diff --git a/src/mame/machine/model1io.cpp b/src/mame/machine/model1io.cpp
index 4b3cd637a3d..3afaf6f2cd9 100644
--- a/src/mame/machine/model1io.cpp
+++ b/src/mame/machine/model1io.cpp
@@ -141,6 +141,8 @@ MACHINE_CONFIG_START( model1io_device::device_add_mconfig )
 	MCFG_315_5338A_IN1_CB(READ8(model1io_device, in1_r))
 	MCFG_315_5338A_IN2_CB(READ8(model1io_device, in2_r))
 	MCFG_315_5338A_IN3_CB(READ8(model1io_device, in3_r))
+	MCFG_315_5338A_IN4_CB(READ8(model1io_device, in4_r))
+	MCFG_315_5338A_OUT4_CB(WRITE8(model1io_device, out4_w))
 	MCFG_315_5338A_OUT5_CB(WRITE8(model1io_device, out5_w))
 	MCFG_315_5338A_IN6_CB(READ8(model1io_device, in6_r))
 
@@ -165,7 +167,8 @@ model1io_device::model1io_device(const machine_config &mconfig, const char *tag,
 	m_eeprom(*this, "eeprom"),
 	m_buttons(*this, "buttons"),
 	m_read_cb(*this), m_write_cb(*this),
-	m_in_cb{ {*this}, {*this}, {*this}, {*this}, {*this}, {*this} },
+	m_in_cb{ {*this}, {*this}, {*this}, {*this} },
+	m_drive_read_cb(*this), m_drive_write_cb(*this),
 	m_an_cb{ {*this}, {*this}, {*this}, {*this}, {*this}, {*this}, {*this}, {*this} },
 	m_output_cb(*this),
 	m_secondary_controls(false)
@@ -182,9 +185,12 @@ void model1io_device::device_start()
 	m_read_cb.resolve_safe(0xff);
 	m_write_cb.resolve_safe();
 
-	for (unsigned i = 0; i < 6; i++)
+	for (unsigned i = 0; i < 4; i++)
 		m_in_cb[i].resolve_safe(0xff);
 
+	m_drive_read_cb.resolve_safe(0xff);
+	m_drive_write_cb.resolve_safe();
+
 	for (unsigned i = 0; i < 8; i++)
 		m_an_cb[i].resolve_safe(0xff);
 
@@ -238,6 +244,16 @@ READ8_MEMBER( model1io_device::in3_r )
 	return m_secondary_controls ? m_in_cb[5](0) : m_in_cb[2](0);
 }
 
+READ8_MEMBER( model1io_device::in4_r )
+{
+	return m_drive_read_cb(0);
+}
+
+WRITE8_MEMBER( model1io_device::out4_w )
+{
+	m_drive_write_cb(data);
+}
+
 WRITE8_MEMBER( model1io_device::out5_w )
 {
 	m_output_cb(data);
diff --git a/src/mame/machine/model1io.h b/src/mame/machine/model1io.h
index 68f302e094f..8260ba72462 100644
--- a/src/mame/machine/model1io.h
+++ b/src/mame/machine/model1io.h
@@ -40,11 +40,11 @@
 #define MCFG_MODEL1IO_IN3_CB(_devcb) \
 	devcb = &downcast<model1io_device &>(*device).set_in_callback(DEVCB_##_devcb, 3);
 
-#define MCFG_MODEL1IO_IN4_CB(_devcb) \
-	devcb = &downcast<model1io_device &>(*device).set_in_callback(DEVCB_##_devcb, 4);
+#define MCFG_MODEL1IO_DRIVE_READ_CB(_devcb) \
+	devcb = &downcast<model1io_device &>(*device).set_drive_read_callback(DEVCB_##_devcb);
 
-#define MCFG_MODEL1IO_IN5_CB(_devcb) \
-	devcb = &downcast<model1io_device &>(*device).set_in_callback(DEVCB_##_devcb, 5);
+#define MCFG_MODEL1IO_DRIVE_WRITE_CB(_devcb) \
+	devcb = &downcast<model1io_device &>(*device).set_drive_write_callback(DEVCB_##_devcb);
 
 #define MCFG_MODEL1IO_AN0_CB(_devcb) \
 	devcb = &downcast<model1io_device &>(*device).set_an_callback(DEVCB_##_devcb, 0);
@@ -94,6 +94,12 @@ public:
 	template <class Object> devcb_base &set_in_callback(Object &&cb, int index)
 	{ return m_in_cb[index].set_callback(std::forward<Object>(cb)); }
 
+	template <class Object> devcb_base &set_drive_read_callback(Object &&cb)
+	{ return m_drive_read_cb.set_callback(std::forward<Object>(cb)); }
+
+	template <class Object> devcb_base &set_drive_write_callback(Object &&cb)
+	{ return m_drive_write_cb.set_callback(std::forward<Object>(cb)); }
+
 	template <class Object> devcb_base &set_an_callback(Object &&cb, int index)
 	{ return m_an_cb[index].set_callback(std::forward<Object>(cb)); }
 
@@ -120,6 +126,8 @@ private:
 	DECLARE_READ8_MEMBER(in1_r);
 	DECLARE_READ8_MEMBER(in2_r);
 	DECLARE_READ8_MEMBER(in3_r);
+	DECLARE_READ8_MEMBER(in4_r);
+	DECLARE_WRITE8_MEMBER(out4_w);
 	DECLARE_WRITE8_MEMBER(out5_w);
 	DECLARE_READ8_MEMBER(in6_r);
 
@@ -130,7 +138,9 @@ private:
 
 	devcb_read8 m_read_cb;
 	devcb_write8 m_write_cb;
-	devcb_read8 m_in_cb[6];
+	devcb_read8 m_in_cb[4];
+	devcb_read8 m_drive_read_cb;
+	devcb_write8 m_drive_write_cb;
 	devcb_read8 m_an_cb[8];
 	devcb_write8 m_output_cb;