 {
 	m_bg_tilemap->set_scrollx(0, data);