diff --git a/src/mame/drivers/saitek_leonardo.cpp b/src/mame/drivers/saitek_leonardo.cpp
index 95009b8cd0c..09cef526c7f 100644
--- a/src/mame/drivers/saitek_leonardo.cpp
+++ b/src/mame/drivers/saitek_leonardo.cpp
@@ -53,6 +53,7 @@ TODO:
 
 #include "bus/saitek_osa/expansion.h"
 #include "cpu/m6800/m6801.h"
+#include "machine/input_merger.h"
 #include "machine/sensorboard.h"
 #include "sound/spkrdev.h"
 #include "video/pwm.h"
@@ -73,6 +74,7 @@ public:
 		driver_device(mconfig, type, tag),
 		m_maincpu(*this, "maincpu"),
 		m_expansion(*this, "exp"),
+		m_stb(*this, "stb"),
 		m_board(*this, "board"),
 		m_display(*this, "display"),
 		m_dac(*this, "dac"),
@@ -90,6 +92,7 @@ private:
 	// devices/pointers
 	required_device<hd6303y_cpu_device> m_maincpu;
 	required_device<saitekosa_expansion_device> m_expansion;
+	required_device<input_merger_device> m_stb;
 	required_device<sensorboard_device> m_board;
 	required_device<pwm_display_device> m_display;
 	optional_device<speaker_sound_device> m_dac;
@@ -102,7 +105,6 @@ private:
 	void leds_w(u8 data);
 	u8 unk_r();
 	void unk_w(u8 data);
-	void exp_stb_w(int state);
 	void exp_rts_w(int state);
 
 	u8 p2_r();
@@ -166,15 +168,9 @@ void leo_state::unk_w(u8 data)
 	// ?
 }
 
-void leo_state::exp_stb_w(int state)
-{
-	// STB-P to P5 IS
-	m_maincpu->set_input_line(M6801_IS_LINE, state ? CLEAR_LINE : ASSERT_LINE);
-}
-
 void leo_state::exp_rts_w(int state)
 {
-	// NAND with ACK-P? (not used by module)
+	// NAND with ACK-P (not used by module)
 }
 
 
@@ -213,7 +209,8 @@ void leo_state::p5_w(u8 data)
 	// d2: expansion NMI-P
 	m_expansion->nmi_w(BIT(data, 2));
 
-	// d3: NAND with STB-P?
+	// d3: NAND with STB-P
+	m_stb->in_w<1>(BIT(data, 3));
 
 	// d5: expansion ACK-P
 	m_expansion->ack_w(BIT(data, 5));
@@ -364,6 +361,9 @@ void leo_state::leonardo(machine_config &config)
 	m_maincpu->in_p6_cb().set(FUNC(leo_state::p6_r));
 	m_maincpu->out_p6_cb().set(FUNC(leo_state::p6_w));
 
+	INPUT_MERGER_ANY_LOW(config, m_stb);
+	m_stb->output_handler().set_inputline(m_maincpu, M6801_IS_LINE);
+
 	config.set_maximum_quantum(attotime::from_hz(6000));
 
 	SENSORBOARD(config, m_board).set_type(sensorboard_device::MAGNETS);
@@ -380,7 +380,7 @@ void leo_state::leonardo(machine_config &config)
 
 	// expansion module
 	SAITEKOSA_EXPANSION(config, m_expansion, saitekosa_expansion_modules);
-	m_expansion->stb_handler().set(FUNC(leo_state::exp_stb_w));
+	m_expansion->stb_handler().set(m_stb, FUNC(input_merger_device::in_w<0>));
 	m_expansion->rts_handler().set(FUNC(leo_state::exp_rts_w));
 }
 
diff --git a/src/mame/drivers/saitek_renaissance.cpp b/src/mame/drivers/saitek_renaissance.cpp
index 83cd1d26bbb..09fcabc6878 100644
--- a/src/mame/drivers/saitek_renaissance.cpp
+++ b/src/mame/drivers/saitek_renaissance.cpp
@@ -33,6 +33,7 @@ TODO:
 
 #include "bus/saitek_osa/expansion.h"
 #include "cpu/m6800/m6801.h"
+#include "machine/input_merger.h"
 #include "machine/sensorboard.h"
 #include "sound/spkrdev.h"
 #include "video/pwm.h"
@@ -55,6 +56,7 @@ public:
 		driver_device(mconfig, type, tag),
 		m_maincpu(*this, "maincpu"),
 		m_expansion(*this, "exp"),
+		m_stb(*this, "stb"),
 		m_board(*this, "board"),
 		m_display(*this, "display"),
 		m_lcd_pwm(*this, "lcd_pwm"),
@@ -76,6 +78,7 @@ private:
 	// devices/pointers
 	required_device<hd6303y_cpu_device> m_maincpu;
 	required_device<saitekosa_expansion_device> m_expansion;
+	required_device<input_merger_device> m_stb;
 	required_device<sensorboard_device> m_board;
 	required_device<pwm_display_device> m_display;
 	required_device<pwm_display_device> m_lcd_pwm;
@@ -94,7 +97,6 @@ private:
 	void leds_w(u8 data);
 	void control_w(u8 data);
 	u8 control_r();
-	void exp_stb_w(int state);
 	void exp_rts_w(int state);
 
 	u8 p2_r();
@@ -189,15 +191,9 @@ u8 ren_state::control_r()
 	return 0;
 }
 
-void ren_state::exp_stb_w(int state)
-{
-	// STB-P to P5 IS
-	m_maincpu->set_input_line(M6801_IS_LINE, state ? CLEAR_LINE : ASSERT_LINE);
-}
-
 void ren_state::exp_rts_w(int state)
 {
-	// NAND with ACK-P? (not used by module)
+	// NAND with ACK-P (not used by module)
 }
 
 
@@ -237,7 +233,8 @@ void ren_state::p5_w(u8 data)
 	// d1: expansion NMI-P
 	m_expansion->nmi_w(BIT(data, 1));
 
-	// d3: NAND with STB-P?
+	// d3: NAND with STB-P
+	m_stb->in_w<1>(BIT(data, 3));
 
 	// d5: expansion ACK-P
 	m_expansion->ack_w(BIT(data, 5));
@@ -354,6 +351,9 @@ void ren_state::ren(machine_config &config)
 	m_maincpu->in_p6_cb().set(FUNC(ren_state::p6_r));
 	m_maincpu->out_p6_cb().set(FUNC(ren_state::p6_w));
 
+	INPUT_MERGER_ANY_LOW(config, m_stb);
+	m_stb->output_handler().set_inputline(m_maincpu, M6801_IS_LINE);
+
 	config.set_maximum_quantum(attotime::from_hz(6000));
 
 	SENSORBOARD(config, m_board).set_type(sensorboard_device::MAGNETS);
@@ -380,7 +380,7 @@ void ren_state::ren(machine_config &config)
 
 	// expansion module
 	SAITEKOSA_EXPANSION(config, m_expansion, saitekosa_expansion_modules);
-	m_expansion->stb_handler().set(FUNC(ren_state::exp_stb_w));
+	m_expansion->stb_handler().set(m_stb, FUNC(input_merger_device::in_w<0>));
 	m_expansion->rts_handler().set(FUNC(ren_state::exp_rts_w));
 }