diff --git a/src/emu/cpu/mcs51/mcs51.c b/src/emu/cpu/mcs51/mcs51.c
index c212047e739..5cac651371c 100644
--- a/src/emu/cpu/mcs51/mcs51.c
+++ b/src/emu/cpu/mcs51/mcs51.c
@@ -286,6 +286,11 @@ mcs51_cpu_device::mcs51_cpu_device(const machine_config &mconfig, device_type ty
 	m_ds5002fp.mcon = 0;
 	m_ds5002fp.rpctl = 0;
 	m_ds5002fp.crc = 0;
+
+	/* default to standard cmos interfacing */
+
+	for (int i=0; i < ARRAY_LENGTH(m_forced_inputs); i++)
+		m_forced_inputs[i] = 0;
 }
 
 
@@ -2079,10 +2084,10 @@ UINT8 mcs51_cpu_device::sfr_read(size_t offset)
 	{
 		/* Read/Write/Modify operations read the port latch ! */
 		/* Move to memory map */
-		case ADDR_P0:   return RWM ? P0 : P0 & IN(MCS51_PORT_P0);
-		case ADDR_P1:   return RWM ? P1 : P1 & IN(MCS51_PORT_P1);
-		case ADDR_P2:   return RWM ? P2 : P2 & IN(MCS51_PORT_P2);
-		case ADDR_P3:   return RWM ? P3 : P3 & IN(MCS51_PORT_P3);
+		case ADDR_P0:   return RWM ? P0 : (P0 | m_forced_inputs[0]) & IN(MCS51_PORT_P0);
+		case ADDR_P1:   return RWM ? P1 : (P1 | m_forced_inputs[1]) & IN(MCS51_PORT_P1);
+		case ADDR_P2:   return RWM ? P2 : (P2 | m_forced_inputs[2]) & IN(MCS51_PORT_P2);
+		case ADDR_P3:   return RWM ? P3 : (P3 | m_forced_inputs[3]) & IN(MCS51_PORT_P3);
 
 		case ADDR_PSW:
 		case ADDR_ACC:
diff --git a/src/emu/cpu/mcs51/mcs51.h b/src/emu/cpu/mcs51/mcs51.h
index 50069ae3070..cb33c4f9252 100644
--- a/src/emu/cpu/mcs51/mcs51.h
+++ b/src/emu/cpu/mcs51/mcs51.h
@@ -72,6 +72,16 @@ enum
 	MCS51_PORT_TX   = 0x20004,  /* P3.1 */
 };
 
+/* At least CMOS devices may be forced to read from ports configured as output.
+ * All you need is a low impedance output connect to the port.
+ */
+
+#define MCFG_MCS51_PORT1_CONFIG(_forced_inputs) \
+	mcs51_cpu_device::set_port_forced_input(*device, 1, _forced_inputs);
+#define MCFG_MCS51_PORT2_CONFIG(_forced_inputs) \
+	mcs51_cpu_device::set_port_forced_input(*device, 2, _forced_inputs);
+#define MCFG_MCS51_PORT3_CONFIG(_forced_inputs) \
+	mcs51_cpu_device::set_port_forced_input(*device, 3, _forced_inputs);
 
 class mcs51_cpu_device : public cpu_device
 {
@@ -82,6 +92,9 @@ public:
 	void i8051_set_serial_tx_callback(write8_delegate tx_func);
 	void i8051_set_serial_rx_callback(read8_delegate rx_func);
 
+	// configuration helpers
+	static void set_port_forced_input(device_t &device, UINT8 port, UINT8 forced_input) { downcast<mcs51_cpu_device &>(device).m_forced_inputs[port] = forced_input; }
+
 protected:
 	// device-level overrides
 	virtual void device_start();
@@ -137,6 +150,8 @@ protected:
 	UINT8   m_irq_active;         /* mask which irq levels are serviced */
 	UINT8   m_irq_prio[8];        /* interrupt priority */
 
+	UINT8	m_forced_inputs[4];	  /* allow read even if configured as output */
+
 	int     m_icount;
 
 	struct mcs51_uart