diff --git a/src/boards/503.c b/src/boards/503.c
new file mode 100644
index 0000000..83bc127
--- /dev/null
+++ b/src/boards/503.c
@@ -0,0 +1,67 @@
+/* FCEUmm - NES/Famicom Emulator
+ *
+ * Copyright notice for this file:
+ *  Copyright (C) 2020
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ *
+ */
+
+#include "mapinc.h"
+#include "asic_mmc3.h"
+
+static uint8 reg;
+static uint8 pad;
+
+static DECLFR(readPad) {
+	return CartBR(A &~3 | pad &3);
+}
+
+static void sync() {
+	if (reg &0x10)
+		setprg32(0x8000, reg >>1);
+	else {
+		setprg16(0x8000, reg);
+		setprg16(0xC000, reg);
+	}
+	MMC3_syncCHR(0x7F, reg <<4 &~0x7F);
+	MMC3_syncMirror();
+	SetReadHandler(0x8000, 0xFFFF, reg &0x20? readPad: CartBR);
+}
+
+static DECLFW(writeReg) {
+	reg = A &0xFF;
+	sync();
+}
+
+static void reset() {
+	reg = 0;
+	++pad;
+	sync();
+}
+
+static void power() {
+	reg = 0;
+	pad = 0;
+	MMC3_power();
+}
+
+void Mapper503_Init(CartInfo *info) {
+	MMC3_init(info, sync, MMC3_TYPE_AX5202P, NULL, NULL, NULL, writeReg);
+	info->Power = power;
+	info->Reset = reset;
+	AddExState(&reg, 1, 0, "EXPR");
+}
diff --git a/src/ines.c b/src/ines.c
index 2c1b2e0..93faa2c 100644
--- a/src/ines.c
+++ b/src/ines.c
@@ -912,6 +912,7 @@ INES_BOARD_BEGIN()
 	INES_BOARD( "Yhc-000",                  500, Mapper500_Init         )
 	INES_BOARD( "Yhc-001",                  501, Mapper501_Init         )
 	INES_BOARD( "Yhc-002",                  502, Mapper502_Init         )
+	INES_BOARD( "ET-170",                   503, Mapper503_Init         )
 	INES_BOARD( "SA-9602B",                 513, SA9602B_Init           )
 	INES_BOARD( "Brilliant Com Cocoma Pack",516, Mapper516_Init        )
 	INES_BOARD( "DANCE2000",                518, UNLD2000_Init          )
diff --git a/src/ines.h b/src/ines.h
index dc04179..c16b741 100644
--- a/src/ines.h
+++ b/src/ines.h
@@ -391,6 +391,7 @@ void Mapper488_Init(CartInfo *);
 void Mapper500_Init(CartInfo *);
 void Mapper501_Init(CartInfo *);
 void Mapper502_Init(CartInfo *);
+void Mapper503_Init(CartInfo *);
 void Mapper516_Init(CartInfo *);
 void Mapper520_Init(CartInfo *);
 void Mapper523_Init(CartInfo *);