diff --git a/external_mem.c b/external_mem.c
index 0ae6327..86e44c9 100644
--- a/external_mem.c
+++ b/external_mem.c
@@ -9,7 +9,7 @@
 #include "ports.h"
 #include <avr/io.h>
 
-#define NUM_ADDRESS_LINES	20
+#define HIGHEST_ADDRESS_LINE	20
 
 // Allow this to be initialized more than once.
 // In case we mess with the port settings,
@@ -22,11 +22,11 @@ void ExternalMem_Init(void)
 
 	// Disable all pull-ups, on both the address and data lines. They aren't needed
 	// for normal operation.
-	Ports_AddressPullups_RMW(0, (1UL << (NUM_ADDRESS_LINES - 1)) - 1);
+	Ports_AddressPullups_RMW(0, (1UL << (HIGHEST_ADDRESS_LINE + 1)) - 1);
 	Ports_DataPullups_RMW(0, 0xFFFFFFFFUL);
 
 	// Configure all address lines as outputs
-	Ports_SetAddressDDR((1UL << (NUM_ADDRESS_LINES - 1)) - 1);
+	Ports_SetAddressDDR((1UL << (HIGHEST_ADDRESS_LINE - 1)) - 1);
 
 	// Sensible defaults for address and data lines:
 	// Write out address zero
diff --git a/tests/simm_electrical_test.c b/tests/simm_electrical_test.c
index adb46b0..e13e104 100644
--- a/tests/simm_electrical_test.c
+++ b/tests/simm_electrical_test.c
@@ -6,11 +6,37 @@
  */
 
 #include "simm_electrical_test.h"
+#include "../ports.h"
+
+#define SIMM_HIGHEST_ADDRESS_LINE	18
 
 int SIMMElectricalTest_Run(void)
 {
 	// Returns number of errors found
 	int numErrors = 0;
 
+	Ports_Init();
+
+	// First check for anything shorted to ground. Set all lines as inputs with a weak pull-up resistor.
+	// Then read the values back and check for any zeros. This would indicate a short to ground.
+	Ports_SetAddressDDR(0);
+	Ports_SetDataDDR(0);
+	Ports_AddressPullups_RMW((1UL << (SIMM_HIGHEST_ADDRESS_LINE + 1)) - 1, (1UL << (SIMM_HIGHEST_ADDRESS_LINE + 1)) - 1);
+	Ports_DataPullups_RMW(0xFFFFFFFFUL, 0xFFFFFFFFUL);
+
+	// TODO: Wait a sec?
+
+	if (Ports_ReadAddress() != (1UL << (SIMM_HIGHEST_ADDRESS_LINE + 1)) - 1)
+	{
+		// TODO: Log all these errors somewhere?
+		numErrors++;
+	}
+
+	if (Ports_ReadData() != 0xFFFFFFFFUL)
+	{
+		// TODO: Log all these errors somewhere?
+		numErrors++;
+	}
+
 	return numErrors;
 }