diff --git a/src/energenie/drv/gpio.h b/src/energenie/drv/gpio.h
index 7437e9a..6226a1a 100644
--- a/src/energenie/drv/gpio.h
+++ b/src/energenie/drv/gpio.h
@@ -10,13 +10,14 @@
 /***** FUNCTION PROTOTYPES *****/
 
 void gpio_init(void);
-void gpio_setin(int g);
-void gpio_setout(int g);
-void gpio_high(int g);
-void gpio_low(int g);
-void gpio_write(int g, int v);
-int  gpio_read(int g);
-//TODO probably need gpio_finished() to unmmap() the memory region and clean up the peripheral?
+void gpio_setin(uint8_t g);
+void gpio_setout(uint8_t g);
+void gpio_high(uint8_t g);
+void gpio_low(uint8_t g);
+void gpio_write(uint8_t g, uint8_t v);
+int  gpio_read(uint8_t g);
+void gpio_finished(void);
+
 #endif
 
 /***** END OF FILE *****/
diff --git a/src/energenie/drv/gpio_arduino.c b/src/energenie/drv/gpio_arduino.c
index 589e77c..ac0a6de 100644
--- a/src/energenie/drv/gpio_arduino.c
+++ b/src/energenie/drv/gpio_arduino.c
@@ -12,7 +12,39 @@
  * boards in the future.
  */
 
-//TODO GPIO_SIMULATED can be replaced with gpio_sim.c in future
+#else if defined(ARDUINO)
+#if defined(ARDUINO_PROMICRO)
+
+//TODO: there might be platform specific macros here to
+//extract register address and bitmask
+//really, what we want is to be able to pass in a uint8_t which is a GPIO number
+//and this get converted (at compile time preferably) into the register access code
+//required to read/write and configure that bit. The actual mapping shoudl probably
+//be in the C code for gpio_arduino
+
+//something a bit like this, but this is not correct yet
+//#define GPIO_0   PORTB,0x01
+//#define GPIO_1   PORTB,0x02
+//#define GPIO_2   PORTB,0x04
+//#define GPIO_3   PORTB,0x08
+//#define GPIO_4   PORTB,0x10
+//#define GPIO_5   PORTB,0x20
+//#define GPIO_6   PORTB,0x40
+//#define GPIO_7   PORTB,0x90
+//#define GPIO_8   PORTC,0x01
+//#define GPIO_9   PORTC,0x02
+//#define GPIO_10  PORTC,0x04
+//#define GPIO_14  PORTC,0x08
+//#define GPIO_15  PORTC,0x10
+//#define GPIO_16  PORTC,0x20
+
+#else
+//#error Unknown Arduino platform
+#endif
+#else
+//#error Unknown platform
+#endif
+
 
 /***** INCLUDES *****/
 
diff --git a/src/energenie/drv/gpio_rpi.c b/src/energenie/drv/gpio_rpi.c
index ce0cd6e..07348d5 100644
--- a/src/energenie/drv/gpio_rpi.c
+++ b/src/energenie/drv/gpio_rpi.c
@@ -146,4 +146,10 @@
 }
 
 
+void gpio_finished(void)
+{
+  //TODO probably need gpio_finished() to unmmap() the memory region and clean up the peripheral?
+}
+
+
 /***** END OF FILE *****/
diff --git a/src/energenie/drv/gpio_sim.c b/src/energenie/drv/gpio_sim.c
index de2c2d7..fc89aef 100644
--- a/src/energenie/drv/gpio_sim.c
+++ b/src/energenie/drv/gpio_sim.c
@@ -1,38 +1,35 @@
 /* gpio_sim.c  D.J.Whale  04/04/2016
  * 
- * A very simple interface to a simulated GPIO port with no platform dependencies.
+ * A very simple interface to a simulated GPIO port with minimal platform dependencies.
  */
 
 /***** INCLUDES *****/
 
-#include <stdio.h>
-//#include <stdlib.h>
-
+#include <stdio.h> // OPTIONAL
 #include "gpio.h"
 
 
 /***** CONFIGURATION *****/
 
+#define GPIO_MAX      20
+#define GPIO_DEBUG
+//#define GPIO_LOOPBACK
 
-/***** CONSTANTS *****/
+
+/* Printf is not available on some platforms, or not very efficient.
+ * These macros make it possible to re-map I/O to more efficient functions.
+ */
+
+#define OUTS(s)  printf("%s", s)
+#define OUTN(n)  printf("%d", (unsigned int)d)
+#define OUTC(c)  putch(c)
+#define NL()     OUTC('\n')
 
