diff --git a/picosim/srcsim/picosim.c b/picosim/srcsim/picosim.c
index 14a1fae5..9a13ab0f 100644
--- a/picosim/srcsim/picosim.c
+++ b/picosim/srcsim/picosim.c
@@ -51,6 +51,7 @@
 
 #include "disks.h"
 #include "rgbled.h"
+#include "picosim.h"
 
 #ifdef WANT_ICE
 static void picosim_ice_cmd(char *cmd, WORD *wrk_addr);
diff --git a/z80core/simcore.c b/z80core/simcore.c
index 518b0d29..73dc074e 100644
--- a/z80core/simcore.c
+++ b/z80core/simcore.c
@@ -11,7 +11,6 @@
 
 #include <inttypes.h>
 #include <stddef.h>
-#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 
@@ -298,6 +297,10 @@ BYTE io_in(BYTE addrl, BYTE addrh)
 	fp_led_data = io_data;
 #endif
 
+#ifdef IOPANEL
+	port_flags[addrl].in = true;
+#endif
+
 	LOGD(TAG, "input %02x from port %02x", io_data, io_port);
 
 	cpu_time -= get_clock_us() - clk;
@@ -353,6 +356,10 @@ void io_out(BYTE addrl, BYTE addrh, BYTE data)
 	fp_led_data = IO_DATA_UNUSED;
 #endif
 
+#ifdef IOPANEL
+	port_flags[addrl].out = true;
+#endif
+
 	cpu_time -= get_clock_us() - clk;
 }
 
@@ -361,7 +368,6 @@ void io_out(BYTE addrl, BYTE addrh, BYTE data)
  */
 void start_bus_request(BusDMA_t mode, Tstates_t (*bus_master)(BYTE bus_ack))
 {
-
 	bus_mode = mode;
 	dma_bus_master = bus_master;
 	bus_request = 1;
@@ -372,7 +378,6 @@ void start_bus_request(BusDMA_t mode, Tstates_t (*bus_master)(BYTE bus_ack))
  */
 void end_bus_request(void)
 {
-
 	bus_mode = BUS_DMA_NONE;
 	dma_bus_master = NULL;
 	bus_request = 0;
diff --git a/z80core/simdefs.h b/z80core/simdefs.h
index e931bb31..0ad9222f 100644
--- a/z80core/simdefs.h
+++ b/z80core/simdefs.h
@@ -8,6 +8,7 @@
 #ifndef SIMDEFS_INC
 #define SIMDEFS_INC
 
+#include <stdbool.h>
 #include <stdint.h>
 
 #define COPYR	"Copyright (C) 1987-2024 by Udo Munk and others"
@@ -92,6 +93,13 @@ typedef uint64_t Tstates_t;	/* uint64 for counting T-states */
 typedef enum { BUS_DMA_NONE, BUS_DMA_BYTE,
 	       BUS_DMA_BURST, BUS_DMA_CONTINUOUS } BusDMA_t;
 
+#ifdef IOPANEL
+typedef struct port_flags {
+	bool in: 1;		/* input port accessed */
+	bool out: 1;		/* output port accessed */
+} port_flags_t;
+#endif
+
 /*
  *	macro for declaring unused function parameters
  */
diff --git a/z80core/simglb.c b/z80core/simglb.c
index 0162ce95..4230f95a 100644
--- a/z80core/simglb.c
+++ b/z80core/simglb.c
@@ -75,17 +75,21 @@ int cpu_needed;			/* don't adjust CPU freq if needed */
 #ifdef FRONTPANEL
 uint64_t fp_clock;		/* simulation clock */
 float fp_fps = 30.0;		/* frame rate, default 30 usually works */
-WORD fp_led_address;		/* lights for address bus */
-BYTE fp_led_data;		/* lights for data bus */
 WORD address_switch;		/* address and programmed input switches */
-BYTE fp_led_output = 0xff;	/* inverted IMSAI/Cromemco programmed output */
 #endif
-#ifdef SIMPLEPANEL
+#if defined(FRONTPANEL) || defined(SIMPLEPANEL)
 WORD fp_led_address;		/* lights for address bus */
 BYTE fp_led_data;		/* lights for data bus */
 BYTE fp_led_output = 0xff;	/* inverted IMSAI/Cromemco programmed output */
 #endif
 
+/*
+ *	Variables for iopanel
+ */
+#ifdef IOPANEL
+port_flags_t port_flags[256];	/* port access flags */
+#endif
+
 /*
  *	Flags to control operation of simulation
  */
diff --git a/z80core/simglb.h b/z80core/simglb.h
index 6b2c6df8..005654d0 100644
--- a/z80core/simglb.h
+++ b/z80core/simglb.h
@@ -56,17 +56,18 @@ extern int	tmax, cpu_needed;
 #ifdef FRONTPANEL
 extern uint64_t fp_clock;
 extern float	fp_fps;
-extern WORD 	fp_led_address;
-extern BYTE 	fp_led_data;
 extern WORD 	address_switch;
-extern BYTE 	fp_led_output;
 #endif
-#ifdef SIMPLEPANEL
+#if defined(FRONTPANEL) || defined(SIMPLEPANEL)
 extern WORD 	fp_led_address;
 extern BYTE 	fp_led_data;
 extern BYTE 	fp_led_output;
 #endif
 
+#ifdef IOPANEL
+extern port_flags_t port_flags[256];
+#endif
+
 extern int	s_flag, l_flag, m_flag, x_flag, i_flag, f_flag,
 		u_flag, r_flag, c_flag;
 #ifdef HAS_CONFIG