diff --git a/src/devices/bus/s100/dj2db.cpp b/src/devices/bus/s100/dj2db.cpp
index b4aa4d303e8..f3cff259db3 100644
--- a/src/devices/bus/s100/dj2db.cpp
+++ b/src/devices/bus/s100/dj2db.cpp
@@ -305,7 +305,7 @@ uint8_t s100_dj2db_device::s100_smemr_r(address_space &space, offs_t offset)
 {
 	uint8_t data = 0;
 
-//  if (!(m_board_enbl & m_phantom)) return 0;
+//  if (!(m_board_enbl & m_phantom)) return 0xff;
 
 	if ((offset >= 0xf800) && (offset < 0xfbf8))
 	{
@@ -368,7 +368,7 @@ uint8_t s100_dj2db_device::s100_smemr_r(address_space &space, offs_t offset)
 	}
 	else
 	{
-		return 0;
+		return 0xff;
 	}
 
 	// LS241 inverts data
@@ -475,7 +475,7 @@ void s100_dj2db_device::s100_mwrt_w(address_space &space, offs_t offset, uint8_t
 
 uint8_t s100_dj2db_device::s100_sinp_r(address_space &space, offs_t offset)
 {
-	return 0;
+	return 0xff;
 }
 
 
diff --git a/src/devices/bus/s100/mm65k16s.cpp b/src/devices/bus/s100/mm65k16s.cpp
index 2cc3edabb58..27cdc021d39 100644
--- a/src/devices/bus/s100/mm65k16s.cpp
+++ b/src/devices/bus/s100/mm65k16s.cpp
@@ -235,7 +235,7 @@ void s100_mm65k16s_device::device_reset()
 
 uint8_t s100_mm65k16s_device::s100_smemr_r(address_space &space, offs_t offset)
 {
-	uint8_t data = 0;
+	uint8_t data = 0xff;
 
 	if (offset < 0xf800)
 	{
diff --git a/src/devices/bus/s100/nsmdsa.cpp b/src/devices/bus/s100/nsmdsa.cpp
index 3a2ab33b594..8efa1c9f392 100644
--- a/src/devices/bus/s100/nsmdsa.cpp
+++ b/src/devices/bus/s100/nsmdsa.cpp
@@ -107,5 +107,5 @@ void s100_mds_a_device::device_reset()
 
 uint8_t s100_mds_a_device::s100_smemr_r(address_space &space, offs_t offset)
 {
-	return 0;
+	return 0xff;
 }
diff --git a/src/devices/bus/s100/nsmdsad.cpp b/src/devices/bus/s100/nsmdsad.cpp
index 21c25e7dcbc..d13f6a7f748 100644
--- a/src/devices/bus/s100/nsmdsad.cpp
+++ b/src/devices/bus/s100/nsmdsad.cpp
@@ -109,5 +109,5 @@ void s100_mds_ad_device::device_reset()
 
 uint8_t s100_mds_ad_device::s100_smemr_r(address_space &space, offs_t offset)
 {
-	return 0;
+	return 0xff;
 }
diff --git a/src/devices/bus/s100/s100.cpp b/src/devices/bus/s100/s100.cpp
index 798cfd67b9e..dd28ca385d3 100644
--- a/src/devices/bus/s100/s100.cpp
+++ b/src/devices/bus/s100/s100.cpp
@@ -137,13 +137,13 @@ void s100_bus_device::add_card(device_s100_card_interface *card)
 
 READ8_MEMBER( s100_bus_device::smemr_r )
 {
-	uint8_t data = 0;
+	uint8_t data = 0xff;
 
 	device_s100_card_interface *entry = m_device_list.first();
 
 	while (entry)
 	{
-		data |= entry->s100_smemr_r(space, offset);
+		data &= entry->s100_smemr_r(space, offset);
 		entry = entry->next();
 	}
 
@@ -173,13 +173,13 @@ WRITE8_MEMBER( s100_bus_device::mwrt_w )
 
 READ8_MEMBER( s100_bus_device::sinp_r )
 {
-	uint8_t data = 0;
+	uint8_t data = 0xff;
 
 	device_s100_card_interface *entry = m_device_list.first();
 
 	while (entry)
 	{
-		data |= entry->s100_sinp_r(space, offset);
+		data &= entry->s100_sinp_r(space, offset);
 		entry = entry->next();
 	}
 
diff --git a/src/devices/bus/s100/s100.h b/src/devices/bus/s100/s100.h
index 139839a7172..6336be291e4 100644
--- a/src/devices/bus/s100/s100.h
+++ b/src/devices/bus/s100/s100.h
@@ -148,7 +148,7 @@ public:
 	// interrupts
 	virtual void s100_int_w(int state) { }
 	virtual void s100_nmi_w(int state) { }
-	virtual uint8_t s100_sinta_r(offs_t offset) { return 0; }
+	virtual uint8_t s100_sinta_r(offs_t offset) { return 0xff; }
 
 	// vectored interrupts
 	virtual void s100_vi0_w(int state) { }
@@ -161,11 +161,11 @@ public:
 	virtual void s100_vi7_w(int state) { }
 
 	// memory access
-	virtual uint8_t s100_smemr_r(address_space &space, offs_t offset) { return 0; }
+	virtual uint8_t s100_smemr_r(address_space &space, offs_t offset) { return 0xff; }
 	virtual void s100_mwrt_w(address_space &space, offs_t offset, uint8_t data) { }
 
 	// I/O access
-	virtual uint8_t s100_sinp_r(address_space &space, offs_t offset) { return 0; }
+	virtual uint8_t s100_sinp_r(address_space &space, offs_t offset) { return 0xff; }
 	virtual void s100_sout_w(address_space &space, offs_t offset, uint8_t data) { }
 
 	// configuration access
diff --git a/src/devices/bus/s100/seals8k.cpp b/src/devices/bus/s100/seals8k.cpp
index 79e90a29d4f..9191f8566df 100644
--- a/src/devices/bus/s100/seals8k.cpp
+++ b/src/devices/bus/s100/seals8k.cpp
@@ -208,7 +208,7 @@ u8 s100_8k_sc_device::s100_smemr_r(address_space &space, offs_t offset)
 	if (board_selected(offset))
 		return m_ram[offset & 0x1fff];
 	else
-		return 0;
+		return 0xff;
 }
 
 
diff --git a/src/devices/bus/s100/wunderbus.cpp b/src/devices/bus/s100/wunderbus.cpp
index 43b7be095a3..9d307d9053b 100644
--- a/src/devices/bus/s100/wunderbus.cpp
+++ b/src/devices/bus/s100/wunderbus.cpp
@@ -303,9 +303,9 @@ void s100_wunderbus_device::s100_vi2_w(int state)
 uint8_t s100_wunderbus_device::s100_sinp_r(address_space &space, offs_t offset)
 {
 	uint8_t address = (m_7c->read() & 0x3e) << 2;
-	if ((offset & 0xf8) != address) return 0;
+	if ((offset & 0xf8) != address) return 0xff;
 
-	uint8_t data = 0;
+	uint8_t data = 0xff;
 
 	if ((offset & 0x07) < 7)
 	{