 
 /***** VARIABLES *****/
 
-
-/****** MACROS *****/
-
-//#define INP_GPIO(g) *(gpio+((g)/10)) &= ~(7<<(((g)%10)*3))
-//#define OUT_GPIO(g) *(gpio+((g)/10)) |=  (1<<(((g)%10)*3))
-//#define SET_GPIO_ALT(g,a) *(gpio+(((g)/10))) |= (((a)<=3?(a)+4:(a)==4?3:2)<<(((g)%10)*3))
-
-//#define GPIO_SET *(gpio+7)  // sets   bits which are 1 ignores bits which are 0
-//#define GPIO_CLR *(gpio+10) // clears bits which are 1 ignores bits which are 0
-
-//#define GPIO_READ(g) ((*(gpio+13)&(1<<g)) != 0)
-
-//#define GPIO_HIGH(g) GPIO_SET = (1<<(g))
-//#define GPIO_LOW(g)  GPIO_CLR = (1<<(g))
+static uint8_t gpio_out[GPIO_MAX] = {0};
+static uint8_t gpio_in[GPIO_MAX]  = {0};
 
 
 void gpio_init()
@@ -41,41 +38,110 @@
 }
 
 
-void gpio_setin(int g)
+void gpio_setin(uint8_t g)
 {
-    printf("gpio:in:%d\n", g);
+#if defined(GPIO_DEBUG)
+    //printf("gpio:in:%d\n", g);
+    OUTS("gpio:in");
+    OUTN(g)
+    NL()
+#endif
 }
 
 
-void gpio_setout(int g)
+void gpio_setout(uint8_t g)
 {
-    printf("gpio:out:%d\n", g);
+#if defined(GPIO_DEBUG)
+    //printf("gpio:out:%d\n", g);
+    OUTS("gpio:out:");
+    OUTN(g);
+    NL();
+#endif
 }
 
 
-void gpio_high(int g)
+void gpio_high(uint8_t g)
 {
-    printf("gpio:high:%d\n", g);
+#if defined(GPIO_DEBUG)
+    //printf("gpio:high:%d\n", g);
+    OUTS("gpio:high:");
+    OUTN(g);
+    NL();
+#endif
+
+    gpio_out[g] = 1;
+
+#if defined(GPIO_LOOPBACK)
+    gpio_in[g] = 1;
+#endif
 }
 
 
-void gpio_low(int g)
+void gpio_low(uint8_t g)
 {
-    printf("gpio:low:%d\n", g);
+#if defined(GPIO_DEBUG)
+    //printf("gpio:low:%d\n", g);
+    OUTS("gpio:low");
+    OUTN(g);
+    NL();
+#endif
+
+    gpio_out[g] = 0;
+
+#if defined(GPIO_LOOPBACK)
+    gpio_in[g] = 0;
+#endif
 }
 
 
-void gpio_write(int g, int v)
+void gpio_write(uint8_t g, uint8_t v)
 {
-    printf("gpio:write:%d=%d\n", g, v);
+#if defined(GPIO_DEBUG)
+    //printf("gpio:write:%d=%d\n", g, v);
+    OUTS("gpio:write:");
+    OUTN(g);
+    OUTC('=');
+    OUTN(v);
+    NL();
+#endif
+
+    gpio_out[g] = v;
+
+#if defined(GPIO_LOOPBACK)
+    gpio_in[g] = v;
+#endif
 }
 
 
-int gpio_read(int g)
+uint8_t gpio_read(uint8_t g)
 {
-    //TODO add a console interface to allow GPIO reads to be injected
-    //either from keyboard, or from a script file
-    return 0; /* always low in simulation */
+#if defined(GPIO_DEBUG)
+    //printf("gpio:read:%d=%d\n")
+    OUTS("gpio:read:");
+    OUTN(g);
+    OUTC('=');
+    OUTN(gpio_in[g]);
+    NL();
+#endif
+    return gpio_in[g];
+}
+
+
+void gpio_finished(void)
+{
+    // Nothing to do
+}
+
+
+void gpio_mock_set_in(uint8_t g, uint8_t v)
+{
+    gpio_in[g] = v
+}
+
+
+uint8_t gpio_mock_get_out(uint8_t g)
+{
+    return gpio_out[g];
 }
 
 
diff --git a/src/energenie/drv/gpio_test.c b/src/energenie/drv/gpio_test.c
index a10a71b..109f720 100644
--- a/src/energenie/drv/gpio_test.c
+++ b/src/energenie/drv/gpio_test.c
@@ -7,6 +7,7 @@
 
 #include "gpio.h"
 
+//TODO this is platform specific (won't work on Arduino?)
 static void delay(struct timespec time)
 {
   nanosleep(&time, NULL);
@@ -14,27 +15,31 @@
 
 static struct timespec delay_1sec = {1, 0};
 
+// To allow platform specific test harnesses
+
+#define GPIO_A 2
+#define GPIO_B 3
 
 void main(void)
 {
   int i;
 
   gpio_init();
-  gpio_setout(2);
-  gpio_setout(3);
+  gpio_setout(GPIO_A);
+  gpio_setout(GPIO_B);
 
   for (i=0; i<10; i++)
   {
-    puts("GPIO 2");
-    gpio_write(2, 1);
+    puts("GPIO A");
+    gpio_write(GPIO_A, 1);
     delay(delay_1sec);
-    gpio_write(2, 0);
+    gpio_write(GPIO_A, 0);
     delay(delay_1sec);
 
-    puts("GPIO 3");
-    gpio_write(3, 1);
+    puts("GPIO B");
+    gpio_write(GPIO_B, 1);
     delay(delay_1sec);
-    gpio_write(3, 0);
+    gpio_write(GPIO_B, 0);
     delay(delay_1sec);
   }
 }